Weathering of the Earth’s surface has commonly been invoked as a driver of global cooling through geologic time. During the Phanerozoic Eon (541–0 million years ago, Ma), for example, the periodic onset of icehouse conditions has variously been attributed to enhanced weathering fluxes associated with mountain building (e.g. the Himalayas) (1), reductions in the global extent of continental arc volcanoes (e.g. the present-day Andes) (2), and uplift of oceanic crust during arc-continent collisions (e.g. present-day Indonesia and New Guinea) (3). These processes, tied to the global plate tectonic cycle, are inextricably linked.  The resulting collinearity (i.e. independent variables are highly correlated) makes it difficult — using conventional statistical techniques — to isolate the contributions of individual geologic processes to global chemical weathering.   An example of this is the Late Cenozoic Ice Age (34–0 Ma) that corresponds both to uplift of the Tibetan Plateau and Himalaya, and a gradual reduction in the extent of the global continental arc system. 

We developed a machine learning framework to analyse the interdependencies between multiple global tectonic and volcanic processes (e.g., continental distribution, extent of volcanic arcs, mid-ocean ridges etc.) and seawater Sr composition (a proxy for weathering flux) over the past 400 million years. We developed a Bayesian network incorporating a novel algorithm that accounts for time lags for each of the predictor variables, and joint conditional dependence (i.e. how variables combine to influence the environmental outcome). Our approach overcomes problems traditionally encountered in geologic time series, such as collinearity and autocorrelation. Our results strongly indicate a first-order role for volcanism in driving chemical weathering fluxes since the mid-Palaeozoic. This is consistent with the strong empirical correlation previously observed between the strontium isotope composition of seawater and continental igneous rocks over the past billion years (4). Our study highlights how geologic processes operate together — not in isolation — to perturb the Earth system over ten to hundred million-year timescales.

References

(1). M. E. Raymo, W. F. Ruddiman, Tectonic forcing of late Cenozoic climate, Nature 359, 117 (1992).

(2). N. R. McKenzie, et al., Continental arc volcanism as the principal driver of icehouse greenhouse variability, Science 352, 444 (2016).

(3). F. A. Macdonald, N. L. Swanson-Hysell, Y. Park, L. Lisiecki, O. Jagoutz, Arc-continent collisions in the tropics set Earth’s climate state, Science 364, 181 (2019).

(4). C. P. Bataille et al., Continental igneous rock composition: A major control of past global chemical weathering, Science Advances 3, e1602183 (2017).  

How to cite: Gernon, T., Hincks, T., Merdith, A., Rohling, E., Palmer, M., Foster, G., Bataille, C., and Muller, D.: Tectonic forcing of global chemical weathering since the mid-Paleozoic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10623, https://doi.org/10.5194/egusphere-egu2020-10623, 2020.

Our understanding of the geological regulation of the carbon cycle has been deeply influenced by the contribution of Bob Berner with his well-known model GEOCARB. Here, we will present a fundamentally different carbon cycle model that explicitly accounts for the effect of the paleogeography using physically based climate simulations and using 22 continental configurations spanning the whole Phanerozoic (GEOCLIM, geoclimmodel.wordpress.com). We will show that several key features of the Phanerozoic climate can be simply explained by the modulation of the carbon cycle by continental drift with the notable exception of the Late Paleozoic Ice Age, which is explained by the intense weathering of the Hercynian mountain range. In particular, the continental drift may have strongly impacted the runoff intensity as well as the weathering flux during the transition from the hot Early Cambrian world to the colder Ordovician world. Another fascinating example is the large atmospheric CO₂ decrease simulated during the Triassic owing to the northward drift of Pangea exposing large continental area to humid sub-tropics and boosting continental weathering. Conversely, our model fails to reproduce the climatic trend of the last 100 Ma. This is due to the highly dispersed continental configurations of the last 100 Ma that optimize the consumption of CO₂ through continental weathering. This discrepancy may be reduced if we account for a larger influence of the Earth degassing flux on the atmospheric CO₂ evolution, which could come from the increase contribution of the pelagic component on the oceanic crust on the global carbonate flux and from the many sub-marine LIPs occurring during the Late Cretaceous.

How to cite: Godderis, Y. and Donnadieu, Y.: The contribution of numerical models to our understanding of the Phanerozoic CO2 history, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19675, https://doi.org/10.5194/egusphere-egu2020-19675, 2020.

The mid-Cretaceous was one of the warmest intervals of the past 140 million years (Myr) driven by atmospheric CO₂ levels around 1000 ppmv. In the near absence of proximal geological records from south of the Antarctic Circle, it remains disputed whether polar ice could exist under such environmental conditions. Here we present results from a unique sedimentary sequence recovered from the West Antarctic shelf. This by far southernmost Cretaceous record contains an intact ~3 m-long network of in-situ fossil roots. The roots are embedded in a mudstone matrix bearing diverse pollen and spores, indicative of a temperate lowland rainforest environment at a palaeolatitude of ~82°S during the Turonian–Santonian (93–83 Myr). A climate model simulation shows that the reconstructed temperate climate at this high latitude requires a combination of both atmospheric CO₂ contents of 1120–1680 ppmv and a vegetated land surface without major Antarctic glaciation, highlighting the important cooling effect exerted by ice albedo in high-CO₂ climate worlds.

How to cite: Klages, J. P., Ulrich, S., Torsten, B., Claus-Dieter, H., Karsten, G., Gerhard, K., Steven, B., Jürgen, T., Juliane, M., Thomas, F., Thorsten, B., Werner, E., Tina, V. D. F., Patric, S. P., Robert, L., Gerrit, L., Niezgodzki, I., Gabriele, U.-N., Maximilian, Z., and Cornelia, S. and the Science Team of Expedition PS104: Temperate rainforests near the South Pole during peak Cretaceous warmth, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-242, https://doi.org/10.5194/egusphere-egu2020-242, 2020.

Reconstructing the environmental consequences of large carbon additions in the past has the potential to improve our understanding and prediction of how the Earth system will respond to human carbon emissions. However, uncertainties over the scale and timing of external carbon additions during past carbon emission events limit quantitative knowledge gained from the geological record. The metals Sr, Os, Li and Ca are essential proxies for changes in volcanic activity and terrestrial weathering rates, and thus for major causes of pre-industrial carbon emission and sequestration, because their isotopic compositions in old continental crust and Earth’s mantle differ significantly. So far, box models and equilibrium-state equations have been the only method to quantitatively relate weathering-derived and magmatic input fluxes to trace metal concentrations and isotopic ratios preserved in ancient sediments. This approach results most commonly in a first order estimate of emitted carbon or weathering changes, but it does not account for the effect of climate feedbacks on metal sources and sinks and associated variations in the residence time of these metals in the ocean. Particularly during fast carbon emissions (e.g. Cenozoic hyperthermals, Oceanic Anoxic Events), the processes which added isotopically traceable metals to the oceans also enchained environmental changes which would have affected metal cycles and residence times, resulting in significant alterations of the recorded isotopic excursion in marine sediments. To disentangle the signals of causes and consequences of environmental change recorded by trace metal isotopes, we simulated various coupled carbon and metal cycle perturbations in the 3D Earth system model of intermediate complexity cGENIE, now containing the first representation of isotope-enabled trace metal dynamics. Here, we present a resulting extended framework to reconstruct metal and carbon fluxes from the geological trace metal record during periods of environmental change.

How to cite: Adloff, M., Greene, S. E., Monteiro, F. M., and Ridgwell, A.: A new framework to quantify carbon cycle perturbations using trace metal isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5962, https://doi.org/10.5194/egusphere-egu2020-5962, 2020.

When it comes to paleoclimate data-model integration, temperature is arguably the most important parameter. Although a range of temperature proxies has been developed over the decades, many of the available methods suffer from large calibration uncertainties, in particular when applied on deep-time intervals. Clumped isotope thermometry is based on thermodynamic principles and therefore can provide accurate temperature constraints for the deeper geological record. Recent analytical developments allow now the analysis of relatively small sample sizes and the application in paleoceanogaphic studies becomes more feasible. I will present new clumped isotope based temperature estimates for the Atlantic deep-sea across the Cenozoic. I will also show that the analysis of small samples now allows us to even resolve seasonal sea surface temperature estimates from high-resolution archives. Deep-sea temperatures as well as seasonally resolved surface temperature estimates are particularly useful for data-model comparison.

How to cite: Ziegler, M.: Cenozoic climate evolution revealed by clumped isotope thermometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20336, https://doi.org/10.5194/egusphere-egu2020-20336, 2020.

A compilation of benthic δ¹⁸O from the whole Atlantic and the Southern Ocean (Atlantic sector), shows two major jumps in the interbasinal gradient of d¹⁸O (Δδ¹⁸O) during the Eocene and the Oligocene: One at ~40 Ma and the second concomitant with the isotopic event of the Eocene-Oligocene transition (EOT), ~33.7 Ma ago. From previously published circulation models, we show that the first Δδ¹⁸O jump reflects the thermal isolation of Antarctica associated with the proto-Antarctic circumpolar current (ACC). The second marks the onset of interhemispheric northern-sourced circulation cell, similar to the modern Atlantic meridional overturning circulation (AMOC). The onset of AMOC-like circulation probably slightly preceded (100-300 ky) the EOT, as we show by the high resolution profiles of δ¹⁸O and δ¹³C previously published from DSDP/ODP sites in the Southern Ocean and South Atlantic. We suggest that while the shallow proto-ACC supplied the energy for deep ocean convection in the Southern Hemisphere, the onset of the interhemispheric northern circulation cell was due to the significant EOT intensification of deepwater formation in the North Atlantic driven by the Nordic anti-estuarine circulation. This onset of the interhemispheric northern-sourced circulation cell could have prompted the EOT global cooling.

How to cite: Abelson, M. and Erez, J.: Was the onset of interhemispheric AMOC slightly prior to Antarctic glaciation at the Eocene-Oligocene transition?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2204, https://doi.org/10.5194/egusphere-egu2020-2204, 2020.

Reconstructing Cenozoic terrestrial paleoclimate is frequently limited by temporal resolution and suitable quantitative tools to reliably assess changes in temperature and aridity. The dynamics of ocean temperatures¹ and chemistry², varying pCO₂³, and faunal assemblages are known to a certain extent, however, terrestrial data on temperatures, which are mostly indirectly derived from fossil assemblages and palynologycal data⁴ are rare. This study contributes to the understanding of the dynamics and variability of terrestrial temperatures during one of the most extreme Neogene climate changes, the Middle Miocene Climate Transition (MCT). The comparison of pCO₂ forecasts for the coming century and reconstructed Mid-Miocene pCO₂ levels suggest that the Mid-Miocene is an important time interval for ascertaining suitable model projections of the future anthropogenic impact on climate. In order to establish an appropriate understanding and modeling of the natural variability of the European/Mediterranean climate system, quantitative climate information of the European continental Mid-Miocene is mandatory. This would facilitate the identification of main drivers of climate evolution in an area which is exposed to the present climate change and its subsequent natural hazards.

This study presents a profound and well-dated terrestrial clumped isotope (Δ₄₇) paleosoil carbonate dataset from Spain that ranges from 13.0 to 15.1 Ma (100 kyr resolution) and hence covers an interval that was previously classified as the MCT. The Δ₄₇ data is supported by stable carbon and oxygen isotope analyses that are in agreement with previously published continental and oceanic records. A distinct decline in apparent Δ₄₇-based temperatures between 13.7 and 14.1 Ma reveals a substantial drop in continental temperatures and indicates changes in seasonality of pedogenic carbonate formation. The major cooling thereby coincides with a change in Milanković periodicities and can be linked to oceanic isotope records⁵. While the transition into the MCT shows a high temperature variability indicating varying environmental conditions, calculated oxygen isotopic values of the soil water point to a rather stable moisture source across the MCT in Southern Europe.

1: Super, J. R., Thomas, E., Pagani, M., et al. (2018) North Atlantic temperature and pCO₂ coupling in the early-middle Miocene. Geology, 46(6), 519-522.

2: Pearson, P. N., and Palmer, M. R. (1999) Middle Eocene seawater pH and atmospheric carbon dioxide concentrations. Science, 284(5421), 1824-1826.

3: Pagani, M., Freeman, K. H., and Arthur, M. A. (1999) Late Miocene atmospheric CO₂ concentrations and the expansion of C4 grasses. Science, 285(5429), 876-879.

4: Lewis, A. R., Marchant, D. R., Ashworth, A. C., et al. (2008) Mid-Miocene cooling and the extinction of tundra in continental Antarctica. Proceedings of the National academy of Sciences.

5: Holbourn, A., Kuhnt, W., Clemens, S., et al. (2013) Middle to late Miocene stepwise climate cooling: Evidence from a high resolution deep water isotope curve spanning 8 million years. Paleoceanography, 28(4), 688-699.

How to cite: Löffler, N., Mulch, A., Krijgsman, W., Krsnik, E., and Fiebig, J.: The continental Middle Miocene Climatic Transition in Southern Europe as derived from clumped isotope analyses, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10864, https://doi.org/10.5194/egusphere-egu2020-10864, 2020.

The South Asian Monsoon (SAM) is one of the most important climatic features of the Asian continent. Proxy-based reconstructions from continuous records in the Indian Ocean suggest a settlement of modern-like monsoon during the Miocene, with a modern winds distribution and strength potentially reached by ~13 Ma. Concurrent with the SAM intensification, a major reorganization of surface ocean currents occurred in the Indian Ocean. The timing of monsoon strengthening overlaps with changes in Indian Ocean and Indonesian Gateway configurations, Himalayas uplift, global cooling, as well as East Antarctic Ice Sheet expansion. Thus, the respective influence of each factor on SAM evolution and Indian Ocean paleoceanography is still poorly understood owing to the modification of multiple forcing mechanisms.

Here we will use a set of experiments with the IPSL-CM5A2 Earth System Model under early to late Miocene configurations in order to tease apart the effects of paleogeography changes, ice-sheet growth and CO₂levels on the Indian Ocean region during the Miocene. We will focus on the impact of increasing SAM winds and precipitation on the oceanographic conditions in the Indian Ocean including not only physical parameters but also biogeochemical ones.

How to cite: Sarr, A.-C., Donnadieu, Y., Bolton, C., and Suchéras-Marx, B.: A modeling study of physical and biogeochemical changes occurring in the tropical Indian Ocean during Miocene times. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9756, https://doi.org/10.5194/egusphere-egu2020-9756, 2020.

Understanding the effect warming has on ice sheets is vital for accurate projections of climate change. A better understanding of how the Antarctic ice sheets have changed size and shape in the past would allow us to improve our predictions of how they may adapt in the future; this is of particular relevance in predicting future global sea level changes. This research makes use of previous reconstructions of the ice sheets, ice core data and Bayesian methods to create a model of the Antarctic ice sheet at the Last Glacial Maximum (LGM). We do this by finding the relationship between the ice sheet shape and water isotope values. 

We developed a prior model which describes the variation between a set of ice sheet reconstructions at the LGM. A set of ice sheet shapes formed using this model was determined by a consultation with experts and run through the general circulation model HadCM3, providing us with paired data sets of ice sheet shapes and water isotope estimates. The relationship between ice sheet shape and water isotopes is explored using a Gaussian process emulator of HadCM3, building a statistical distribution describing the shape of the ice sheets given the isotope values outputted by the climate model. We then use MCMC to sample from the posterior distribution of the ice sheet shape and attempt to find a shape that creates isotopic values matching as closely as possible to the observations collected from ice cores. This allows us to quantify the uncertainty in the shape and incorporate expert beliefs about the Antarctic ice sheet during this time period. Our results suggests that there may have been a thicker West Antarctic ice sheet at the LGM than previously estimated.

How to cite: Turner, F., Wilkinson, R., Buck, C., Jones, J. M., and Sime, L.: Antarctic Uncertainty: Learning more about past ice sheet shapes with Bayesian methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5526, https://doi.org/10.5194/egusphere-egu2020-5526, 2020.

As a significant terrestrial carbon reservoir, peatland has great potential to affect the global carbon cycle and global climate. However, our understanding of broad-scale mechanisms that control the long-term global peatland expansion and carbon accumulation rates is still limited. Here we present a new data synthesis of global coal deposit location, thickness changes, carbon concentration, and distribution area changes, along with new carbon pool estimates of global peatlands from Devonian through geological time. By identifying orbital cycles in coal seams, we show that the long-term rate of carbon accumulation (LORCA) in peatland calculated from published data is controlled by pO₂, pCO₂, temperature, precipitation, and total solar irradiance. We use this relationship and latitudinal temperature gradients to reconstruct the equations between LORCA with latitude on different geological time. The results suggest that there are three main sets of high carbon pool and high carbon accumulation rate of global peatlands in Late Paleozoic, Early-Middle Jurassic, and Late-Cretaceous to Early Cenozoic under low tectonic activity and high terrestrial plant diversity background. In addition, we measure the shortest distances between all coal locations and coastlines based on the new Scotese’s paleo-Atlas for the past 400 million years, in order to exhibit the extent of peatland expansion into inland. The result shows the Early-Middle Jurassic period has the longest average distance, which is probably due to the high sea level that minimizes the development of peat swamps on coastal areas and facilitated the moisture to move into deeper inland under the Jurassic Greenhouse climate condition. This study highlights that combining comprehensive coal-related database with paleoclimate, tectonics, and evolution of land plants provides insights into the mechanisms of the long-term behavior of the peatland expansion and carbon reservoir through deep time.

