The land ice contribution to global mean sea level has not yet been predicted for the latest generation of socio-economic scenarios, nor with coordinated assessment of uncertainties from the various computer models involved (climate, Greenland and Antarctic ice sheets, and global glaciers). Two recent projects generated a large suite of projections but used previous generation scenarios and climate models and could not fully explore uncertainties. Here we estimate probability distributions for their projections, using statistical emulation, and find uncertainty does not diminish if greenhouse gas concentrations are reduced: the sea level contribution of land ice is 28 [5, 57] cm from 2015 to 2100 under no mitigation (median and 90% range), and 16 [-5, 46] cm under very stringent mitigation. Greenland is projected to contribute around 2.5 cm/ºC of global warming, and Alaskan and Arctic glaciers a total of around 2 cm/ºC, but Antarctic uncertainties are too large to determine temperature-dependence. Knowing future global mean temperature exactly for a given socio-economic scenario would reduce the uncertainty for glaciers by up to two thirds (6 cm) but have little effect for ice sheets. Quantifying how ice sheet margins respond to ocean warming would reduce uncertainty by up to one third (Antarctica 15 cm; Greenland 7 cm). The remaining uncertainty for a given scenario is dominated by the climate and glaciological models themselves. Improved modelling and observations of polar regions, rather than global warming and glaciers, would therefore have the greatest effect in reducing uncertainty in future sea level rise.

How to cite: Edwards, T. and the ISMIP6, GlacierMIP and friends: Quantifying uncertainties in the land ice contribution to sea level from ISMIP6 and GlacierMIP, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11241, https://doi.org/10.5194/egusphere-egu2020-11241, 2020.

We have compared the surface mass (SMB) and energy balance of the Earth System model (ESM) CESM (Community Earth System Model) with those of the regional climate model (RCM) MAR (Modèle Atmosphérique Régional) forced by CESM over the present era (1981 — 2010) and the future (2011 — 2100 with SSP585 scenario).

Until now, global climate models (GCM) and ESMs forcing RCMs such as MAR didn’t include a module able to simulate snow and energy balance at the surface of a snow pack like the SISVAT module of MAR and were therefore not able to simulate the SMB of an ice sheet. Evaluating the added value of an RCM compared to a GCM could only be done by comparing atmospheric outputs (temperature, wind, precipitation …) in both models. CESM is the first ESM including a land model capable of simulating the surface of an ice sheet and thus to directly compare the SMB of an RCM and an ESM the first time.

Our results show that, if the SMB and is components are very similar in CESM and MAR over the present era, they quickly start to diverge in our future projection, the SMB of MAR decreasing more than that of CESM. This difference in SMB evolution is almost exclusively explained by a much larger increase of the melter runoff in MAR compared to CESM whereas the temporal evolution of snowfall, rainfall and sublimation is comparable in both runs.

How to cite: Lang, C., Amory, C., Delhasse, A., Hofer, S., Kittel, C., van Kampenhout, L., Lipscomb, W., and Fettweis, X.: Comparison of the surface mass and energy balance of CESM and MAR forced by CESM over Greenland: present and future, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18037, https://doi.org/10.5194/egusphere-egu2020-18037, 2020.

The Greenland ice sheet has been one of largest sources of sea-level rise since the early 2000s. The total mass balance of the ice sheet is typically determined using one of the following methods: estimates of ice volume change from satellite altimetry, measurements of changes in gravity, and by considering the difference between solid ice discharge and surface mass balance (often referred to as the input–output method). In spite of an overall agreement between the different methods, uncertainties remain regarding the relative contribution from individual processes, and to date the basal melt has never been explicitly included in total mass balance estimates. Here, we present the first estimate of the contribution from basal melting to the total mass balance. We partition the basal melt into three terms; melt caused by frictional heat, geothermal heat and viscous heat dissipation, respectively. Combined, the three terms contribute approximately 25 Gt per year of basal melt to the total mass loss equivalent to 5% of the average solid ice discharge (average value of 1986-2018 discharge). This is equivalent to the ice discharge from the entire northeastern sector. We find that basal melting also accounts for between 5% and 30% of observed thinning in most major glacier outlets. Over our observation period (winter 2017/18), close to 2/3 of the basal melt is due to frictional heating from fast moving ice. This term is expected to increase in the future, as ice streams are likely to expand and speed up in response to rising temperatures.

How to cite: Karlsson, N. B., Solgaard, A. M., Mankoff, K. D., Box, J. E., Citterio, M., Colgan, W. T., Kjeldsen, K. K., Korsgaard, N. J., Vandecrux, B., Benn, D., Hewitt, I., and Fausto, R. S.: Basal Melt of the Greenland Ice Sheet: The Invisible Mass Budget Term, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6943, https://doi.org/10.5194/egusphere-egu2020-6943, 2020.