Keywords: Greenhouse climate, Peatland expansion, Carbon pool, Cabon accumulation rates, Coal

How to cite: Zhang, Z., Wang, C., Lv, D., and Wang, T.: Greenhouse climate forces expansion of peatlands into inland areas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3988, https://doi.org/10.5194/egusphere-egu2020-3988, 2020.

Oceanic anoxic events (OAEs) are abrupt events of widespread deposition of organic-rich sediments and extensive seafloor anoxia. Mechanisms usually invoked as drivers of oceanic anoxia are various and still debated today. They include a rise of the CO2 atmospheric level due to increased volcanic activity, a control by the paleogeography, changes in oceanic circulation or enhanced marine productivity. In order to assess the role of these mechanisms, we use an IPCC-class model, the IPSL-CM5A2 Earth System Model, which couples the atmosphere, land surface, and ocean components, this last one including sea ice, physical oceanography and marine biogeochemistry which allows to simulate oceanic oxygen.

We focus here on OAE2, which occurs during the Cretaceous at the Cenomanian-Turonian boundary (93.5 Ma), and is identified as a global event with evidence for seafloor anoxia in the Atlantic and Indian Oceans, the Southwest Tethys Sea and the Equatorial Pacific Ocean. Using a set of simulations from 115 to 70 Ma, we analyze the long-term paleogeographic control on oceanic circulation and consequences on oceanic oxygen concentration and anoxia spreading. Short-term controls such as an increase of pCO₂, nutrients, or orbital configurations are also studied with a second set of simulations with a Cenomano-Turonian (90 Ma) paleogeographic configuration. The different simulated maps of oxygen are used to study the evolution of marine productivity and oxygen minimum zones as well as the spreading of seafloor anoxia, in order to unravel the interlocking of the different mechanisms and their specific impact on anoxia through space and time.

How to cite: Donnadieu, Y., Laugie, M., Ladant, J.-B., Raisson, F., and Bopp, L.: Modeling the impact of oceanic circulation and marine productivity on Cretaceous seafloor anoxia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20746, https://doi.org/10.5194/egusphere-egu2020-20746, 2020.

Geological evidence suggests that Earth's past featured periods during which the planet was largely or even entirely covered by ice, a state termed "snowball Earth".  Model based studies confirm that one of Earth's equilibrium states is a fully glaciated planet (hard snowball) but it is not clear how this state could have been left once it had been established. We use simulations with the Max-Planck-Institute for Meteorology's Earth system model to investigate the conditions that enable the transition out of the snowball-state. We show that the high albedo of pure snow would have prevented deglatiation, even for extremely high atmospheric CO2 concentrations. Terminal deglaciation is only triggered for surface albedos corresponding to old, darkened snow or sea-ice. Here, increasing snowfall rates, resulting from the intensification of the hydrological cycle with rising CO2 concentrations, would have prohibited the gradual build-up of dust that leads to a darkening of the surface.  Only when assuming dust deposition fluxes at least similar to present-day fluxes, can the deglation be triggered for plausible atmospheric CO2 concentrations.

How to cite: de Vrese, P., Stacke, T., Brovkin, V., and Caves Rugenstein, J.: The simulated transition from a hard snowball Earth, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19916, https://doi.org/10.5194/egusphere-egu2020-19916, 2020.

Ocean anoxic event 2 (OAE2) was a large perturbation in the Earth's ocean carbon system, occurring at approximately 93.5 Ma, and is characterised by widespread black shales deposition in sediment records. This record has been interpreted as evidence of large anoxia in the global ocean for a long period, resulting in large scale extinction of marine life. However, the exact causes of OAE2, and how it initially developed, are not fully understood. We modelled the period leading up to OAE2 using the HadCM3L global climate model with full ocean (HADOCC) and terrestrial carbon cycle (TRIFFID) modules. We compared our results to equivalent simulations using late Cretaceous (Maastrichtian) paleogeographies. This allowed us to analyse the effects of continental configuration on the development to the OAE. Our results show that restricted ocean circulation, caused by the paleobathymetry, is necessary for anoxic conditions to develop but is not sufficient alone. This suggests that continental configuration is highly important in determining the ability of the oceans to develop an OAE and may explain why they only occur during some times during Earth history.

How to cite: Manning, A., Valdes, P., Monteiro, F., and Williams, J.: Role of paleogeography in preconditioning the Late Cretaceous Oceanic Event (OAE2) in a full global circulation Earth System model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15254, https://doi.org/10.5194/egusphere-egu2020-15254, 2020.

Antarctic bryozoans are important colonial marine invertebrates in terms of their origin, palaeoenvironment and climatic approaches. The changes of the bryozoan fossil records during the last 55 Ma years  are well-defined by their biodiversity, taxonomic composition and colony growth-forms. The late Early Eocene biota from the shallow-marine–estuarine clastic succession of the lower part (Telm1-2) of the La Meseta Formation of Seymour Island are represented by the prolific, spectacular in size, massive multilamellar colonies dominated by the cerioporids as well as diverse ascophorans  cheilostomes (Hara, 2001). The free-living lunilitiform, disc-shaped colonies, which occur in the middle part of the La Meseta Formation (Telm4-Telm5), are characteristic for the warm, shallow-self environment and bottom temperature, which ranges from 10 to 29°C. The presence of the bimineralic skeletons of this fauna (such as Lunulites, Otionellina, and Uharella) with the traces of aragonite is indicative for the temperate shelf environment, sandy and often shifting substrate. Lunulitids are inhabited by the circumpolar to warm-temperate waters, at the present day. Contrary to that, the bryozoans  from the upper part of the LM (Telm6-7) are  represented by the scarce lepraliomorphs accompanied by the crustaceans, brachiopods and gadiform fish remains. The individuality of the Eocene bryozoan assemblages are well-correlated with the EECO, MECO and EOT climatic events, based on the other marine macrofaunal marine fossil records (see also Ivany et al. 2008). The lower Pliocene bryofauna recently described  from the Cockburn Island Formation  is composed of the rich encrusting shallow-water, membraniporiform zoaria (Hara and Crame, in review, 2020). The biota of thePectenConglomerate are indicative of the interglacial conditions during the deposition of the Cockburn Island Formation. At the present day bryozoans with the preponderance of cheilostomes are the most significant marine benthic community, thriving successfully in cool-water Antarctic  conditions.

References,

Hara, U., 2001. Bryozoans from the Eocene of Seymour Island, Antarctic Peninsula, Palaeontogia Polonica , 60:33-155.

Hara, U., Mors, T., Hagstrom, and Reguero M. A., 2018. Eocene bryozoans  assemblages from the La Meseta Formation of Seymour Island. Geological Quarterly, 62: 705-728. 

Hara., U., and Crame, J.A. 2019. Paleobiodiversity of the Lower Pliocene bryozoan  benthic community and its response to interglacial conditions.  Geological Review (in review).

Ivany L.C., Lohmann, K. C. Hasiuk, F., Blake D.B., Glass A., Aronson R.B., and Moody R.M. 2008. Eocene climate record of the high southern latitude continental shelf: Seymour Island, Antarctica. Geological Society of America Bulletin, v. 120, no. 5-6: 659-678.

How to cite: Hara, U.: Cenozoic bryozoan biota and their response to climatic changes in Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22540, https://doi.org/10.5194/egusphere-egu2020-22540, 2020.

The Middle Eocene represents the last ice-free period of the Cenozoic. Vegetation proxy data (wood, leaves, palynomorphs) discovered in the Antarctica peninsula and neighbouring islands or hosted in sedimentary sequences deposited on the continental margin reveal the presence of paratropical rain forests which thrived along the Antarctica coast during the Early Eocene. During the Middle and Late Eocene these flora have been progressively replaced by temperate Nothofagus-dominated rainforests (Contreras et al., 2013). Jacques et al. (2012) proposed, using a physiognomic approach (CLAMP), that a warm temperate and wet climate (with a marked summer rainy season) prevails until the middle Eocene (43±2 Ma) on the tip of the Antarctica Peninsula.

            To better constrain the climate in Antarctica and understand processes governing the polar climate during the Middle Eocene, we performed a set of experiments using the IPCC-like Earth System Model (IPSL-CM5A2-VLR) forced with a Middle Eocene (~40 Ma) paleogeography reconstruction and a 4 times pre-industrial atmospheric CO₂ level (1120ppm). To highlight the importance of the seasonality, we launched 6 orbital configurations exploring end-members situations. To complete the procedure, simulated sea surface temperatures and sea ice extents were then employed as boundary conditions to force the Atmospheric General circulation model LMDz6 (run at higher spatial resolution) coupled with a soil and vegetation model ORCHIDEE to simulate the corresponding vegetation over Antarctica. The 6 end-members Earth's orbital configuration allows exploring the full climatic spectrum which would have been recorded by proxy data. Simulated changes in atmospheric circulation will be discussed and the simulated climate and vegetation will be confronted to paleoclimatic indicators and vegetation data.

How to cite: Fluteau, F., Tardif, D., Le Hir, G., Donnadieu, Y., Sepulchre, P., Ladant, J.-B., Poblete, F., and Dupont-Nivet, G.: The climate in Antarctica during the Middle Eocene: a modelling perspective, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18748, https://doi.org/10.5194/egusphere-egu2020-18748, 2020.

Simulating the proxy-derived surface warming and reduced meridional temperature gradient of the early Eocene greenhouse climate still represents a challenge for most atmosphere-ocean general circulation models. A profound understanding of uncertainties associated with the respective model results is thereby essential to reliably identify any similarities or misfits to the proxy record. Besides incomplete knowledge of past greenhouse gas concentrations and other boundary conditions, structural and parametric uncertainties are the main factors that determine our confidence in paleoclimate simulation results.

The recent publication of coordinated model experiments that apply identical paleogeographic boundary conditions for key time periods of the early Eocene (DeepMIP) allows a systematic analysis of inter-model differences and therefore of structural uncertainties in the simulated surface warming. Here we additionally explore the parametric uncertainty of the early Eocene climatic optimum (EECO) surface warming within one DeepMIP model. For this we performed perturbed parameter ensemble (PPE) simulations with HadCM3B at different atmospheric CO₂ concentrations following the DeepMIP protocol. Twenty-one parameter sets based on changes in six atmospheric parameters, a sea-ice parameter and the ocean background diffusivity were branched off from the respective DeepMIP control simulations and integrated for a further 1500 model years. The selected parameter sets are based on previous results demonstrating their ability to simulate a pre-industrial global-mean surface temperature within ±2 °C of the standard configuration.

Preliminary results indicate a large spread of the simulated low-latitude surface warming in the PPE and therefore significant changes of the large-scale meridional temperature gradient for the EECO. Some ensemble members develop numerical instabilities at CO₂ concentrations of 840 ppmv and above, most likely in consequence of high temperatures in the tropical troposphere. We further compare the magnitude of the parametric uncertainty of the HadCM3B perturbation experiments with the structural differences found in the DeepMIP multi-model ensemble and explore the sensitivity of the results to the strength of the applied greenhouse gas forcing. Model skill of the PPE members is tested against the most recent DeepMIP compilations of marine and terrestrial proxy temperatures.

How to cite: Steinig, S., Bragg, F. J., Irvine, P. J., Lunt, D. J., and Valdes, P. J.: Estimating structural and parametric uncertainties in the simulated early Eocene surface warming, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8370, https://doi.org/10.5194/egusphere-egu2020-8370, 2020.

The North American Great Plains are characterized by a sharp aridity gradient at around the 100^th meridian with a more humid climate to the east and a more arid climate to the west. This aridity gradient shapes the region's agriculture and economy, and recent work suggests that arid conditions on the Great Plains may expand eastward with global warming. The abundant Neogene sediments of the Ogallala Formation in the Great Plains present an opportunity to reconstruct regional hydroclimate conditions at a time when pCO₂ and global temperatures were much higher than today, providing insight into the aridity and ecosystem response to warming. We present new paleosol carbonate δ¹³C and δ¹⁸O data (n=366) across 37 sites spanning the Great Plains and compile previously published measurements (n=381) to evaluate the long-term hydroclimatic and ecosystem changes in the region during the late Neogene. This study combines a spatial and temporal analysis of carbon and oxygen isotope data with reactive-transport modeling of oxygen isotopes constrained by climate model output, providing critical constraints on the paleoenvironmental and paleoclimatological evolution of the Neogene Great Plains. Carbonate δ¹⁸O demonstrate remarkable similarity between the spatial pattern of paleo-precipitation δ¹⁸O and modern precipitation δ¹⁸O. Today, modern precipitation δ¹⁸O over the Great Plains is set by the mixing between moist, high-δ¹⁸O moisture delivered by the Great Plains Low-Level Jet and drier, low-δ¹⁸O westerly air masses. Thus, in the absence of countervailing processes, we interpret this similarity between paleo and modern δ¹⁸O to indicate that the proportional mixing between these two air masses has been minimally influenced by changes in global climate and that any changes in the position of the 100^th meridian aridity gradient has not been forced by dynamical changes in these two synoptic systems. In contrast, prior to the widespread appearance of C₄ plants in the landscape of the Great Plains, paleosol carbonate δ¹³C show a robust east-to-west gradient, with higher values to the west. We interpret this gradient as reflective of lower primary productivity and hence soil respiration to the west. Close comparison with modern primary productivity data indicates that primary productivity has declined and shifted eastward since the late Neogene, likely reflecting declining precipitation and/or a reduction in CO₂ fertilization during the late Neogene. Finally, δ¹³C increases across the Miocene-Pliocene boundary, which, consistent with previous studies, we interpret as a shift from a C₃ to a C₄ dominated landscape. We conclude that, to first order, the modern aridity gradient and the hydrologic processes that drive it are not strongly sensitive to changes in global climate and any shifts in this aridity gradient in response to rising CO₂ will be towards the west, rather than towards the east.

How to cite: Manser, L., Kukla, T., and Caves Rugenstein, J. K.: Long-term stability of large-scale hydroclimate processes in the North American Great Plains revealed by a Neogene stable isotope study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11141, https://doi.org/10.5194/egusphere-egu2020-11141, 2020.

It has been well demonstrated that the variations of orbital parameters, known as Milankovitch theory, are one of the most important drivers of the Earth’s climate system. However, the way how the changes in orbital forcing imprint the glacial-interglacial cycles recorded in paleo-proxies, such as stable water isotopes in ice cores and speleothems, is still unclear. One way to progress in this question is to make direct comparisons of isotopic data with simulation results from isotope-enabled General Circulation Models (GCMs). We use here such a model, the Japanese atmospheric GCM MIROC5-iso[1], to perform simulations under different idealized paleoclimate conditions. For that, corresponding orbital parameters and greenhouse gases concentrations are set. Prescribed sea surface temperature and sea ice coverage boundary conditions from the fully coupled atmosphere-ocean GCM MIROC (MIROC-AOGCM) experiments are used, after an adaptation to the MIROC5-iso grid. Because earlier version of MIROC-AOGCM has been widely used for paleoclimate modeling purposes, the climatological mean states of MIROC5-iso under preindustrial conditions are evaluated against simulation results from different versions of MIROC-AOGCM (MIROC4m, which is a slightly updated version of MIROC3.2(med), and MIROC5 [2]). In addition, several interglacial periods and idealized paleoclimate experiments will be investigated and implications for the interpretation of water isotope response to the changes in orbital forcing will be discussed.

[1] Okazaki and Yoshimura, J. Geophys. Res. Atmos, 124, 8972–8993, 2019.

[2] Watanabe et al., J. Climate, 23, 6312–6335, 2010.

How to cite: Kino, K., Okazaki, A., Cauquoin, A., and Yosnimura, K.: Investigation of the response of water isotope records to the changes in orbital forcing with the isotope-enabled AGCM MIROC5-iso, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12420, https://doi.org/10.5194/egusphere-egu2020-12420, 2020.

Perhaps the most important feedback to orbital climate change is CO₂ storage in the deep ocean.  By regulating atmospheric CO₂, ocean carbon storage synchronizes glacial climate in both hemispheres, and drives the full magnitude of glacial-interglacial climate change.  However few data exist that directly track the deep ocean’s carbon chemistry over a glacial cycle.  Here, we present geochemical reconstructions of deep ocean circulation, redox, and carbon chemistry from sediment cores making up a detailed depth profile in the South Atlantic, alongside a record of Southern Ocean surface water CO₂, spanning the last glacial cycle.  These data indicate that initial glacial CO₂ drawdown is associated with a major increase in surface ocean pH in the Antarctic Zone of the Southern Ocean, cooling at depth, enhanced deep ocean stratification, and carbon storage.  Deep ocean carbon storage and deep stratification are further enhanced when CO₂ falls at the onset of Marine Isotope Stage 4, and are also pronounced during the LGM, illustrating a link between orbital scale climate stages and deep ocean carbon.  However our data also illustrate non-linear feedbacks to orbital forcing during glacial terminations, which show abrupt decreases in pH in Southern Ocean surface and subsurface waters, as CO₂ is rapidly expelled from the deep ocean at the end of the last ice age.

How to cite: Rae, J., Foreman, A., Crumpton-Banks, J., Burke, A., Charles, C., and Adkins, J.: Ocean carbon storage and release over a glacial cycle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17480, https://doi.org/10.5194/egusphere-egu2020-17480, 2020.