With recent developments in the modelling of Antarctica and its interactions with the ocean several coupled model frameworks now exist.  This talk will focus on presenting the Framework for Ice Sheet - Ocean Coupling (FISOC), developed to provide a flexible platform for performing coupled ice sheet - ocean modelling experiments. We present progress and preliminary results using FISOC to couple the Regional Ocean Modelling System (ROMS) with Elmer/Ice, a full-Stokes ice sheet model. Idealised experiments have been used that also contribute to the WCRP Marine Ice Sheet-Ocean Model Intercomparison Project (MISOMIP).  A recent focus is on testing emergent behaviour of the coupled system and the model numerics. The talk will outline future technological applications and developments conducted as part of a broader international consortium effort. These efforts include coupling to sub-glacial hydrology, sea ice and atmospheres to form a complete system-downscaling technology from which to examine the influence of future climate on ice sheet evolution and hence sea level and global climate impacts. Developments to apply the technology to the Greenland Ice Sheet are presently underway.

How to cite: Galton-Fenzi, B., Gladstone, R., Zhao, C., Gwyther, D., Moore, J., and Zwinger, T.: Progress towards coupling ice sheet and ocean models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11738, https://doi.org/10.5194/egusphere-egu2020-11738, 2020.

The Greenland Ice Sheet (GrIS) mass loss has been accelerating at a rate of about 20 +/- 10 Gt/yr² since the end of the 1990's, with around 60% of this mass loss directly attributed to enhanced surface meltwater runoff. However, in the climate and glaciology communities, different approaches exist on how to model the different surface mass balance (SMB) components using: (1) complex physically-based climate models which are computationally expensive; (2) intermediate complexity energy balance models; (3) simple and fast positive degree day models which base their inferences on statistical principles and are computationally highly efficient. Additionally, many of these models compute the SMB components based on different spatial and temporal resolutions, with different forcing fields as well as different ice sheet topographies and extents, making inter-comparison difficult. In the GrIS SMB model intercomparison project (GrSMBMIP) we address these issues by forcing each model with the same data (i.e., the ERA-Interim reanalysis) except for two global models for which this forcing is limited to the oceanic conditions, and at the same time by interpolating all modelled results onto a common ice sheet mask at 1 km horizontal resolution for the common period 1980-2012. The SMB outputs from 13 models are then compared over the GrIS to (1) SMB estimates using a combination of gravimetric remote sensing data from GRACE and measured ice discharge, (2) ice cores, snow pits, in-situ SMB observations, and (3) remotely sensed bare ice extent from MODerate-resolution Imaging Spectroradiometer (MODIS). Our results reveal that the mean GrIS SMB of all 13 models has been positive between 1980 and 2012 with an average of 340 +/- 112 Gt/yr, but has decreased at an average rate of -7.3 Gt/yr² (with a significance of 96%), mainly driven by an increase of 8.0 Gt/yr² (with a significance of 98%) in meltwater runoff. Spatially, the largest spread among models can be found around the margins of the ice sheet, highlighting the need for accurate representation of the GrIS ablation zone extent and processes driving the surface melt. In addition, a higher density of in-situ SMB observations is required, especially in the south-east accumulation zone, where the model spread can reach 2 mWE/yr due to large discrepancies in modelled snowfall accumulation. Overall, polar regional climate models (RCMs) perform the best compared to observations, in particular for simulating precipitation patterns. However, other simpler and faster models have biases of same order than RCMs with observations and remain then useful tools for long-term simulations. It is also interesting to note that the ensemble mean of the 13 models produces the best estimate of the present day SMB relative to observations, suggesting that biases are not systematic among models. Finally, results from MAR forced by ERA5 will be added in this intercomparison to evaluate the added value of using this new reanalysis as forcing vs the former ERA-Interim reanalysis (used in SMBMIP). 

How to cite: Fettweis, X. and the The GrSMBMIP team: GrSMBMIP: Intercomparison of the modelled 1980-2012 surface mass balance over the Greenland Ice sheet, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10792, https://doi.org/10.5194/egusphere-egu2020-10792, 2020.

Projections of sea level contribution from the Greenland and Antarctic ice sheets rely on atmospheric and oceanic drivers obtained from climate models.  The Earth System Models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) generally project greater future warming compared to the previous CMIP5 effort. Here we use four CMIP6 models and a selection of CMIP5 models under two future climate scenarios to force multiple ice sheet models as part of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). We find that the projected sea level contribution at 2100 from the multi ice sheet models under the CMIP6 scenarios falls within the CMIP5 range for the Antarctic ice sheet but is significantly increased for the Greenland ice sheet.  

How to cite: Payne, T., Nowicki, S., and Goelzer, H. and the ISMIP6 team: Contrasting contributions to future sea level under CMIP5 and CMIP6 scenarios from the Greenland and Antarctic ice sheets, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11667, https://doi.org/10.5194/egusphere-egu2020-11667, 2020.

Global warming has become a world concerned issue which draws increasingly attention of the scientific community. Sea-level rise is an important indicator of Global warming as it integrates many factors of climate change including ice sheet melting.  The accurate assessment of the Antarctic ice sheet mass balance is applied to deeply explore the impact of minor change in Antarctic ice sheet on sea level rise. Based on multi-source remote sensing product, we finely estimated the mass balance of the Antarctic ice sheet and discussed dynamics and climatological causes of the fluctuations from 2005 to 2015 by IOM (Input-Output-Method).