The Antarctic Circumpolar Current (ACC) is the largest current system in the world, linking the Pacific, Atlantic and Indian Ocean basins. However, the variability of the ACC, which plays a fundamental role on global ocean circulation and climate variability, is still poorly constrained. This information is crucial for understanding the role of the ACC on global ocean circulation in response to global warming. Here, we reconstruct changes in the ACC over the past 155,000 years based on sediment grain size variations recorded in a highly-resolved marine sedimentary record from the central Drake Passage near the Polar Front. Our results show significant changes in the ACC during the last glacial cycle and a remarkable boundary between the glacial and interglacial periods. Substantial decreases (~33% to ~47%) in the ACC flow speed from interglacial to glacial period, which corroborates and extends results of previous studies along the subantarctic northern limit of the ACC into the central Drake Passage. This strong variation of ACC likely plays a significant role in regulating Pacific-Atlantic water mass exchange via the “cold water route” and could significantly affect the Atlantic Meridional Overturning Circulation. Superimposed on these glacial-interglacial changes, we found strong millennial-scale variations in ACC current speed, increasing in amplitude close to full glacial conditions. We hypothesise that the central ACC increases its sensitivity to Southern Hemisphere millennial-scale climates oscillations, likely associated with westerlies’ wind stress and Antarctic sea ice extent once glacial conditions fully formed.

How to cite: Wu, S., Lamy, F., Kuhn, G., Lembke-Jene, L., Zhang, X., Haas, C., Nowaczyk, N., W. Arz, H., and Tiedemann, R.: Millennial-scale variability in Antarctic Circumpolar Current and its impacts during the last glacial cycle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6778, https://doi.org/10.5194/egusphere-egu2020-6778, 2020.

Indian Ocean surface salinity dynamics are thought to play an important role in shaping glacial-interglacial climate through controlling Agulhas leakage efficiency. It is proposed that a strong Agulhas leakage supplies warm and salty Indian ocean surface waters to Atlantic surface currents influencing convective potential at North Atlantic deep-water formation sites. Here, we present new planktonic foraminiferal Mg/Ca and stable isotope-derived salinity reconstructions for the last 1.2Ma from the northern Mozambique channel. We find salinity increases well before terminations, followed by early decrease before glacial inception. We present a possible link between the hydrography in the northern Mozambique channel and whole ocean salinity changes due to unique surface circulation in the Indian ocean. Despite being a mostly tropical and subtropical ocean, salinity in the modern tropical Indian Ocean is fresher than at comparable latitudes in the Atlantic or Pacific. This is due to the inflow of freshwater from the Indonesian throughflow and recycling via an active Agulhas leakage. We show that salinity in the glacial western Indian Ocean was significantly higher due to a reduced ITF and a weaker Agulhas leakage. We hypothesise that opening and closing of these two gateways influences the development/diminishment of a strong subtropical Indian Ocean gyre which controls sea surface salinity and temperature of tropical Indian Ocean water masses and subsequently the efficiency of the Agulhas Leakage.

How to cite: Nuber, S., Rae, J. W. B., Andersen, M. B., de Boer, B., Zhang, X., Hall, I. R., and Barker, S.: The Role of Changing Indian Ocean Salinity in shaping Pleistocene Climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9828, https://doi.org/10.5194/egusphere-egu2020-9828, 2020.

The geometry of large-scale deep ocean circulation is closely linked to processes occurring in the Southern Ocean (SO). The SO is the ‘window’ through which much of the world’s ocean interior interacts with the atmosphere, and understanding the complex relationships coupling SO dynamics to deep circulation can provide valuable insights into biogeochemical and physical processes important to global climate. Of particular interest is how these processes interacted with, and behaved under different climate states, such as the glacial-interglacial cycles of the Pleistocene (0-2.8 Ma), and the intensification of Northern Hemisphere glaciation during the transition from the warm Mid-Pliocene (3.3-3.1 Ma) to the early Pleistocene. Here, we utilise new composite sediment core records (41^oS, 25^oE, 2700-2900 m water depth) to reconstruct deep chemical and physical ventilation at the Agulhas Plateau, as well as the competing presence of warm Subtropical waters vs cold Subantarctic waters in the surface, over the past ~3 Ma. We present records of the ‘sortable silt’ flow speed proxy, the stable isotope (δ¹⁸O, δ¹³C) composition of benthic foraminifera, bulk sediment element concentrations, and the accumulation of ice-rafted debris (IRD). The sortable silt proxy demonstrates that deep physical ventilation is largely decoupled from deep chemical ventilation as indicated by benthic δ¹³C, with higher flow speeds coincident with more depleted δ¹³C. Furthermore, deep ventilation is related to changes in the terrigenous sediment composition: deep flow speeds and δ¹³C vary concurrently with bulk sediment geochemistry (K/Al, Ti). At the Agulhas Plateau, we interpret deep chemical ventilation and near-bottom flow speeds to reflect changes in the advection of northern-sourced deep waters (e.g. North Atlantic Deep Water and its glacial equivalent) and meridional variability in the location of the deep-reaching Antarctic Circumpolar Current (ACC) and its associated fronts. The presence of IRD at the Agulhas Plateau is controlled primarily by the equatorward survivability far-travelling Antarctic icebergs, and therefore represents the relative presence of cold, iceberg-bearing Subantarctic Zone (SAZ) surface waters. Generally, at times of high near-bottom flow speed and more ‘southern’ terrigenous sediment composition, IRD is higher, implying a meridional expansion of the SAZ. Together, these proxy records provide a continuous and long-term insight into the evolution of coupled surface-deep conditions at the Agulhas Plateau. We postulate that these conditions may reflect the wider geometry of ocean circulation in the SO, documenting the interactions between the ACC and circum-Antarctic fronts with the upwelling, conversion, and export of deep water masses. Our records represent the first multi-proxy reconstruction of this system across climate transitions of the past ~3 Ma, allowing us to explore its evolution across a range of timescales, from million-year to orbital-scale. Furthermore, by measuring multiple proxies on the same samples, we are able to determine the relative phasing between different processes independent of chronostratigraphic uncertainties, for example the timing of SAZ changes vs perturbations in deep ocean circulation at the site.   

How to cite: Starr, A., Hall, I. R., Barker, S., van der Lubbe, J., Hemming, S. R., Jimenez-Espejo, F. J., and Lathika, N.: The Evolution of Subantarctic Fronts, Deep Ocean Ventilation and Flow Vigour at the Agulhas Plateau: Surface-Deep Coupling Across Climate Transitions of the past 3 Ma, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10495, https://doi.org/10.5194/egusphere-egu2020-10495, 2020.

The formation of North Atlantic Deep Water (NADW) in the North Atlantic is an important modulator of the climate system, as it drives the global termohaline circulation, responsible for the distribution of heat, salts and nutrients across the oceans. ODP Site 1063 (4584 m), on the deep Bermuda Rise, is located in the mixing zone between NADW and Antarctic Bottom Water (AABW) and appears to be a good location to study how ocean circulation and climate interconnect. Here we present a new record based on Nd isotope ratios that covers ~1 Ma at that Site. Our data shows Nd isotope ratios during parts of interglacials that are much lower than present day NADW. These results are coherent with recent published studies on the last interglacial–glacial cycle that show that the deep North Atlantic Nd isotope ratios are also lower than NADW during the early interglacial. However, Nd isotope values from the shallower DSDP Site 607 (3427 m), within the core of NADW, have remained similar to modern NADW during interglacials over the same time interval. Site 607 is thought to represent the deep North Atlantic, as shown by an Atlantic meriodional transect that displays Nd isotopes ratios for glacial and interglacial maxima over the last ~1 Ma. We suggest that Nd isotope ratios at Site 1063 do not fully represent the North Atlantic endmember of the AMOC during interglacials, but regional or local processes. However, glacial values at Site 1063 fitting those of Site 607 suggest that Nd isotope ratios represent, indeed, water mass mixing during glacial periods. The low Nd-isotope ratios in the deep Bermuda Rise during interglacials would be the result of particle-seawater exchange derived from the arrival of freshly ground, poorly weathered bedrock from the Canadian shield to the North Atlantic during major ice sheet retreats, such as deglaciations as well as stadial-to-interstadial transitions. Consequently, a deep, regionally constrained layer of seawater is tagged with this extreme Nd isotope signature that is not representative of the AMOC. We suggest that a benthic nepheloid layer, whose development is driven by a deep-recirculating gyre system regulated by the interaction between the northward flowing Gulf Stream and the southward flowing deep western boundary current, facilitates the periodical masking of the deep Atlantic Nd isotope signature at Site 1063. The intermittence of the masking allows for a speculation on how the deep-recirculating gyre system might have changed over the last ~1 Ma glacial-to-interglacial cycles.

How to cite: Jaume-Seguí, M., Kim, J., Knudson, K. P., Yehudai, M., Goldstein, S. L., Bolge, L., Ferretti, P., and Pena, L. D.: Glacial-to-interglacial variations in the deep water at the Bermuda Rise inferred from a Nd isotope record covering the last million years , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18340, https://doi.org/10.5194/egusphere-egu2020-18340, 2020.

The causes for major climate transitions in the Upper Pleistocene are based on the assumption that orbital forcing, i.e. the increase in northern hemisphere summer insolation (NHSI), initiates glacial ice sheets to melt away leading to the formation of warm interglacials. Good examples are plentiful available, e.g. major glacial terminations (T) such as T1, 2, or 5. Besides these major climate transitions there are also other glacial terminations across marine substage boundaries that, although seemingly of minor scale, had nevertheless massive climate impacts either globally (MIS4/3, MIS7d/7c) or regionally (MIS5b/5a). While an interglacial decrease in NHSI seems to run in parallel with early glacial inception - as can be noted for the later Holocene and MIS5e - the onset of T2 vs. T1 has long been controversially discussed with respect to its orbitally forced timing. This study therefore explores the involvement of other mechanisms. Primarily, these have to do not so much with internally produced feedback processes but are the consequence of temperature changes to be found in the low-latitudes. Transferred northward through the atmosphere and ocean these changes then feed ice sheet growth and determine its geographical configuration of different magnitudes, also eventually leading to a glacial maximum. During the past, climate transitions from a glacial into an interglacial world therefore did not start with the end of a glacial maximum. It is the time just prior to that particular maximum when the major change occurred.

How to cite: Bauch, H.: Exploring the nature and timing of glacial climate transitions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5427, https://doi.org/10.5194/egusphere-egu2020-5427, 2020.

From the combination of orbital theory with benthic δ¹⁸O it has been suggested which obliquity cycles led to interglacials during the Quaternary (e.g. Tzedakis et al., 2017). Here, we define interglacials, as deduced for the last 800 kyr (Past Interglacials Working Group of PAGES, 2016), by the absence of substantial northern hemispheric land ice outside of Greenland. When applied to land-ice distribution derived from a 3D-ice-sheet model-based deconvolution of the LR04-benthic δ¹⁸O stack into its temperature and sea-level components (de Boer et al., 2014) we find an irregular pattern of interglacials not only, as suggested so far, in the late Pleistocene but across most of the last 2.6 Myr. In the early Pleistocene eight obliquity cycles miss the onset of new interglacials, therefore increasing the average interglacial periodicity to 60 kyr. Both prolonged glacials (due to skipped terminations) and prolonged interglacials (so-called continued interglacials) are the reasons for these new irregularities. This finding adds new irregularities to the already known glacial/interglacial pattern during the last 1 Myr that include eleven obliquity cycles without new interglacials. Only in the Mid-Pleistocene in-between interglacials reappear regularly once in each obliquity cycle (every 41 kyr) with an exception around 1.1 Myr BP in which the onset of two successing interglacials is more than 100 kyr apart. This finding suggests that the notation of the Quaternary as an obliquity driven period with a growing influence of ice volume on the timing of deglaciations is too simple, or that our definition of interglacials, that seems to be suitable for the last 1.6 Myr, is not applicable to the whole Quaternary.

References:

How to cite: Köhler, P. and van de Wal, R.: Land ice distribution suggests an irregular pattern of interglacials across most of the Quaternary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1340, https://doi.org/10.5194/egusphere-egu2020-1340, 2020.

Within the Late Pleistocene, a ‘termination’ is the name given to the rapid (~10kyr) deglacial transition marking the end of a (~100kyr) glacial cycle. These massive events involve all the critical elements of Earth’s climate system: global temperatures, precipitation patterns, ice sheet extent, ocean and atmospheric circulation systems, atmospheric composition and biological activity. Investigations into the mechanisms of glacial termination have been many and it is now thought that abrupt shifts in the ocean/atmosphere system play a ubiquitous and critical role in deglaciation. However, significant uncertainties remain concerning the timing and magnitude of deglacial changes and the likelihood that they will be interrupted by ‘terminal oscillations’ such as the Bølling-Allerød / Younger Dryas oscillation during Termination 1. In this presentation we will address these uncertainties in the light of recent developments in the understanding of glacial terminations as the ultimate expression of the interaction between millennial and orbital timescale variations in Earth’s climate. Innovations in numerical climate simulation and new geologic records that enable us to test these simulations allow us to highlight new avenues of research as well as to emphasise the importance of lingering uncertainties in key climatic parameters such as sea level variability through time.

How to cite: Knorr, G. and Barker, S.: Glacial Termination: Going, Going, Gone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4227, https://doi.org/10.5194/egusphere-egu2020-4227, 2020.

We use a global array of ~120 sea-surface temperature (SST) records based on Mg/Ca, alkenone, and faunal proxies to reconstruct global and regional temperature change over the last 5 Myr. All records are placed on the LR04 age model. Here we report the reconstructions and discuss their implications for characterizing global climate evolution (frequency, variance, transitions) over this interval and its relationship to changes in CO2, orbital forcing, and mean ocean temperature. Average global temperature has cooled by ~6.5oC since 5 Ma, with significant breakpoints tentatively identified at ~3.38 Ma, 1.34 Ma, and 0.88 Ma. We also invert the global reconstruction to reconstruct global sea level for the last 5 Myr.

How to cite: Clark, P. U., Shakun, J., Rosenthal, Y., Bartlein, P., Koehler, P., and Mix, H.: Reconstructions of Global and Regional Temperature Change for the Last 5 Myr, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12928, https://doi.org/10.5194/egusphere-egu2020-12928, 2020.

Understanding the evolution of, and the interactions between, ice sheets and the global climate over geological time is important for being able to constrain earth system sensitivity. However, direct observational evidence of past CO₂ concentrations only exists for the past 800,000 years. Records of benthic d¹⁸O date back millions of years, but contain signals from both land ice volume and ocean temperature. In recent years, inverse forward modelling has been developed as a method to disentangle these two signals, resulting in mutually consistent reconstructions of ice volume, temperature and CO₂. We use this approach to force a hybrid ice-sheet – climate model with a benthic d¹⁸O stack, reconstructing the evolution of the ice sheets, global mean sea-level and atmospheric CO₂ during the late Pliocene and the Pleistocene, from 3.6 Myr ago to the present day. The resulting reconstructions of CO₂ and sea level agree well with the ice core record and different sea-level proxies, indicating that this model set-up yields useful information for colder-than-present climates. For the warmer-than-present climates of the Late Pliocene, different proxies for both CO₂ and sea level are contradictory, making model validation difficult. During the early Pleistocene, 2.6 – 1.2 Myr ago, we simulate 40 kyr glacial cycles with CO₂ ranging between 270 – 280 ppmv during interglacials and 210 – 240 ppmv during glacial maxima. After the Mid-Pleistocene Transition (MPT), when the glacial cycles change from 40 kyr to 80/120 kyr cyclicity, these values change to 260 to 280 ppmv during interglacials, and 180 – 200 ppmv during glacial maxima.

How to cite: Berends, T., de Boer, B., and van de Wal, R.: Reconstructing the evolution of ice sheets, sea level and atmospheric CO2 during the past 3.6 million years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8762, https://doi.org/10.5194/egusphere-egu2020-8762, 2020.

The late Pliocene corresponds to a large cooling over Northern Hemisphere associated with sporadic occurrences of glaciations. The most important event occurred during the marine isotope stage M2 (MIS M2, 3.312–3.264 Ma) when a large glaciation took place with a sea level drop from 20 to 60 m, but its duration is short and the summer insolation forcing change at 65°N is weak. De Schepper et al (2013) invoked to explain the onset and termination of this glaciation with the opening and closing of the Central American Seaway (shallow CAS). Based on their hypothesis, we have intensively studied the onset mechanism of  MIS M2 through a series of sensitivity experiments using the IPSL AOGCM and the asynchronous coupling with an Ice sheet model (GRISLI). Our results demonstrate that the shallow CAS helps to precondition the low-latitude oceanic circulation and affects the related northward energy transport, but cannot alone explain the onset of the M2 glaciation, the most important contribution on MIS M2 are from the large change of pCO₂ as well as the internal feedbacks of vegetation and ice sheet. Moreover, we have also investigated the period from the late Pliocene to the early Pleistocene (3-2.5 Ma) through a transient-like simulation using the same AOGCM and ISM. This enables to simulate the Greenland Ice Sheet (GRIS) onset and development using the pCO₂ reconstructions from different proxies. All these simulations were analyzed with emphasis on cryosphere and focused on the Northern Hemisphere (mid-to-high latitudes). Here we used the same modeling simulations but with a focus over the tropical Africa. We first depict the large changes of temperatures and hydrological cycle produced over this area during these two periods and compare our data to reconstructions. Moreover, by prescribing our climate results as inputs for the vegetation model (Biome4), we compare more directly the simulated plant functional types (PFTs) with that constructed by the pollen data. In addition, we further quantify the respective impact of various driving factors on these PFTs variations.

How to cite: Tan, N., Yule, E., Ramstein, G., Barboni, D., Raj, R., and Dumas, C.: Simulations of large climate transition occurring at high and low latitudes during the late Pliocene (3.3 Ma) and the Plio/Pleistocene (3-2.5 Ma) boundary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20914, https://doi.org/10.5194/egusphere-egu2020-20914, 2020.

The intensification of Northern Hemisphere glaciation (iNHG) at 3.4-2.5 million years ago (Ma) represents the last great transition in Cenozoic climate state with the development of large scale ice sheets in the Northern Hemisphere that waxed and waned with changes in insolation. Declining atmospheric CO₂ levels are widely suggested to have been the main cause of iNHG but the CO₂ proxy record is too poorly resolved to provide an adequate test of this hypothesis. The boron isotope-pH proxy, in particular, has shown promise when it comes to accurately estimating past CO₂ concentrations and is very good at reconstructing relative changes in CO₂ on orbital timescales. Here we present a new orbitally resolved record of atmospheric CO₂ (1 sample per 3 kyr) change from Integrated Ocean Drilling Program Site 999 (12.74˚N, -78.74 ˚E) spanning ~2.6–2.4 Ma based on the boron isotope (δ¹¹B) composition of planktic foraminiferal calcite, Globingerinoides ruber (senso stricto, white).  We find that δ¹¹B values of G. ruber show clear glacial-interglacial cycles with a magnitude that is similar to those of the Mid-Pleistocene at the same site and elsewhere.  This new high-resolution view of CO₂ during the first large glacial events of the Pleistocene confirms the importance of CO₂ in amplifying orbital forcing of climate and offers new insights into the mechanistic drivers of natural CO₂ change. 

How to cite: Brown, R., Chalk, T., Wilson, P., Rohling, E., and Foster, G.: High resolution CO2 record of the great Plio-Pleistocene glaciations using boron isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10925, https://doi.org/10.5194/egusphere-egu2020-10925, 2020.

The Pliocene to early Pleistocene yields a close analogy to near-future climate, with atmospheric pCO₂ between pre-industrial and anthropogenically perturbed levels as they may be reached in few decades. A sedimentary archive that is well suited to study Plio-Pleistocene climate dynamics in the terrestrial realm has recently become available through the ICDP-sponsored HOTSPOT project on the evolution of the Snake River Plain (Idaho, USA). At the Mountain Home site, HOTSPOT drilling has yielded the MHAFB11 core that comprises 635 m of fine-grained lacustrine sediments (Shervais et al. 2013). Based on the yet available paleomagnetic age control, these sediments span from the late Pliocene to the early Pleistocene, which makes them the first archive in continental North America that covers this time interval at one site. Based on their geographic position, the sediments from paleo-Lake Idaho can contribute to a better understanding of climate variability across the Plio-Pleistocene transition in western North America, notably with respect to the hypothesis that enhanced moisture transport into the higher latitudes of North America from ~2.7 Ma onwards allowed the initiation of Northern Hemisphere glaciation (Haug et al., 2005).

To gain insight into the paleoclimatic evolution of northwestern North America during the late Pliocene to early Pleistocene, we have palynologically analyzed 131 samples from the 732–439 m depth interval (corresponding to an age of ~2.8 to ~2 Ma) of the MHAFB11 core. The obtained palynological dataset, which has a mean temporal resolution of ~7 ka, documents that a Pinus-dominated coniferous forest biome prevailed in the catchment area of paleo-Lake Idaho throughout the study interval. However, percentages of pollen from conifer taxa decrease in the latest Pliocene before reaching consistently lower values in the early Pleistocene at ~2.4 Ma. In contrast, pollen taxa representing an open vegetation (e.g., Artemisia, Asteraceae) and deciduous trees (e.g., Quercus, Betula and Alnus) become increasingly abundant in the early Pleistocene (at ~2.4 Ma). We interpret this vegetation shift to an open mixed conifer/deciduous forest to be caused by wetter climate conditions. This interpretation is supported by quantitative climate estimates, which show a gradual increase in mean annual precipitation in the early Pleistocene. This trend towards wetter conditions supports the notion that enhanced moisture transport to northern North America from the subarctic Pacific Ocean contributed to the onset of Northern Hemisphere glaciation at ~2.7 Ma (Haug et al., 2005).

References:

Haug, G.H., Ganopolski, A., Sigman, D.M., Rosell-Mele, A., Swann, G.E., Tiedemann, R., Jaccard, S.L., Bollmann, J., Maslin, M.A., Leng, M.J. and Eglinton, G., 2005. North Pacific seasonality and the glaciation of North America 2.7 million years ago. Nature, 433, 821-825.

Shervais, J.W., Schmitt, D.R., Nielson, D., Evans, J.P., Christiansen, E.H., Morgan, L.A., Shanks, P. W.C., Prokopenko, A.A., Lachmar, T., Liberty, L.M., Blackwell, D.D., Glen, J.M., Champion, D., Potter, K.E., Kessler, J., 2013. First Results from HOTSPOT: The Snake River Plain Scientific Drilling Project, Idaho, U.S.A. Scientific Drilling, 3, 36-45.

How to cite: Allstädt, F., Koutsodendris, A., Appel, E., Rösler, W., Prokopenko, A., Reichgelt, T., and Pross, J.: Reconstruction of environmental and climatic change during the late Pliocene and early Pleistocene in northwestern North America based on a new drill core from paleo-Lake Idaho , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14645, https://doi.org/10.5194/egusphere-egu2020-14645, 2020.

Over the past 1.5 million years, Earth’s climate has shifted from a predominantly 41 thousand year (kyr) dominated climate cycle to one dominated by longer and larger glacial-interglacial cycles, known as the Mid-Pleistocene Transition (MPT). The MPT occurs over a period of several hundreds of thousands of years, with little change to Earth’s external orbital forcing, thus implicating internal climate feedbacks. Here we interrogate the current capacity, and future potential, of boron isotope records to provide high quality carbon cycle information for the Pleistocene. We also present a compilation of boron isotope-derived pH-CO₂ records from low-latitude ocean drill cores which closely follow the evolution of atmospheric CO₂ over the ice core interval but extend it to 1.5 million years ago with a resolution of up to ~1 sample per 3 kyr. This new, near continuous δ¹¹B-derived CO₂ record is compared against other independent CO₂ data from blue-ice cores and records of ocean and climate change., This confirms there is a decline in mean CO₂ across the MPT which manifests as a lengthening and deepening of glacial CO₂, and highlights the distinct difference in the nature of CO₂ cycles in the 41-kyr world.

How to cite: Chalk, T., Hain, M., Foster, G., Nuber, S., Rohling, E., Barker, S., Cherry, S., and Wilson, P.: Orbital CO2 cycles and the Mid-Pleistocene Transition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11682, https://doi.org/10.5194/egusphere-egu2020-11682, 2020.

The Mid-Pleistocene Transition (MPT, ~1.3-0.7 Ma) is one of the most drastic climatic transition in the recent climatic history of our planet. During this transition, glacial-interglacial variability shifted from 41- to 100-ka cycles, without notable changes in the orbital forcing. Internal forcing mechanisms in Earth’s climate likely shifted the system towards particularly more extreme glacial periods. A decrease in the atmospheric CO₂ contemporary to a severe weakening of the Atlantic deep-ocean circulation around 900 ky suggests that weakened deep-ocean circulation facilitated the capture of CO₂ into the deep ocean and thus contributed to the switch towards more intense and longer glacial periods.

ODP Site 668B, in the deep eastern equatorial Atlantic, has been previously used to reconstruct the atmospheric CO₂ evolution across the MPT using boron isotopes in surface dwelling foraminifera. Here we present new high resolution proxies from the same site covering the last 2 Ma. In particular, benthic foraminifera stable isotopes and trace elements (B/Ca, Mg/Ca, Cd/Ca), as well as Nd isotope data (εNd) from Fe-Mn encrusted foraminifera shells. Using the newly improved chronology based on benthic foraminifera stable isotopes we show that our new εNd data covaries substantially with the atmospheric pCO₂ data and shows a glacial-interglacial variability through the entire record, with εNd values matching typical glacial-interglacial range values in the North-Atlantic basin (~-11 to ~-14). Between ~1 to 2 Ma, when the 41-ka-cycles were dominant, εNd data also covaries with carbonate ion saturation index (ΔCO₃²-) as derived from the new B/Ca data, Bottom Water Temperatures (BWT, Mg/Ca) and, with deep ocean nutrient content (phosphate derived from Cd/Ca). Results indicate a higher fraction of warmer, less corrosive and nutrient-poor northern-sourced waters (higher BWT, higher ΔCO₃^2-, lower Cd/Ca, lower εNd) reaching the deep-equatorial Atlantic during interglacial periods compared to glacial periods. Interestingly, this covariation does not stand after ~0.9Ma. Even though εNd and BWT data suggest an increased contribution of southern-sourced waters to the site during glacial periods after 0.9Ma, as shown by a gradual decrease in glacial BWT (>1°C) and increasing glacial εNd values (~1ε units), both B/Ca and Cd/Ca show a distinctive low frequency variability superimposed to the glacial-interglacial variability. These oscillations can be interpreted as infiltrations and/or overflows of southern-sourced waters across the mid-ocean ridge into the SE Atlantic basin that do not completely follow glacial-interglacial periodicity. We propose that bathymetrical constrains exert a control on the chemistry of the deep waters in the deep eastern equatorial Atlantic with potential impacts on global climate. Partially isolated sub-basins such as the SE Atlantic could have effectively acted as carbon reservoirs over longer time scales than glacial-interglacial changes.

How to cite: Pena, L. D. and Jaume-Seguí, M.: Deep water mass geometry in the south east Atlantic across the Mid-Pleistocene Transition: bathimetric vs oceanographic controls , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13196, https://doi.org/10.5194/egusphere-egu2020-13196, 2020.

Abrupt climate events are generally believed to be characteristic of glacial (intermediate-to-large cryosphere) climate states, requiring either sizeable ice-sheets or large freshwater pulses to act as triggers for abrupt climate changes to occur. Amplification occurs when these triggers bear upon the Atlantic Meridional Overturning Circulation (AMOC). However, the focus on glacial climate states in abrupt climate change research has led to an underrepresentation of research into interglacial periods. It thus remains unclear whether high-magnitude climate variability requires large cryosphere-driven feedbacks or whether it can also occur under low ice conditions. Here we present a high resolution analysis of surface and deep water components of the AMOC spanning the transition from Marine Isotope Stage (MIS) 19c to 19a to test if orbital boundary conditions similar to our current Holocene can accommodate abrupt climate events. Sediment core DSDP 610B (53°13.297N, 18°53.213W), located approximately 700-km west of Ireland, was specifically chosen due to its high sedimentation rate during interglacial periods, excellent core recovery over the Quaternary and its unique geographical location. Above the core site, the dominant oceanographic feature is the North Atlantic Current and at 2417-m water depth, 610B is influenced by Wyville Thomson Overflow Water flowing southwards. A multiproxy approach including paired grain size analysis, planktic foraminifer assemblage counts, and ice-rafted debris counts within the same samples allows us to resolve the timing between both surface and bottom components of the AMOC and their response to abrupt climate events during MIS-19 in the eastern subpolar gyre. This study is societally relevant as future freshwater inputs from a melting Greenland ice sheet may impact ocean circulation, potentially causing shifts in climate for many European countries.

How to cite: Holmes, D. and Morley, A.: Are Cryosphere-Driven Feedbacks a Requisite for Abrupt Climate Events?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19225, https://doi.org/10.5194/egusphere-egu2020-19225, 2020.

Anthropogenic CO₂ release into the atmosphere leads to temperature projections for 2100 only experienced on Earth since many million years. However, those periods are poorly known due to low temporal and spatial data and ill-defined climate forcings. However past warm periods (interglacials), occurring during the Quaternary, under variable boundary conditions (e.g. greenhouse gases concentration, sea level and ice sheets size, insolation and orbital forcing), can provide invaluable information on the dynamics and processes behind natural warm climates. Here we present records for the sea surface temperature based in Uk’37-SST at orbital and millennial-scale over the last 1.25 Ma, with special focus on the past interglacials of two SW Iberian margin sedimentary sequences recovered during IODP Expedition 339,  Sites U1385 (37°34.285′N, 10°7.562′W;  2589m) and U1391 (37°21.5322′N, 9°24.6558′W; 991m). We also performed a data-model comparison to explore the dynamics related with the role of obliquity on the Atlantic Meridional Overturning Circulation (AMOC) changes. Our data  show that Interglacials are characterized by an interval of maximum warmth followed by a temperature decline punctuated by millennial-scale SST oscillations. In most cases the first stadial marks the beginning of a glacial inception that is characterized by an abrupt SST decrease, followed by high frequency SST oscillations, and large amounts of freshwater input. This suggests a climatic change from interglacial to glacial conditions linked to the start of ice sheets growth (enrichment of d18O) and the AMOC slowdown resulting in an enhanced cooling of the mid-latitudes.

How to cite: Rodrigues, T., Zeng, X., Padilha, M., Oliveira, D., O. Grimalt, J., and Abrantes, F.: The role of obliquity forcing on the interglacial climate instabilities in the mid-latitudes of the North Atlantic , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19780, https://doi.org/10.5194/egusphere-egu2020-19780, 2020.

The Last Glacial Maximum (LGM) is marked by significant cooling of the global ocean, which was recently estimated to 2.6°C using noble gases trapped in ice cores (1). This cooling is not equally distributed throughout the world oceans, since global ocean circulation models predict regional temperature anomalies during the LGM of up to 7°C (annually and zonally averaged) when compared to modern interior ocean temperature (2). The oceans deep interior thus became haline stratified (3) due to the drop in temperature to near freezing and the global increase in salinity from ice sheet growth. In contrast to a deepening of the modern thermocline as a result of anthropogenic global warming, cooling causes the thermocline to rise in the sub-tropics as more polar waters enter the mid-depth ocean.

Here we present glacial thermocline temperature reconstructions since the LGM based on the Li/Mg ratio in aragonite skeletons of precisely dated cold-water corals. Corals have been collected from 300-1000m water depths from sites in the northern and southern Atlantic (62°N to 25°S) and demonstrate synchronous 5 - 7°C glacial cooling, and a dramatic shoaling of the thermocline. Through the deglaciation the warming of the upper thermocline ocean occurs early in the southern hemisphere followed by fluctuating warming and thermocline deepening in the northern Hemisphere, which supports the oceanic climate seesaw proposed by Stocker and Johnson in 2003 (4). We thus propose dramatic changes in export of polar waters towards the Equator and augmented subsurface ocean stratification leading to a mostly polar Atlantic with a shallow permanent thermocline. This shoaling possibly increased the rate of nutrient recycling causing higher biological surface ocean activity and the cooling promoted carbon storage. During the glacial, we assume an atmospheric forcing, such as equatorward displacement of the Hadley circulation, to steer the glacial polar water advance as mid-depth boundary currents in the northern and southern hemisphere to effectively spread the cold water through the entire mid-depth Atlantic.

References:

1. Bereiter et al.: Mean global ocean temperatures during the last glacial transition. Nature 553, 39-44 (2018).
2. Ballarotta et al.: Last Glacial Maximum world ocean simulations at eddy-permitting and coarse resolutions: do eddies contribute to a better consistency between models and palaeoproxies?, Clim. Past 9, 2669-2686 (2013).
3. Adkins et al.: The Salinity, Temperature, and d18O of the Glacial Deep Ocean. Science 298, 1769-1773 (2002).
4. Stocker and Johnsen: A minimum thermodynamic model for the bipolar seesaw, Paleoceanography 18, 1087 (2003).

How to cite: Lausecker, M., Hemsing, F., Krengel, T., Förstel, J., Schröder-Ritzrau, A., Border, E., Orejas, C., Titschack, J., Wienberg, C., Hebbeln, D., Wefing, A.-M., Montagna, P., Douville, E., Matos, L., Raddatz, J., and Frank, N.: Was the Atlantic a predominantly Polar Ocean during the last glacial?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21063, https://doi.org/10.5194/egusphere-egu2020-21063, 2020.

Small air inclusions in ice cores represent a direct archive of past atmospheric compositions, allowing us to measure the concentration of the three most potent non-condensable Greenhouse Gases (GHG) CO₂, CH₄ and N₂O as far back as 800,000 years before present (kyr BP). These records demonstrate that transitions from glacial to interglacial conditions are accompanied by a substantial net increase of CO₂, CH₄ and N₂O in the atmosphere (Lüthi et al. 2008, Loulergue et al. 2008, Schilt et al. 2010). A sound understanding of the interplay between the reorganization of the climate system and the perturbation of GHG inventories during glacial terminations is partly limited by the temporal resolution of the records derived from ice cores. In fact, with the exception of the last deglaciation (23-9 kyr BP) centennial-scale GHG variability remained uncaptured for precedings glacial terminations.

In this work, we exploit the exceptionally long temporal coverage of the EPICA Dome C (EDC) ice core to reconstruct, for the first time, centennial-scale fluctuations of CH₄ mole fractions from 145 to 125 kyr BP, encompassing the entire penultimate deglaciation (138-128 kyr BP). With a temporal resolution of ~100 years, our new record is now unveiling all climate-driven signals enclosed into the EDC ice core, exploiting the maximum resolution possible at Dome C (). This offers us the opportunity to study the timing and rates of change of CH₄ in unprecedented details.

Preliminary analysis reveals that the deglacial CH₄ rise is a superimposition of gradual millennial-scale increases (~0.01-0.02 ppb/year) and abrupt and partly intermittent centennial-scale events (~80-200 ppb in less than a millennium). We will investigate processes modulating the observed changes in the CH₄ cycle, compare the structure of our record with the CH₄ profile of the last deglaciation (Marcott, 2014) and contrast it with the EDC CO₂ and N₂O records over the penultimate glacial termination now available in similar resolution.

How to cite: Schmidely, L., Silva, L., Nehrbass-Ahles, C., Han, J., Shin, J., Schmitt, J., Fischer, H., and Stocker, T.: Centennial-scale evolution of methane during the penultimate deglaciation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2844, https://doi.org/10.5194/egusphere-egu2020-2844, 2020.

Paleo-records of primary productivity (PP) changes from the Arabian Sea (AS) have revealed the major influence of monsoon-wind intensity in controlling productivity variations at different timescales, through mixed-layer dynamics and upwelling activity. Much less is known, however, about past changes in paleo-PP in the Bay of Bengal (BoB).

       In the present study, we have reconstructed PP over the last 26,000 years from a sediment core located on the northeastern (NE-) BoB. Paleo-PP was estimated by a PP empirical equation using the relative abundance of Florisphaera profunda, a deep dwelling coccolithophore that develops in the lower euphotic zone. Our record does not reveal any obvious difference of PP between the Last Glacial Maximum (LGM) and the late Holocene, but strong oscillations characterize the deglaciation. Our NE-BoB record is anti-phased to PP records in the AS, and positively correlated to surface seawater salinity (SSS) changes reconstructed from the same core since the LGM. We propose that the strong correlation to salinity variations reflects the role of salinity-stratification related to monsoon precipitation on PP at both orbital- and millennial-scales. Outputs of a climatic transient simulation (TraCE-21) and runs obtained with the Earth System Model IPSL-CM5 support the above interpretation of a strong control of past PP variations by local hydrological changes in the NE-BoB. Our data also highlight the potential teleconnection of the Atlantic Meridional Overturning Circulation strength and Indian Monsoon intensity during the deglaciation.

How to cite: Zhou, X., Duchamp-Alphonse, S., Kageyama, M., Bassinot, F., Beaufort, L., and Colin, C.: Primary productivity dynamics in the northeastern Bay of Bengal over the last 26,000 years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10253, https://doi.org/10.5194/egusphere-egu2020-10253, 2020.

During the last glacial period atmospheric CO₂ and temperature in Antarctica varied together on millennial timescales, with CO₂ abruptly increasing by 10-20 ppm in <1000 years in some cases. The exact causes of these rapid CO₂ changes during a cold glacial climate remain unclear. Here we examine the role of ocean carbon storage and atmospheric exchange by applying the boron isotope-pH (CO₂) proxy to Globigerina bulloides from core site TAN110628 located in the Pacific Sector of the Southern Ocean.  By reconstructing the surface carbonate system at TAN110628 at high temporal resolution (1 sample every 1 kyr) from 30 to 64 kyr we are able to fully constrain the nature of carbon leakage from the Sub Antarctic Zone of the Southern Pacific Ocean associated with these millennial-scale changes in atmospheric CO₂.  This provides unique insights into the causes of abrupt changes in atmospheric CO₂ during Marine Isotope Stage 3 and the last termination. 

How to cite: Shuttleworth, R., Bostock, H., and Foster, G.: Millennial-scale oceanic CO2 release during marine isotope stage 3, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17440, https://doi.org/10.5194/egusphere-egu2020-17440, 2020.

Glacial Termination V is one of the most extreme glacial-interglacial transitions of the past 800 ka [1]. However, the changes in orbital forcing from Marine Isotope Stage (MIS) 12 to 11 are comparatively weak. In addition, MIS 11c is exceptionally distinct compared to other interglacials with for example a longer duration [2] and a higher-than-present sea level [3] despite a relative low incoming insolation. Therefore, the term “MIS 11 paradox” was coined [4]. However, only little is known about the Atlantic overturning circulation during this time interval [e.g. 5,6].

Here, we present Atlantic-wide deep water circulation patterns spanning the glacial maximum of MIS 12, Termination V, and MIS 11. Therefore, sediment cores throughout the Atlantic were analyzed regarding their Nd isotopic composition of authigenic coatings to reconstruct the provenance of the prevailing bottom water masses.

During the glacial maximum of MIS 12, the deep Atlantic Ocean was bathed with a higher amount of southern sourced water compared to the following interglacial. Termination V is represented by a sharp transition in the high-accumulating sites from the North Atlantic with a switch to northern sourced water masses. MIS 11 is characterized through an active deep water formation in the North Atlantic with active overflows from the Nordic Seas, only disrupted by a short deterioration. A strong export of northern sourced water masses to the South Atlantic points to an overall strong overturning circulation.

[1] Lang and Wolff 2011, Climate of the Past 7: 361-380.

[2] Candy et al. 2014, Earth-Science Reviews 128: 18-51.

[3] Dutton et al. 2015, Science 349: aaa4019.

[4] Berger and Wefer 2003, Geophysical Monograph 137: 41-60.

[5] Dickson et al. 2009, Nature Geoscience 2: 428-433.

[6] Vázquez Riveiros et al. 2013, EPSL 371-372: 258-268.

How to cite: Link, J. M. and Frank, N.: Deep water circulation patterns in the Atlantic during MISs 12-11, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20750, https://doi.org/10.5194/egusphere-egu2020-20750, 2020.

At present Earth’s climate is warming and the frequency of large wildfires appears to be increasing (Westerling and Bryant, 2008). Long term trends in climate and the effect on wildfire are understudied and examining the geological record can aid current understanding of natural variability of wildfire over longer time scales. The Early Jurassic is a period of overall global warmth, and therefore serves as a suitable modern-day analogue to understand changes in the Earth System. The Early Jurassic was characterized by major climatic and environmental perturbations which can be seen preserved at high resolution on orbital timescales. Recent research has indicated from Quaternary deposits that wildfires respond to orbital forcings (Daniau et al., 2013). This study tests whether wildfire activity corresponds to changes over Milankovitch timescales in the deep past.

        A high-resolution astrochronology exists for the Upper Pliensbachian in the Llanbedr (Mochras Farm) borehole (NW Wales). Ruhl et al. (2016) show that elemental concentration recorded by hand-held X-ray fluorescence (XRF), changes mainly at periodicities of ~21,000 year, ~100,000 year and ~400,000 year, and which can be related to visually described sedimentary bundles.

        We have quantified the abundance of fossil charcoal at a high resolution (10-15 cm) to test the hypothesis that these well-preserved climatic cycles influenced fire activity throughout this globally warm period. Our results suggest that variations in charcoal abundance are coupled to Milankovitch forcings over periods of ~21,000 and ~400,000 years. Supplementary to the charcoal record, a high-resolution clay minerology dataset has been generated, which indicates the presence of the 400ky cycle. Decreased hydrology on land, corresponds to increased charcoal production. We suggest that these changes in fire relate to changes in seasonality and monsoonal activity that drove changes in vegetation that are linked to variations in the orbital forcing.

How to cite: Hollaar, T., Baker, S., Deconinck, J.-F., Mander, L., Ruhl, M., Hesselbo, S., and Belcher, C.: Orbital pacing of large fluctuations in wildfire activity during the Pliensbachian, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5383, https://doi.org/10.5194/egusphere-egu2020-5383, 2020.

The nature and dynamics of Pliocene climate has been a focus of intense study for many years. This is because the Pliocene has a unique potential to inform science/society about how the Earth system responds to forcing of direct relevance to future climate change. We examine large-scale climate features derived from the second phase of the Pliocene Model Intercomparison Project. PlioMIP2 is composed of simulations derived from sixteen coupled atmosphere-ocean and Earth System Models of a variety of vintages (IPCC AR3/4 to 6). This represents one of the largest ensembles of models ever assembled to represent a particular interval in Earth history. Each model has been set up to include the very latest Pliocene boundary conditions provided by the U.S. Geological Survey Pliocene Research Interpretation and Synoptic Mapping Project (PRISM4). As well as examining large-scale features of the PlioMIP2 model ensemble we further examine trends in model sensitivity versus model age in order to ascertain if newer versions of models are becoming more sensitive to Pliocene boundary conditions. We examine this across the PlioMIP2 ensemble as a whole and within individual model families, and examine what this implies in terms of the potential for individual models, or families of models, to represent patterns of surface temperature change reconstructed from geological proxies.

How to cite: Haywood, A. and Tindall, J.: Are models becoming more sensitive to Pliocene boundary conditions?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19313, https://doi.org/10.5194/egusphere-egu2020-19313, 2020.

The warm Pliocene serves as an analogue for predicted warming over the next century. However, large uncertainties exist for atmospheric circulation and land surface conditions during the Pliocene. Dust transported by wind to locations of accumulation (terrestrial or marine) can provide a record of wind intensity and/or direction. Few dust flux records spanning the Plio-Pleistocene exist. As such, there is ample opportunity to use marine sediments to reconstruct changes in atmospheric conditions during a warmer-than-present world, as well as across the onset/intensification of Northern Hemisphere Glaciation (NHG). During this time, East Asia’s interior, the second largest source of mineral dust today, experienced aridification, occurring alongside a major reorganization of the subarctic North Pacific circulation which led to stratification of the surface ocean. Here, we present two North Pacific marine sediment records of extraterrestrial (ET) ³He-derived terrigenous dust flux proxies (⁴He_Terr and Th), along with a record of multiple paleoproductivity proxies (Ba_xs, Opal, and C³⁷_Total) for the period spanning ~2.5-4.5 Ma. Our results show that dust flux to the western North Pacific was relatively low and constant through the Pliocene up until ~2.7 Ma, with minor peaks during cooler phases from ~2.9-3.1 Ma. At ~2.7 Ma, concurrent with the intensification of NHG and formation of a permanent halocline cap in the subarctic North Pacific, dust fluxes increase dramatically. The central North Pacific record shows a less drastic shift in dust, but generally displays higher fluxes after ~3 Ma. Dust fluxes in East Asia and the North Pacific are consistent during this time interval, as are global dust fluxes from the North Atlantic, South Atlantic and North Pacific. Western North Pacific dust, SST, and paleoproductivity records point to northward-shifted and weakened Northern Hemisphere westerlies during the warm Pliocene, with evidence for strengthening and southward movement of the westerlies during glacials after ~2.7 Ma. Changes in both winds and dust production mechanisms are likely working in tandem to produce the coherent global dust signals.

How to cite: Abell, J. T., Winckler, G., Anderson, R. F., and Herbert, T.: Reconstructing the intensity and location of Northern Hemisphere westerlies during the Plio-Pleistocene using marine sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5897, https://doi.org/10.5194/egusphere-egu2020-5897, 2020.

The response of the climate system to astronomical parameters is an important scientific issue, but the internal processes and feedbacks need to be better understood. This study investigates the differences of the climate response to the astronomical forcing between the Northern (NH) and Southern (SH) hemispheres based on a more than 90,000-year long transient simulation using the model LOVECLIM. Our results show that the response of sea ice and sea surface temperature (SST) to precession and obliquity are different between the two hemispheres. Precession plays a dominant role on the NH sea ice. This is mainly due to its response to the local summer insolation and also, to a less degree, the influence of the northward oceanic heat transports. However, obliquity plays a dominant role on the SH sea ice through its influence on insolation and the westerly winds. As far as the SST is concerned, it shows a strong precession signal at low latitudes in both hemispheres. For the SST in the mid and high latitudes, obliquity plays a dominant role in the SH whereas precession is more important in the NH. This is largely due to the different response to insolation and feedbacks related to the different land-ocean distribution in the two hemispheres. Near the Equator, besides the precessional signal, the SST also shows strong half-precessional signal, which can be explained by the unique characteristics of the insolation variations at the Equator.

How to cite: Wu, Z., Yin, Q., Guo, Z., and Berger, A.: Different response of sea surface temperature and sea ice to precession and obliquity between the two hemispheres, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4765, https://doi.org/10.5194/egusphere-egu2020-4765, 2020.

Uncertainties on the radiometric ages of Devonian stage boundaries are currently on the order of several millions of years. A cyclostratigraphic approach is the foremost way forward to improve the Devonian geological time scale. To do so requires well-preserved continuous records, as well as reliable paleoclimatic proxies.  The NY Route 199 section, from Kingston, in the Hudson Valley of eastern New York, is a road cut outcrop, which exposes most of the Schoharie Formation. It corresponds to the upper portion of the Emsian Stage (upper Lower Devonian, ~400 to ~394 Ma), with essentially continuous deposition. The lithology consists of a mixed siliciclastic-carbonate succession with overall increasing carbonate upsection, showing various degrees of bioturbation (traces includes primarily Zoophycos, Planolites and Chondrites); colors range from white to beige, brown or dark grey. The quality of most of the outcrop is so remarkable that the color variations by themselves permit recognition of Milankovitch cycles, with prominent bundles of light and dark beds. One type of cycle expression is represented by a succession of about six darker beds nested between lighter beds, which is interpreted as six precession cycles within a short eccentricity cycle (precession in the Devonian was ~17 kyr).

Samples were collected every 2 cm through 38 m of the section for magnetic susceptibility measurements. On top of these measurements, we provide elemental geochemistry, carbon isotopes and hysteresis measurements (every 50 cm) to constrain the depositional setting and the diagenesis. Hysteresis measurements show that despite being remagnetized (throughout the Appalachians, these Paleozoic rock sequences are all remagnetized during the Variscan-Alleghenian Orogeny), the magnetic susceptibility reflects depositional information. The geochemistry and carbon isotopes give insight into the occurrence of oxic/reducing conditions and detrital inputs. Milankovitch cycles are visible on the outcrop and in the magnetic susceptibility record, allowing a precise floating timescale framework to be constructed for this interval.

How to cite: Da Silva, A.-C., Bartholomew, A., Brett, C., Hilgen, F., Ver Straeten, C., and Dekkers, M.: Exceptionally preserved Milankovitch cycles in Lower Devonian argillaceous limestone of the Hudson Valley, New York State (USA) , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7565, https://doi.org/10.5194/egusphere-egu2020-7565, 2020.

The Late Paleozoic Ice Age (LPIA), one of the best known and prolonged glaciation events in Earth's history, resulted in the widespread deposition of glacial sediments over Gondwana (Crowell, 1999). Some of the most important LPIA deposits of the multiple glacial-deglacial episodes (Isbell et al., 2003) were preserved in the Itararé Group of the Paraná Basin (Brazil). This unit presents continental and marine glacially-influenced deposits formed by advances and retreats of glaciers and consists in an opportunity to better understand the mechanisms forcing climate shifts during the LPIA. In low latitudes, the deposition of the Carboniferous cyclothems was controlled by long- and short-eccentricity (Davydov et al., 2010). In high latitudes, orbital-scale climate cycles may also be preserved in the sedimentary succession. We aim to recognize whether or not orbital and millennial-scale climate cycles are preserved in the sedimentary succession of a core drilled in the southeastern border of the Paraná Basin. Here, we present the first cyclostratigraphic study based on X-ray fluorescence records from a 27 m-long interval of LPIA rhythmites of the Rio do Sul Formation (top of the Itararé Group). The sedimentary succession is composed of lithological couplets of fine-grained siliciclastic sediments, locally displaying subtle plane-bedding. Such rhythmites are characterized by abrupt contacts between couplets and normal grading internally. TiO₂ and Fe₂O₃ vary in phase and display well-defined cyclicities in the stratigraphic domain. The TiO₂ series presents millennial and orbital scale periodicities. Variations in the concentrations of the analyzed terrigenous components are likely indicative of glacial-interglacial changes, reflected by advances and retreats of glaciers under drier and wetter climate conditions, respectively. Here we show that these high latitude glacial-interglacial cycles were probably paced by short-eccentricity, as previously suggested for Carboniferous cyclothems in low latitude deposits, and highlight the importance of millennial-scale climate cycles forcing high latitudes glacial-related deposits, similar to patterns seen in Pleistocene records.

References:

Crowell, J. C. (1999). Pre-Mesozoic Ice Ages: Their Bearing on Understanding the Climate 375 System. Geologic Society of America Memoir 192, pp. 1–112.

Davydov, V. I., Crowley, J. L., Schmitz, M. D., & Poletaev, V. I. (2010). High-precision U-Pb zircon age calibration of the global Carboniferous time scale and Milankovitch band cyclicity in the Donets Basin, eastern Ukraine. Geochemistry, Geophysics, Geosystems, 11.

Isbell, J. L., Miller, M. F., Wolfe, K. L., & Lenaker, P. A. (2003). Timing of late Paleozoic glaciation in Gondwana: Was glaciation responsible for the development of Northern Hemisphere cyclothems? In Geologic Society of America Special Paper 370, pp. 5–24.

How to cite: Kochhann, M., Cagliari, J., Kochhann, K., and Franco, D.: Climate variability during the Late Paleozoic Ice Age in the southwestern Gondwana: records of orbital and millennial-scale cycles in the Carboniferous rhythmite of the Paraná Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11886, https://doi.org/10.5194/egusphere-egu2020-11886, 2020.

Fifty-one years of scientific ocean drilling through the International Ocean Discovery Program (IODP) and its predecessors generated a treasure trove of Cenozoic climate and carbon cycle dynamics. Yet, it remains unclear how climate system and carbon cycle interacted under changing geologic boundary conditions. Here, we present the carbon isotope (d¹³C) megasplice, documenting deep-ocean d¹³C evolution since 35 million years ago (Ma). We juxtapose the d¹³C megasplice with its d¹⁸O counterpart and determine their phase-difference on ~100-kyr eccentricity time-scales. This analysis uncovers that 2.4-Myr eccentricity modulates the in-phase relationship between d¹³C and d¹⁸O during the Oligo-Miocene (34-6 Ma), potentially related to changes in continental weathering. At 6 Ma, a striking switch from in-phase to anti-phase behaviour occurs, signalling a threshold in the climate system. We hypothesize that Arctic glaciation and the emergence of bipolar ice sheets enabled eccentricity to exert a major influence on the size of continental carbon reservoirs. Our results suggest that a reverse change in climate - carbon cycle interaction should be anticipated if CO₂ levels rise further and we return to a world of unipolar ice sheets.

How to cite: De Vleeschouwer, D., Drury, A. J., Vahlenkamp, M., Liebrand, D., Rochholz, F., and Pälike, H.: Thirty-five million years of changing climate – carbon cycle dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21479, https://doi.org/10.5194/egusphere-egu2020-21479, 2020.

Variations in solar insolation exert a fundamental control on the high-latitude climate–cryosphere system. Controversy, however, exists about the relative importance of orbital eccentricity versus axial tilt (obliquity) in driving pre-Quaternary Antarctic ice sheet variability. This problem is particularly acute during the late Oligocene-to-early Miocene interval (Oligo-Miocene, ~27-21 Ma), because several benthic foraminiferal oxygen isotopes (δ¹⁸O) records show strong pacing by obliquity, while others primarily show eccentricity pacing. The differences in orbital pacing are impossible to reconcile with the globally congruent imprint of ice volume on benthic δ¹⁸O on orbital time scales. Here we present a new astronomically tuned δ¹⁸O record generated at Integrated Ocean Drilling Program (IODP) Site U1406 (north-western Atlantic Ocean), a key area in modern-day thermohaline circulation. Clear imprints of both obliquity and eccentricity on the δ¹⁸O record are observed at Site U1406 throughout the study interval, irrespective of changes in sedimentation rate. The eccentricity variations at Site U1406 are remarkably similar to those seen in all other δ¹⁸O records, suggesting that eccentricity exerts a strong control on the high-latitude climate–cryosphere system via the modulation of the precession cycle. In contrast, the δ¹⁸O sensitivity to obliquity is globally variable, suggesting the influence of temperature in different bottom-water masses.

How to cite: van Peer, T., Taylor, V., Liebrand, D., Brzelinski, S., Möbius, I., Bornemann, A., Friedrich, O., Bohaty, S., Xuan, C., Lippert, P., and Wilson, P.: Eccentricity-paced ice sheet variability and obliquity-driven bottom-water changes during the Oligocene-Miocene, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15256, https://doi.org/10.5194/egusphere-egu2020-15256, 2020.

The early Pliocene, with atmospheric CO₂ concentrations at levels similar to today, is seen as a case study for Earth’s future climate evolution. During this period the progressive closing of the Central American Seaway led to increased poleward heat and salt transport within the Atlantic with North Atlantic Deep Water (NADW) becoming warmer and saltier and resulting in an enhanced Atlantic Meridional Overturning Circulation (AMOC). In order to evaluate how stable the Pliocene AMOC really was, we are producing surface and deep-water records for IODP Site U1313 (41°N, 33°W, 3412m) for the interval from 3.3 to 4.1 Ma. This site is ideally located to monitor past AMOC changes with North Atlantic Drift waters at the surface and NADW, exported by the deep western boundary current, in the deep. Surface water conditions are reconstructed based on the stable isotope data of planktonic foraminifer species Globigerinoides ruber (white) or Globigerinoides extremus with centennial-scale resolution and on sea-surface temperatures (U^k_37' alkenone thermometer) with an average 4 ky resolution. Stable isotope records of the benthic foraminifer genus Cibicidoides reveal changes in the deep water.

Besides the interglacial/glacial cycles, higher frequency oscillations are recorded in both the planktonic and benthic foraminifer stable isotope records. Varying surface water conditions, especially during Late Pliocene interglacial periods, are reflected in the Globigerinoides isotope data and appear to be linked to salinity changes since they are not recorded in the sea-surface temperature data. The high-frequency oscillations in the planktonic isotope records are related to precession (insolation) forcing, especially its harmonics in the 5.5 ky and 11 ky ranges. The benthic δ¹³C values indicate nearly continuous NADW presence and confirm a strong AMOC throughout the studied interval, also during most of the glacial periods. Excluding the pronounced M2 glacial, glacial stage Gi 6 had a stronger impact on the AMOC, as revealed by cooler, less ventilated surface waters and a less ventilated NADW, than Gi 2 and Gi 4. Overall, the AMOC was strong throughout, but experienced high frequency oscillations at a level similar to the middle Pleistocene interglacial periods.

How to cite: Voelker, A. H. L., Sierro, F. J., Naafs, B. D. A., Andersen, N., and Kuhnert, H.: Early to Late Pliocene climate change in the mid-latitude North Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10667, https://doi.org/10.5194/egusphere-egu2020-10667, 2020.

The Sahel region is one of the most vulnerable regions on Earth to anthropogenically-driven climate change, but also one of the least equipped to deal with the consequences. Predictions of precipitation levels over the forthcoming centuries diverge, not only in magnitude, but also in the sign of change. One key aspect of this uncertainty comes from the role of Atlantic Ocean sea surface temperatures (SST), which are known to exert a strong control over precipitation in the Sahel and are implicated in both the major drought of the late 20^th century and extreme droughts associated with the Heinrich events of the last glacial. To better understand how Sahelian hydroclimate may respond to SST variability in a warmer world, we turn to the Pliocene epoch, when atmospheric CO₂ levels were comparable to present.

We studied sediments from Ocean Drilling Project Site 659, which is situated in the subtropical North Atlantic beneath the major modern summer Saharan dust plume. Our new dust accumulation rates and X-ray fluorescence core scan data indicate that there were major shifts between highly arid conditions and humid intervals with vegetated or “Green Sahara” conditions over much of northern Africa, driven by both solar insolation and glacial-interglacial variability. We also report three unusually long Plio-Pliocene humid intervals (each lasting ca. 100 kyr) characterised by very low dust emissions, that we term “Green Sahara Megaperiods (GSMPs)”. All three of these GSMPs occur at times when insolation variability was weak, resulting in values close to the long-term mean. This observation strongly suggests that factors other than insolation drove the sustained humidity of GSMPs. We present paired alkenone SST estimates and multi-species planktonic foramaniferal isotope records from 3.5–2.3 Myr ago to explore the extent to which the GSMPs were accompanied by intervals of extended warmth in the surface waters of the North Atlantic Ocean.

How to cite: Wilson, P., Jewell, A., Crocker, A., Buchanan, S., Mitsunaga, B., Westerhold, T., Röhl, U., Russell, J., and Herbert, T.: Green Sahara Megaperiods during the Pliocene: What was the role of North Atlantic Ocean temperature?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20327, https://doi.org/10.5194/egusphere-egu2020-20327, 2020.

Models from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) show that the mid-Pliocene Warm Period (mPWP) was a warmer and wetter world than today. However, there is not strong model agreement as to how tropical precipitation was different in the mPWP. Although PlioMIP2 models agree that there was more precipitation associated with the African Monsoon and the Asian Monsoon, away from these regions models do not show a consistent and robust change in precipitation between the mPWP and the preindustrial.

Here we use the HadGEM2 model to explore changes in tropical precipitation between the mPWP and the preindustrial, particularly those associated with the position and strength of the Intertropical Convergence Zone (ITCZ). Reasons for these changes within HadGEM2 will be discussed. We will also expand our discussion of the ITCZ to the PlioMIP2 ensemble in order to show the differing factors that could influence ITCZ characteristics in a warmer world.

How to cite: Tindall, J. and Haywood, A.: Modelling Tropical Precipitation in the mid-Pliocene Warm Period, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19088, https://doi.org/10.5194/egusphere-egu2020-19088, 2020.

Today in the North Pacific only intermediate water forms because of a strong halocline, but Pacific Meridional Overturning Circulation (PMOC) may have existed in the past. The mid-Pliocene warm period (3.264-3.025 Ma) is a time of sustained warmth where atmospheric carbon dioxide concentrations were similar to today and the northern hemisphere was relatively ice free – making it a pseudo-analogue for future climate change. North Pacific sedimentological and climate modeling evidence suggests a PMOC formed during this time.  To determine the spatial extent of a PMOC during the mid-Pliocene warm period, we constructed a depth transect of sites between 2400 to 3400 m water depth on Shatsky Rise by measuring stable isotopes of Cibicidoides wuellerstorfi. We compare these new results with previously published records and calculate anomalies using the OC3 water column and core-top data products. The δ¹³C spatial pattern is consistent with a modest PMOC of intermediate depth (core ~2000 m) extending to the equator during the mid-Pliocene warm period. Ventilation of the North Pacific by a PMOC has broad implications for deep ocean carbon storage as the North Pacific contains the oldest, carbon-rich waters today. Future work will include minor and trace element analyses to determine the temperature and carbon characteristics of the PMOC water mass and comparisons with PlioMIP modeling outputs.

How to cite: Ford, H. L., Burls, N., and Hodell, D.: Pacific Meridional Overturning Circulation during the Mid-Pliocene Warm Period, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21860, https://doi.org/10.5194/egusphere-egu2020-21860, 2020.

The South Asian or Indian Summer Monsoon (ISM) brings seasonal winds and rains to the Indian subcontinent and affects billions of people.  It is likely that the global monsoon will strengthen in a 1.5 °C warming scenario (IPCC special report (2018)), however our ability to predict ISM behaviour in the future is restricted due to lack of understanding of its behaviour under varying climatic conditions before instrumental records began.  Thus, reconstructing the palaeo-monsoon using proxies gives insight into past and potentially future controls on the ISM.  We present new data covering the interval ~5 to ~2 million years ago (Ma), during the Pliocene and early Pleistocene when the long-term Cenozoic cooling trend culminated in intense northern hemisphere glaciations from 2.7 Ma.  At this time, global temperatures are suggested to have been 2-3 °C warmer than today and atmospheric CO₂ was over 400 ppm (similar to today). 

This study focuses on sediments from Site U1443 ( 5°N, 90°E), drilled during International Ocean Discovery Program (IODP) Expedition 353 in the Bay of Bengal (BoB) for the Pliocene – early Pleistocene.  We present X-ray fluorescence (XRF)-derived bulk sediment geochemical data and suggest that erosional flux (terrigenous elements/total counts) as well as productivity (Br/Cl) varied in response to runoff strength, precipitation, and wind stress at the study site to reconstruct ISM variability.  Additionally, new nannofossil assemblage and morphometric data, collected using the automated system SYRACO, are used to reconstruct BoB stratification and productivity and thereby assess ISM dynamics.  A new benthic oxygen isotope-based age model will allow us to place the Site U1443 records into the context of existing climate and monsoon records and evaluate ISM response due to external and internal climate forcing factors.

How to cite: Gray, E., Anand, P., Bolton, C., Murayama, M., and Badger, M.: Reconstructing Past Indian Summer Monsoon Productivity and Stratification During the Late Pliocene and Early Pleistocene , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-399, https://doi.org/10.5194/egusphere-egu2020-399, 2020.

Seasonality is a dominant factor in the Earth’s climate system, but proxy reconstructions on this time scale are sparse. Corals provide an excellent archive to reconstruct environmental conditions on seasonal time scale using geochemical proxies. Here, we use subfossil (~6.2-7.1 ka BP) Siderastrea siderea and Pseudodiploria labyrinthiformis corals from a pristine Mid-Holocene reef, located in Panamá, southwestern Caribbean. Mid-Holocene insolation seasonality in the Northern Hemisphere was stronger than at present. We investigate the resulting changes in SST and hydrological seasonality using coral Sr/Ca, δ¹⁸O and δ¹³C. To evaluate, if the coral heads can be utilised for geochemical analyses, they have been screened for diagenetic alteration (2D-XRD, thin section analysis). Obtained modern coral Sr/Ca-SST based annual cycle corresponds well with in situ measured SST. Fossil coral Sr/Ca-SST based cycles exceed the modern one by up to 50%. Fossil coral δ¹⁸O seasonal amplitudes are higher than the modern one by up to 30% and show a reduction in the mean gradient between wet and dry period, attributable to the northward shift of the Intertropical Convergence Zone. Increased SST and δ¹⁸O seasonality are consistent with model simulated SSTs (Kiel Climate Model) and model-based calculated pseudocoral δ¹⁸O, but the model underestimates the seasonality increase in the Mid-Holocene.

How to cite: Skiba, V., Struck, U., Reuning, L., Garbe-Schönberg, D., Frank, N., Leinfelder, R., O'Dea, A., and Zinke, J.: Coral reconstructed Mid-Holocene seasonality in the southwestern Caribbean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1092, https://doi.org/10.5194/egusphere-egu2020-1092, 2020.

The ocean is the largest heat capacitor of the earth climate system and a main source of atmospheric moist static energy. Especially, upper ocean heat content changes in the tropics can be taken as the heat engine of global climate. Here we provide an orbital scale perspective on changes in OHC obtained from a transient simulation of the Community Earth System Model under orbital insolation and GHG forcings. Considering the vertical stratification of the upper ocean, we calculate OHC for the mixed layer and the upper thermocline layer according to the isotherm depths of 26℃ and 20℃ respectively. Generally, our simulated OHC are dominated by thickness changes rather than temperature changes of each layer. In details, there are three situations according to different forcings:

(1) Higher GHG induces positive mixed layer OHC anomalies inside the western Pacific warm pool but with neglected anomalies outside it. For the upper thermocline layer, there are negative OHC anomalies inside the warm pool and positive anomalies in the subtropical Pacific of two hemispheres. For the total OHC above 20℃ isotherm depth, positive anomalies mainly come from the mixed layer between 15ºS-15ºN and from the thermocline between 15º-30º. Lower obliquity induces similar spatial patterns of OHC anomalies as those of higher GHG, but total OHC anomalies are more contributed by upper thermocline anomalies.

(2) Lower precession results in positive mixed layer OHC anomalies in the core of warm pool (150ºE-150ºW, 20ºS-10ºN) and the subtropical northeastern Pacific, but with negative anomalies in other regions of the tropical Pacific. Upper thermocline layer OHC anomalies have similar patterns but with opposite signs relative to the mixed layer in regions between 15ºN-30ºS. As a combination, positive total OHC anomalies occupy large areas of 130ºE-120ºW from 30ºS to10ºN, while negative anomalies dominate the subtropical north Pacific, the western and eastern ends of the tropical Pacific.

If confirmed by paleoceanographic proxies, our simulated OHC results can be served as the first guide map of anomalous energetic storage & flows in the earth climate system under orbital forcings.

How to cite: Wang, Y., Jian, Z., Dang, H., Liu, Z., Jin, H., Zhang, S., Luo, L., and Wang, X.: Upper ocean heat content (OHC) changes in the tropical Pacific induced by orbital insolation and greenhouse gases (GHG), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6318, https://doi.org/10.5194/egusphere-egu2020-6318, 2020.

Lake Towuti is a tectonic lake on central Sulawesi, Indonesia. It is located within the Indo Pacific Warm Pool, a convection cell which has major impact on tropical climate and the ability to project its influence on a global scale (Chiang, 2009; De Deckker, 2016). Pre-site surveys using seismic methods and piston cores indicated that sediments in Lake Towuti provide best conditions to obtain a long-term paleoclimate record in this key region (Russel et al., 2014).  

During an ICDP-project in 2015, downhole logging equipment of the Leibniz Institute for Applied Geophysics was used at two drill-sites to record a series of chemical and physical parameters (spectral gamma ray, magnetic susceptibility, resistivity, sonic velocity, dipmeter, ultrasonic imaging of the borehole wall). Continuous lithological logs based on downhole logging data were constructed using cluster analysis. Although the spatial resolution of constructed logs is not as detailed as core descriptions, good correlation to core descriptions and differentiation between the upper lacustrine facies and the lower pre-lacustrine facies (Russell et al., 2016) show that cluster analysis is a powerful tool in giving an instant overview of in situ sediments and determining their physical properties.

Cyclostratigraphic methods in downhole logging can help developing a better understanding of sedimentation rates and thus improving age-depth models for lacustrine sediments (Molinie and Ogg, 1990; Hinnov, 2013; Baumgarten et al., 2015). In case of Lake Towuti, a magnetic susceptibility log from the upper lacustrine facies (0-98 meters below lake floor) was analysed to calculate changes in sediment influx. A careful pre-processing of the data is crucial to secure undisturbed amplitude spectra. This includes the identification and exclusion of event-layers (tephra and turbidite-like mass movement deposits) from the log. Also side effects of those layers to surrounding sediments were diminished from the record.

Sedimentation rates for certain parts were calculated and complement the preliminarily age model derived from ¹⁴C- (Russel et al., 2014) and tephra-dating (A. Deino, personal communication, December, 2018). Further refining of the model and omission of an interpretation of long cyclicities results in the most detailed age-depth model for Lake Towuti, and thus is a fundamental step towards our understanding of paleoclimate processes in this region.

How to cite: Ulfers, A., Hesse, K., and Wonik, T.: Paleoenvironmental indications and cyclostratigraphic studies of sediments from tropical Lake Towuti obtained from downhole logging, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21806, https://doi.org/10.5194/egusphere-egu2020-21806, 2020.

The Milutin Milankovic Astronomical Model of Ice Ages revisited

As per the M.M.-Model, the 3 combined precessional effects have a cycle of ca. 21,000 years;

the cycle of the axis tilting from 21.8⁰ to 24.4⁰ runs ca. 41,000 years; the cycle from circle to ellipse, back to circle spans ca. 90,000 - 120,000 years. Science predicts the inception of a new ice age, fearing that the period the system achieved its best parameters is already behind us.

However, the data from other sources, from Plato and the Greek Sibyls to the New Testament and beyond, predict the imminence of a Golden Age, with optimal weather patterns, following a prophesied earthshaking and a few other astrophysical and geophysical woes. This is most consistent with what Milankovic’s true parameters would predict, once certain hidden variables are dealt with.

Besides the pull of sun, moon and planets, affecting the motions of the earth and insolation levels on a regular basis of solar system motions, we must factor in periodic entries of special "controller" comets whose purpose is to exercise potent "sucking" power, which helps re-calibrate the motions of the earth. Such comets do not cause impacts, but earth-shaking all the same, due to the reaction of the earth to such potent attraction. We have evidence of many comets entering the system (the "myths", Plato's Timaeus, Critias, Politicus, etc.) and geological evidence of the effects of such cometary activity.

Depending on the comet's size and its motion parameters, we get ekpyrosis and cataclysm, at global levels. Plate tectonic activity due to a major earthshaking `fatal attraction' will most definitely influence the axis' obliquity, once the `dust is settled'. If at the same time we get a minor impact that generates Flooding and enhanced volcanic activity, the results are more pronounced.

What is the periodicity of such "controller" comets which enter the inner solar system and change so drastically our motion parameters? The notion of `aeon' as per Heraclitus of Ephesus deserves our attention.

The Heraclitus aeon is a period of 10,800 years. Besides being twenty times the `age of Phoenix' calculated at 540 years, the aeon of 10,800 years is the most accurate unit for measuring the cycles of the Milankovic model. In fact,

10,800 x 2 = 21,600 our best approximation to the combined effects of all three precessional cycles.

10,800 x 4 = 43,200 (the cycle of axis tilting)

10,800 x 10 =108,000 the best calibration, so far, of the "eniautos" from circle to ellipse and back.

It is no `co-incidence' that makes the numbers fit so neatly. Not to mention the `ancient myths' which loved periods of 540 years; or 432,000; the combined effects of 25,920 and 108,000 etc.

Very soon we shall witness such a`controller comet', making the year 360 days long. It will provide the parameters for a new Golden Age for the survivors of the Floods and the Ekpyrosis. Golden Ages and years of 360 days with enhanced insolation come at a high price in the Drama Of Evolution.

How to cite: Otto, H.: The Milutin Milancovic Astronomical Model of Ice Ages Revisited, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22130, https://doi.org/10.5194/egusphere-egu2020-22130, 2020.

With the recent demonstration that millennial timescale Dansgaard-Oeschger oscillations of MIS 3 are predictable in a modern coupled climate model following a Heinrich event-like reduction of AMOC strength (eg. Peltier and Vettoretti, 2014), the stage was set for a renewed attack upon the physics of H-events themselves (see Velay-Vitow et al, 2019, JGR-Oceans). This predicts that the freshwater forcing of the AMOC by individual H-events will be on the order of 0.1 Sv and to be maintained for a period between 500 years and 1500 years in accord with data-based inferences (Hemming, 2004). Whereas in the original analysis of H-event induced D-O oscillations the D-O initiating H-event appeared simply as a sharp reduction in AMOC strength in the spin-up of the coupled model, in the work to be reported we transform the pseudo H-event into one that involves explicit freshwater forcing applied at a strength and over a range of times in accord with observational constraints. This has enabled a detailed analysis of the global climate impacts of these events as represented in the coupled climate model that we continue to employ. A critical focus of this analysis is upon the phase relationship between events recorded in the oxygen isotopic records from Greenland and Antarctic ice cores, analyses which demonstrate that this phase relationship is set by the D-O initiating Heinrich event. We also address the expected global climate impacts of stadial-interstadial transitions and provide an initial discussion of these impacts with those recorded in speliothems and other archives.

How to cite: Peltier, R., velay-Vitow, J., and Chandan, D.: Heinrich Stadials: Globa Climate Impacts and the "Bipolar Seesaw" Phase Relationsip, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1955, https://doi.org/10.5194/egusphere-egu2020-1955, 2020.

Using high precision and centennial resolution ice core information on atmospheric nitrous oxide concentrations and its stable nitrogen and oxygen isotopic composition enables us to quantitatively reconstruct changes in the terrestrial and marine N₂O emissions over the last 21,000 years as well as over Heinrich Stadial (HS) 4.

We show that over the deglaciation N₂O emissions from land and ocean increased in parallel by 1.8 ± 0.3 TgN yr^-1 and 0.7 ± 0.3 TgN yr^-1, respectively. However, close to 50% of the terrestrial increase is accomplished within less than 200 years at the end of HS1 starting essentially in parallel with the co-occurring CH₄ increase. A similarly rapid but smaller increase is observed at the end of HS0 and suggested at the end of HS4, showing that terrestrial N₂O emissions respond strongly and rapidly to the northward shift in the Intertropical Convergence Zone connected to the resumption of the Atlantic Meridional Overturning Circulation (AMOC). However, little change in terrestrial N₂O emissions is observed during the onsets of Heinrich Stadials. Assuming that N₂O loss from terrestrial ecosystems is directly connected to nitrogen turnover in soils, the fast increase at the end of Heinrich Stadials suggests that terrestrial ecosystems did not become nitrogen-limited on this relatively short time scales, as also supported by model runs in our LPX-Bern dynamic vegetation/biogeochemical model. However, changes in number of moles of N₂O lost to the atmosphere per mole nitrogen turned over in soils (yield factor) may also contribute to the atmospheric N₂O changes.

Marine N₂O emissions also respond to Heinrich events and AMOC changes, however more gradually and less strongly compared to terrestrial emissions both in our data-based reconstruction and the Bern3D coupled ocean/biogeochemistry model. In fact, reconstructed marine emissions start to slowly increase many centuries before the rapid warmings, connected to a re-equilibration of subsurface oxygen concentrations in response to previous AMOC changes. At the onset of HS1 marine emissions decreased by about 0.5 TgN yr^-1, concomitantly with changes in atmospheric CO₂ and δ¹³C(CO₂), and started to re-increase after about 1500 years, when also rapid CO₂ and CH₄ jumps occurred, pointing to Southern Ocean and low-latitude circulation changes. A similar decrease as at the start of HS1 is found after the onset of HS0, but little N₂O emission change is suggested by N₂O concentrations and their isotopic signature at 39.5 kyr before present when Heinrich Event 4 presumably occurred, as suggested by a co-occurring intermittent CH₄ peak and a sudden increase in CO₂.

How to cite: Fischer, H., Schmitt, J., Bock, M., Seth, B., Joos, F., Spahni, R., Battaglia, G., Stocker, B., Lienert, S., Prentice, C., Otto-Bliesner, B., Liu, Z., Schilt, A., and Brook, E.: N2O changes during Heinrich Stadials - Isotopic source deconvolution over HS0, HS1 and HS4 and its implication for the marine and terrestrial nitrogen cycle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5638, https://doi.org/10.5194/egusphere-egu2020-5638, 2020.

Paleoclimate records from the North Atlantic show some of the most iconic signals of abrupt climate change during the ice ages. Here we use radiocarbon as a tracer of ocean circulation and air-sea gas exchange to investigate potential mechanisms for the abrupt climate changes seen in the North Atlantic over the last deglaciation. We have created a stack of North Atlantic surface radiocarbon reservoir ages over the past 20,000 years, using new synchronized age models from thirteen sediment cores refined with thorium normalization between tie-points. This stack shows consistent and large reservoir age increases of more than 1000 years from the LGM into HS1, dropping abruptly back to approximately modern reservoir ages before the onset of the Bolling-Allerod. We use the intermediate complexity earth system model cGENIE to investigate the potential drivers of these reservoir age changes. We find that sea ice, circulation and CO₂ all play important roles in setting the reservoir age. We use these coherently dated records to revisit the sequence and timing of climatic events during HS1 and the last deglaciation, and show that Laurentide Heinrich Events are a response to stadial conditions, rather than their root cause.

How to cite: Burke, A., Greenop, R., Rae, J., Rees-Owen, R., Reimer, P., and Heaton, T.: Dating rapid climate change in the North Atlantic during Heinrich Stadial 1, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18942, https://doi.org/10.5194/egusphere-egu2020-18942, 2020.

The last deglaciation in the northern hemisphere was interrupted by several short cold setbacks of which the Younger Dryas (YD) was the last and most pronounced. This abrupt and extreme cold period provides valuable insights into regional climate and environmental responses. To decipher the rate of such rapid changes continuous climate archives of annually laminated (varved) sediments are crucial.

Lake Gościąż (central Poland) exhibits an iconic varved lake sediment record that is one of the few European lake records preserving varves throughout the complete YD. To re-investigate this archive, 10 new sediment cores have been obtained along a N-S transect through the deepest part of the lake basin. We used a combination of continuous microfacies analyses, XRF element core scanning, µ-XRF mapping, and high-resolution chironomid-inferred mean July air temperature as well as analyses of stable oxygen and carbon isotopes.

Lacustrine sedimentation begins in the late Allerød, is briefly interrupted by a slump during the early YD and proceeds continuously afterwards. Here, we present a first continuous microfacies investigation of the complete YD in Lake Gościąż. Varve composition during the YD is the most complex and variable one, featuring primarily diatom frustules, calcite, re-worked and re-suspended material. Contrastingly, the simpler structured varves during the early Preboreal and late Allerød are characterized predominantly by calcite, rhodochrosite and dissolved organic matter. The change in microfacies at both YD transitions occurs not simultaneously with the other proxy responses.

Causes of and differences in proxy responses in regard to the dynamics of environmental change during a major change in climate are discussed. Further, we conduct a proxy comparison at both YD transitions and provide a detailed documentation of the transitions through µ-XRF mapping.

This study is a contribution to the Virtual Institute of Integrated Climate and Landscape Evolution Analysis (ICLEA) of the Helmholtz Association (grant number VH-VI-415). It is further a contribution to a scientific project financed by the National Science Centre, Poland – No. UMO-2015/19/B/ST10/03039.

How to cite: Müller, D., Tjallingii, R., Plessen, B., Płóciennik, M., Ramisch, A., Neugebauer, I., Schwab, M. J., Słowiński, M., Błaszkiewicz, M., and Brauer, A.: Dynamic environmental response to the Younger Dryas cooling in the sediment record of Lake Gościąż, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-742, https://doi.org/10.5194/egusphere-egu2020-742, 2020.

In light of the current rate of anthropogenic climate change, it is becoming increasingly critical to enhance knowledge of past abrupt climate events and subsequent responses of the Earth system. One period that can provide such insight is the last ~28 kyr, with several abrupt changes occurring over the course of the deglaciation. The Portuguese Margin has been an ideal location to study the impacts of these abrupt climate events on marine and terrestrial environments.  The combined effect of the narrow continental shelf and close proximity to the Tagus and Sado rivers, lead to the rapid delivery of a high quantity of sediment, including our pollen and biomarker proxies, to the Tagus Abyssal Plain. Joint terrestrial and palaeoceanographic analyses from the same sediment samples enable an in situ assessment of the relative timing of changes in palaeoceanographic and terrestrial proxies.

Here we document the response of western Iberian vegetation to millennial and centennial-scale changes, particularly changes in moisture availability, over the deglaciation and Holocene, by combining (for the first time at a Portuguese Margin site) pollen and leaf-wax isotopic biomarker records (δ¹³C and δD) from core SHAK06-5K. A high-resolution pollen record (every 2cm) and lower-resolution n-alkane δ¹³C and δD records spanning 28kya are compared with high-resolution XRF sediment and planktonic foraminiferal d¹⁸O analyses from the same core.  The sequence is supported by high-resolution age control, based on 40 Accelerator mass spectrometry (AMS) ¹⁴C dates from monospecific samples of the planktonic foraminifera, Globigerina bulloides.

Our pollen record indicates the rapid response of regional vegetation to centennial changes and millennial-scale climate events, with forest expansion during the warm interglacial/ interstadial Bølling-Allerød and Holocene, and forest contraction and steppe expansion during cold glacial/ stadial conditions of the Last Glacial Maximum and Younger Dryas. Comparing our pollen and n-alkane biomarker data with the XRF Zr:Sr ratio and planktonic foraminiferal δ¹⁸O records, a clear synchroneity can be seen in the timing of millennial-scale changes in all records.  The millennial-scale changes in our leaf wax n-alkane δD and δ¹³C records can be explained by both vegetation composition and growing season water availability. 

How to cite: Cutmore, A., Ausin, B., Eglinton, T., Maslin, M., and Tzedakis, C.: Reconstructing abrupt climate changes of the last deglaciation & Holocene: Pollen & biomarker analyses from the Portuguese Margin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21845, https://doi.org/10.5194/egusphere-egu2020-21845, 2020.

The INTIMATE group has, for a number of years, outlined the most robust approaches for comparing high resolution palaeoclimate archives in order to understand the regional pattern of response to climate change, and hence test models of climate forcing. These have tended to focus on the Last glacial and Early Holocene, until recently. However, in the later Holocene there are similar climatic oscillations with a variety of proposed mechanisms and regional responses. One such climatic oscillation, the 2.8 ka BP event, is a cold spell thought to be driven by a grand solar minimum with potential impacts on atmospheric dynamics and hydrology across parts of Western Europe¹. At present there are only a small number of independently-dated, high-resolution records for this event, limiting the extent to which a regional expression of this event can be understood. This is a problem, as there is significant interest in understanding the impact of solar minima on recent and future climates².

High resolution, multiproxy records of this event are limited in the UK, however, annually laminated sediments from Diss Mere, Norfolk, UK, provide an excellent opportunity to improve our understanding of the environmental impacts of this climatic oscillation on ecosystems of the region. Here we consider multiple proxies including diatoms, chironomids, pollen, ẟ¹⁸O and ẟ¹³C isotopes, integrated through a highly constrained age model based on varve counts, radiocarbon dating, tephrochronology and Bayesian modelling³. Our analyses highlight distinct responses linked to the associated cooling of the 2.8 ka BP oscillation. These include an opening of the landscape around the lake, documented in our pollen record, accompanied by diatom community changes, linked to alterations in temperature, nutrients, turbidity and water clarity. These are potentially a result of increased landscape instability changing the nutrients entering the lake water and its clarity, while increased wind shear due to a more open environment, is linked to the changes in turbulence. Chironomid inferred temperatures also indicate cooling during this period. We compare the Diss palaeorecord with another annually-resolved lake record for this event, Meerfelder Maar, Germany, and with peat bog sites in Ireland, where the event is also associated with tephra layers, to outline the similarities and differences in the regional response to this solar induced event. These results are particularly significant for studies of environmental/ecological impact¹ of grand solar minima on future climates in a warming world, through the potential for palaeodata climate model comparisons².

References:

1.      Martin-Puertas, C. et al. (2012). Nat. Geo. 5, 397-401.

2.      Ineson, E. et al. (2015) Nat. Commun. 6, 7535.

3.      Martin-Puertas, C. et al. (2020) European Geosciences Union.

How to cite: Harding, P., Langdon, C., Walsh, A., Biddulph, G. E., Blockley, S. P. E., Milner, A. M., Langdon, P., and Martin-Puertas, C.: The 2.8 BP Event: a high-resolution multiproxy perspective from Diss Mere, Norfolk, UK., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11311, https://doi.org/10.5194/egusphere-egu2020-11311, 2020.

The paleoclimatic changes that occurred in the Southern and Northern hemispheres during the last deglaciation are thought to have affected the continental tropical regions. However, the respective impact of North and Southern climatic changes in the tropics are still poorly understood. In the High Tropical Andes, the Antarctic Cold Reversal (ACR, 14.3-12.9 ka BP) was reported to be more represented than the Younger Dryas (12.9-11.7 ka BP) among morainic records. However, in the Altiplano basin (Bolivia), two cold periods of the North Hemisphere (Heinrich Stadial 1a (16.5-14.5 ka) and Younger Dryas) are synchronous with (i) major advances or stillstands of paleo-glaciers and with (ii) the highstands of the giant palaeo-lakes Tauca and Coipasa. Therefore, additional results are needed to disentangle between potential North and South Hemisphere climatic influence on the glacial dynamics in the region.

We present new Cosmic Ray Exposure (CRE) ages from glacial landforms of the Bolivian Andes that extend pre-existing datasets for four different sites spreading from 16 to 21°S. We reconstruct the Equilibrium Line Altitudes (ELA) associated with each moraine with the AAR method and use them in an inverse algorithm that combines both the palaeo-glaciers and palaeo-lake budgets to derive temperature and precipitation reconstructions. Our temperature reconstruction (ΔT vs. Present) shows a consistent trend through the four glacial sites with a progressive warming from ΔT= -5°C (17 ka BP) to –2.5°C (15-14.5 ka BP, at the end of the Tauca highstand). This is followed by a return to colder conditions, around -4°C, during the ACR (15.5-12.9 ka BP). The Coipasa highstand is coeval with another warming trend followed by ΔT stabilization at the onset of the Holocene (circa 10 ka BP), around -3°C. Precipitation is mainly characterized by increases during the lake highstands, modulated by the distance from the glacial sites to the center of the paleolakes that are moisture sources (recycling processes).

These new results highlight the decorrelation of the glacier dynamics to the temperature signal in regions that are characterized by high precipitation variability. They also provide a theoretical frame to explain how both regional and global forcings can imprint the paleo-glacial records. Our results strongly suggest that during the last deglaciation (20 – 10 ka BP), in the Tropical Andes, atmospheric temperatures follow the Antarctic variability, while precipitation is driven by the changes occurring in the Northern Hemisphere.

How to cite: Martin, L., Blard, P.-H., Lavé, J., Lupcker, M., Charreau, J., Jomelli, V., and Bourles, D.: Antarctic-like temperature variations in the Tropical Andes recorded by glaciers during the last deglaciation (20 – 10 ka BP), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9550, https://doi.org/10.5194/egusphere-egu2020-9550, 2020.

The high sensitivity of climate variability to the mean position of the intertropical convergence zone at different time scales is well known. However, due to a lack of absolutely dated high-resolution proxy records, the long-term changes in the tropical Atlantic oceanic and atmospheric circulation system prior to the late Holocene are still not well constrained. Paleo climate reconstructions and model studies suggest a very complex response of the northern hemispheric tropical rain belts in the western tropical Atlantic depending on the nature of the forcing, surface type and surrounding continent-ocean configuration.

Here we present a high resolution multi-proxy speleothem record from Cueva Larga (Puerto Rico) covering the last Glacial between 46 and 15 ka BP. Precise ²³⁰Th/U-dating reveals growth rates between 50 up to more than 1000 µm/year which allow for the investigation of multi-decadal to millennial scale variability in the stable isotope (δ¹⁸O and δ¹³C) and elemental records.

The analysed proxies document a pronounced response of regional precipitation to abrupt centennial to millennial scale climatic excursions across the last Glacial, such as Heinrich Stadials and Dansgaard/Oeschger oscillations. Here, we observe a strong agreement between our paleo-precipitation reconstruction and climate proxy records which are indicative of the strength of the Atlantic meridional overturning circulation and northern hemispheric temperature changes. The coherence of speleothem δ¹⁸O values with sedimentary ²³¹Pa/²³⁰Th also on sub-millennial timescales supports a persistent link of regional precipitation variability to ocean circulation variability. Spectral analysis further suggests that multi-decadal to centennial variability persisted in the western tropical Atlantic hydro-climate not only during stadial and interstadial conditions, but also during the last Glacial maximum, supporting the hypothesis that the Atlantic low-latitude regions respond to internal modes of climate variability on these time scales regardless of the global climate state.

The compilation of our dataset from Puerto Rico with other paleo-precipitation records allows for the reconstruction of past changes in position, strength and extent of the intertropical convergence zone in the western tropical Atlantic and reveal the existence of spatio-temporal gradients in response to millennial to orbital climate change.

How to cite: Warken, S., Vieten, R., Winter, A., Spötl, C., Miller, T., Frank, N., Jochum, K. P., Mielke, A., Schandl, J., Schröder-Ritzrau, A., Mangini, A., and Scholz, D.: Last Glacial multi-decadal to millennial-scale precipitation variability inferred from Puerto Rican speleothems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19591, https://doi.org/10.5194/egusphere-egu2020-19591, 2020.

High-latitude climate variations during Greenland Interstadials (GI) are expected to transfer globally in a complex way through interactions of fast atmospheric as well as slower cryospheric and oceanic processes. Prerequisite for an investigation of the evolution of GI is a climate independent synchronization of the considered paleoenvironmental archives. Measuring and aligning globally common production rate variations of the cosmogenic radionuclide ¹⁰Be in different archives provides a tool for such synchronizations and the investigation of environmental gradients in space and time, with minimized uncertainties in the relative timing.

A ¹⁰Be time-series at < 40-year resolution was measured along with new proxy records down to sub-mm step size from Black Sea sediment core M72/5-22-GC8 around GI-10 (~41 ka BP). We synchronized our ¹⁰Be time-series to that from Central Greenland ice cores based on the globally common production rate variations using the globally optimal fit.

Comparing the synchronized environmental proxy records points to a bipartite response of the Black Sea sediment record at the onset of GI-10. First, synchronous with the abrupt temperature increase in Greenland, costal sea ice decreases on the Black Sea, reflected by reduced sedimentary ice rafted debris contents. Second and with a lag of ~190 years, abrupt increases in the K/Ti proxy point to enhanced regional precipitation causing higher riverine sediment supply into the basin.

This bipartite structure might be connected to both differential thresholds of proxy responses in Black Sea sediments to locally abrupt environmental forcing and/or a bipartite climate transition in the region in response to GI-10. The latter could possibly be explained by an initial fast atmospheric-transmitted warming in the Black Sea region synchronous to the onset of GI-10, followed by a shift from predominantly continental to Mediterranean weather systems ~190 years later, after regional oceanic adjustments. However, further investigations during more GIs are necessary to test the robustness of these results.

How to cite: Czymzik, M., Nowaczyk, N., Dellwig, O., Wegwerth, A., Muscheler, R., Christl, M., and Arz, H.: Bipartite response in the Black Sea sediment record to Greenland-Interstadial 10, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8561, https://doi.org/10.5194/egusphere-egu2020-8561, 2020.

To better understand the response of Central European vegetation to rapid climate change during the late Quaternary, we have revisited the Füramoos peat bog in southwestern Germany. Located between two moraine ridges of Rissian age and comprising a near-complete sedimentary sequence from late Marine Isotope Stage (MIS) 6 to 1, this peat bog represents the longest continuous pollen record from the last glacial-interglacial cycle north of the Alps. The Füramoos site has been in the focus of several palynological studies in the past, showing that it presents an excellent archive to study the impact of Dansgaard-Oeschger (D-O) events on the Central European ecosystems (e.g., Müller et al., 2003). However, these previous studies were only of limited temporal resolution, which has yet precluded detailed insight into the ecosystem response to short-term climate change. We present a new, highly resolved pollen record (temporal resolution: 80–200 yrs) and XRF core scanning data from Füramoos spanning the past ~130 ka based on two new drill cores that consist of peat and lake sediments (Kern et al., 2019).

Our results show that closed temperate forests thrived at Füramoos during full interglacials characterized by Alnus, Corylus, Quercus, and Ulmus. The major difference between the past two interglacials is that Fagus dominates during MIS 1 whereas it is mostly absent during MIS 5e. During MIS 5, the vegetation evolved from closed temperate (MIS 5e) to boreal forests (dominated by Betula, Picea, and Pinus; MIS 5d–5a). The youngest part of the last interglacial (MIS 5d–5a) is marked by six distinct forests contractions (decreases in arboreal pollen by ~30–50%) before the establishment of a steppe vegetation that prevailed throughout the Last Glacial (MIS 2–4). In addition, seven transient increases in tree-pollen percentages document the expansion of boreal forests during MIS 2–4; they are associated with synchronous increases of Si, Ti, K and Fe contents as evidenced in XRF data.

We attribute the forest contractions during MIS 5d–5a to the cooling events C19–C24 known from marine records in the North Atlantic and terrestrial records from southern Europe. Moreover, the forest expansions during MIS 2–4 are associated with warm and moist conditions occurring during D-O events 7–12, and 14. In contrast, D-O events 13 and 15–19 don’t leave an imprint on the vegetation although their presence is clearly documented in the XRF data. Our findings emphasize that the sediments from Füramoos are exceptionally well suited to reconstruct ecosystem dynamics in Central Europe yielding unprecedented insight into the vegetation response to short-term climatic forcing north of the Alps during the past 130 kyrs.

Müller, U.C., Pross, J., Bibus, E., 2003. Vegetation response to rapid climate change in Central Europe during the past 140,000 yr based on evidence from the Füramoos pollen record. Quaternary Research 59, 235–245.

Kern, O.A., Koutsodendris, A., Mächtle, B., et al., 2019. X-ray fluorescence core scanning yields reliable semiquantitative data on the elemental composition of peat and organic-rich lake sediments. Science of the Total Environment 697, 134110.

How to cite: Kern, O., Allstädt, F., Koutsodendris, A., Mächtle, B., Schukraft, G., Heiri, O., and Pross, J.: Central European vegetation and climate dynamics during the past 130 ka at Füramoos, SW Germany, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19955, https://doi.org/10.5194/egusphere-egu2020-19955, 2020.

Chronologies for marine sediments are usually constructed by tuning marine proxies for global ice volume (δ18O) to the well understood variations in the Earth's orbit, by the identification of event horizons (e.g. tephra or biostratigraphic markers) and/or by radiocarbon dating. However, these techniques are not universally applicable. Optically stimulated luminescence dating (OSL) is potentially widely applicable to marine cores and may offer significant advantages over more conventional chronometric techniques. However, methodological considerations regarding the application of OSL techniques have yet to be systematically explored. Using material from Ocean Drilling Program (ODP) cores 658B and 659A, we assess the applicability of OSL dating to deep ocean sediments. For these cores, severe uranium-series disequilibrium is found, but the cause and character of this disequilibrium is spatially and temporally variable. Uranium-series disequilibrium causes the environmental dose rate to vary over time, and an iterative dose rate calculation is required to generate accurate ages. For the last glacial-interglacial cycle, these calculations yield OSL ages which are in good agreement with independent age estimates, suggesting that the application of luminescence dating techniques to deep-sea sediments merits further investigation.

How to cite: Armitage, S., Sahy, D., Tindall, J., and Pinder, R.: Direct dating of marine sediments using optically stimulated luminescence techniques: Insights from ODP cores 658B and 659A., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7620, https://doi.org/10.5194/egusphere-egu2020-7620, 2020.

The sedimentation rate in the northeastern South China Sea (SCS) is high and it therefore offers an opportunity for a high-resolution paleoceanographic study. This study is based on high-resolution AMS ¹⁴C dating on forams and oxygen isotope data of two planktonic foraminifera species (Globigerinoides ruber and Neogloboquadrina dutertrei) from the sediment core, MD18-3568, collected from the northeastern SCS, to reconstruct upper-ocean stratification since 35 ka.

The marine sediment core MD18-3568 is located on the accretionary wedge off SW Taiwan at a water depth of 1,315 m, the whole core is dominated by hemipelagic sediments and is of 20.7 m in length. Samples for AMS ¹⁴C dating were selected at roughly 2 ka interval with a total of 16 samples. The ages show a continuously younging-upward trend with bottom of this core around 35,000 years BP. Samples for high-resolution oxygen isotope measurements were selected at a nominal 500-year age interval. The difference in δ¹⁸O between G. ruber (mixed layer dwelling species) and N. dutertrei (thermocline dwelling species) is used to reconstruct the upper ocean stratification with large difference indicating significant ocean stratification and vice versa. The results show moderate upper ocean stratification during 35-24 ka, and it became less stratified during the Last Glacial Maximum (LGM, 23-19 ka). During the deglacial stage, the stratification gradually became stronger until the early Holocene (12-9 ka), and it has kept strong upper-ocean stratification since 9 ka. Literature has documented less rainfall intensity during the LGM and heavy rainfall during the Holocene in southern Taiwan. We interpret the upper-ocean stratification in the NE South China Sea near Taiwan is linked to the amount of freshwater inputs from Taiwan. Less Taiwan freshwater input during the LGM led to a weak stratified upper ocean and a large amount of freshwater input from Taiwan led to a strong upper-ocean stratification during the Holocene.

How to cite: Wang, T.-C., Lin, A. T.-S., Mii, H.-S., Horng, C.-S., and Colin, C.: Upper-ocean stratification of the NE South China Sea during the last 35 ka: Implications from oxygen isotope records from planktonic foraminifera, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3222, https://doi.org/10.5194/egusphere-egu2020-3222, 2020.

The nature and expression of climate change in the Eastern Mediterranean, the Levant and further into Arabia is of considerable interest across a range of communities. This is in part due to the need to understand the potential for future climate forcing on environments given the complex range of climatic forcing factors that play out in the region. These include the role of prevailing winds across the Mediterranean, Northerly winds pushing down into the region during cold glacial conditions, and the influence of the Afro-Arabian Monsoon. The last glacial to interglacial period is a critical window to examine such processes, as a range of climatic signals are recorded, many of which have been proposed as correlatives of events seen in the North Atlantic. Dating issues are as ever an issue when trying to precisely compare different climate archives. To address such, the INTIMATE event stratigraphy has been developed for the North Atlantic region, with recent extensions into parts of the Mediterranean. This couples the stratigraphic framework of the Greenland Ice core records as a regional stratotype, with  a number of tephra horizons in the North Atlantic and Europe, aiding the process of correlation. The last INTIMATE event stratigraphy coupled the extended GICC05 timescale for Greenland back to 128 b2k (Blockley et al., 2014). This paper reports on attempts to test the potential for tephrochronology to be extended into the Levant and potentially Arabia, through the identification of tephra layers in sediment focussing archives, such as archaeological cave sequences. We have examined tephra presence in archaeological sites, principally in Israel, that record sediment deposition from ~30ka BP through to >100ka BP. Analyses of these records show that tephra is present in almost all of the studied sites (e.g., Kebara, Tabun, Amud, Shovakh). Moreover, tephra in these sequences can be chemically correlated to known volcanic systems, demonstrating the potential going forward to analyse long lake and marine records around the region for cryptotephra. At the same time clear challenges are emerging. Firstly, there is a range of chemistry in many of the layers and careful analyses is needed to pick apart the geochemical signal and to identify reworking, as opposed to chemically heterogeneous ash layers from a single volcano. This process is complicated by the relatively limited range of published geochemical data from some volcanic centres. This presentation will outline the current state of knowledge of key volcanic centres, particularly in the Aegean and Turkey, alongside the new Levantine data, to consider the steps needed to establish a secure extension of the INTIMATE approach into this region.

Blockley, S., et al., 2014. Quaternary Science Reviews. 106, 88-100. doi:10.1016/j.quascirev.2014.11.002.

How to cite: Blockley, S., White, D., Timms, R., Lincoln, P., Armitage, S., and Stringer, C.: Challenges and opportunities extending the INTIMATE tephra event stratigraphy into the Levant and Arabia., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9051, https://doi.org/10.5194/egusphere-egu2020-9051, 2020.

The last glacial was an interval characterized by a sequence of abrupt millennial-scale events well documented mainly from the Greenland and Antarctica ice-cores. Although the triggers are not fully understood, most of the works agree that they occurred in consonance with oscillations in the strength of the Atlantic Meridional Overturning Circulation (AMOC). Paleoceanographic reconstructions have shown that cold millennial-scale stadials were accompanied by high temperatures in the subsurface to intermediate waters of the Atlantic Ocean that may have acted in both the basal melting of ice-sheets and in the rapid atmospheric warming during the onset of warm interstadials. Assuming that recent transient models indicated an accentuated response of the subsurface western South Atlantic to the millennial-scale disturbances, here we present a paleoceanographic reconstruction in this area based on the deep-dwelling planktic foraminifer Globorotalia inflata. Our high-resolution oxygen isotope (d¹⁸O) presents a sequence of millennial-scale variability that strongly resembles the structure of the Greenland Dansgaard-Oeschger cycles, mainly during Marine Isotope Stage (MIS) 5. On the other hand, during MIS 3, this millennial-scale feature is absent or weakly represented. Cross-spectral analyzes indicate a meaningful north-to-south forcing over the western South Atlantic subsurface during early-glacial. Mg/Ca-derived temperature and ice-volume free seawater d¹⁸O (d¹⁸O_IVF-SW) executed for the MIS 5 interval demonstrated that the subsurface western South Atlantic was warmer and saltier (colder and fresher) during early glacial stadial (interstadials). We hypothesized that a wide reorganization of the northward heat transport throughout the last glacial occurred, in which regions so far south as 24 ºS worked as prominent heat reservoirs in periods of weakened AMOC during MIS 5 but not necessarily during MIS 3. Our data suggest that future impacts over the AMOC along the Brazilian margin will likely be recognized in the subsurface layers of the western South Atlantic.

How to cite: Santos, T., Ballalai, J., Franco, D., Oliveira, R., Lessa, D., Venancio, I., Chiessi, C., Kuhnert, H., Johnstone, H., and Albuquerque, A. L.: Modes of response of the subsurface western South Atlantic to the last glacial Dansgaard-Oeschger cycles , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10124, https://doi.org/10.5194/egusphere-egu2020-10124, 2020.

Pollen analysis is one of the methods that allow revealing ecological and climatic changes in the
past based on vegetation reconstruction. Spitsbergen (Svalbard) archipelago, as well as other
regions of the Arctic, is difficult for creation of regional models of vegetation and climate
development during the Holocene. This is primarily due to the limited distribution, low thickness
and relative young ages (usually this is the late Holocene) of organogenic deposits, which are
most suitable for palynological studies.
Nordenskiöld Land is located in the central part of the West Spitsbergen Island and different the
most favorable climatic conditions. The largest number of sites suitable for paleobotanical
researches is located here. The Coles valley has length about 12 km, well-developed profile and
situated on the north shore of Nordenskiöld Land. The field campaign with studying of
floodplain peat sediments from Coles River valley was carried out in August 2018. Two sites
(K18-15, K18-16) were studied on the remains of first terrace. Excavated deposits are
represented by leafy peat of varying degrees of decomposition with silt lenses. The laboratory
studies of sediments included radiocarbon dating, pollen and non-pollen palynomorph analyses.
They were carried out in Laboratory of St-Petersburg State University and Russian chemical-
analytical Lab on the Spitsbergen archipelago.
The pollen analysis of two sections from Coles River valley allowed us to reconstruct
paleovegetation changes. Samples from K18-15 site contain more mineral components and more
pollen and spores than samples from K18-16 site. This is probably due to the inflow of pollen
with water. The main components of spore-pollen spectra are Poaceae, Cyperaceae, Salix and
Betula sect. Nanae. The relationship between these taxa shows a different degree of moisture of
the study area under the dominance of the grass - sedge tundra. Thus, a significant influence on
the formation of spores and pollen spectra in the studied deposits is played by the dynamics of
the sedimentation.
Results of radiocarbon dating showed that studied deposits formed during mid and late
Holocene.
A generalization of all available palynological data on the Nordenskjöld land made it possible to
construct a scheme of dwarf birch (Betula sect. Nanae) distribution during the Middle and Late
Holocene. A comparison of received data with our previous data and published data from
Nordenskiöld Land shows the asynchronous of appear and distribution of shrubs on these area
from ~5000 to ~2500 yrs ago.