In our study, the calculation method of ice flux on the grounding line is improved. We also precisely evaluate the ice flux as an output component. The result shows that: (1) The Antarctic ice sheet was continuously losing mass during the period of 2005-2016. (2) The mass loss of the Antarctic ice sheet was dominated by West Antarctica when East Antarctica was in a positive mass balance, but some basins also occurred significant mass loss. The Antarctic peninsula fluctuated in a state of zero balance. (3) The change in the mass balance of the ice sheet was dominated by the surface mass balance as a whole, and was mainly affected by the interannual variation of climatological factors. From a small-scale perspective, ice shelf thinning and glacier calving causes the change of ice flux on the grounding line. That change leads to the severe mass loss in the region it happened. Therefore the mass loss in the year of the disintegration event happened increases.

How to cite: Lin, Y.: Antarctic Ice Sheet mass balance over the past decade from 2005 to 2016 , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12835, https://doi.org/10.5194/egusphere-egu2020-12835, 2020.

      The Greenland ice sheet (GrIS) surface melt-water runoff dominates recent ice mass loss under global warming. We present runoff simulations during 1950-2500 over GrIS under RCP (Representative Concentration Pathways) 4.5, RCP8.5 and their extensions scenarios using three modified degree-day models, forced with five CMIP5 (Coupled Model Intercomparison Project) Earth System Models (CanESM2, BNU-ESM, HadGEM2-ES, MIROC-ESM and MIROC-ESM-CHEM). The degree-day factors are tuned at two sites on Greenland to best match the results by surface energy and mass balance model SEMIC. The modeled SMB over Greenland by modified degree-day models agree well with SEMIC in 21th century, then is applied to do projections for the 2100-2500 period. We also consider equilibrium line altitude evolution, surface topography changes and runoff-elevation feedback in the post-2100 simulations. The ensemble mean projected GrIS runoff is equivalent to sea-level rise of 7 cm (RCP4.5) and 10 cm (RCP8.5) by the end of the 21st century relative to the period 1950-2005, and 25cm (RCP4.5) and 121cm (RCP8.5) by 2500. Runoff-elevation feedback increases extra runoff of 7% (RCP4.5, RCP8.5) by 2100 and 23% (RCP4.5) and 22% (RCP8.5) by 2500. Sensitivity experiments show that 150% and 200% snowfall in post-2100 period would lead to 10% and 20% runoff increase under RCP4.5, 5% and 10% for RCP8.5, respectively.

How to cite: Yue, C., Zhao, L., and Moore, J. C.: Greenland Ice Sheet surface runoff projections to AD2500 using degree-day model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7865, https://doi.org/10.5194/egusphere-egu2020-7865, 2020.

Interannual variations associated with El Niño-Southern Oscillation can alter the surface-pressure distribution and moisture transport over Antarctica, potentially affecting the contribution of the Antarctic ice sheet to sea level. Here, we combine satellite gravimetry with auxiliary atmospheric datasets to investigate interannual ice-mass changes during the extreme 2015-16 El Niño. Enhanced precipitation during this event contributed positively to the mass of the Antarctic Peninsula and West Antarctic ice sheets, with the mass gain on the peninsula being unprecedented within GRACE’s observational record. Over the coastal basins of East Antarctica, the precipitation-driven mass loss observed in recent years was arrested, with pronounced accumulation over Terre Adélie dominating this response. Little change was observed over Central Antarctica where, after a brief pause, enhanced mass-loss due to weakened precipitation continued. Overall, precipitation changes over this period were sufficient to temporarily offset Antarctica’s usual (approximately 0.4 mm yr^-1) contribution to global mean sea level rise.

How to cite: Bingham, R. and Bodart, J.: The Impact of the Extreme 2015-16 El Niño on the Mass Balance of the Antarctic Ice Sheet, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2900, https://doi.org/10.5194/egusphere-egu2020-2900, 2020.

The mass balance of the Greenland ice sheet (GrIS, units Gt per year) equals the surface mass balance (SMB) minus solid ice discharge across the grounding line. As the latter is definite positive, an important threshold for irreversible GrIS mass loss occurs when long-term average SMB becomes negative. For this to happen, runoff (mainly meltwater, some rain) must exceed mass accumulation (mainly snowfall minus sublimation). Even for a single year, this threshold has not been passed since at least 1958, the first year with reliable estimates of SMB components, although recent years with warm summers (e.g. 2012 and 2019) came close. Simply extrapolating the recent (1991-present) negative SMB trend into the future suggests that the SMB = 0 threshold could be reached before ~2040, but such predictions are extremely uncertain given the very large interannual SMB variability, the relative brevity of the time series and the uncertainty in future warming. In this study we use a cascade of models, extensively evaluated with in-situ and remotely sensed (GRACE) SMB observations, to better constrain the future regional warming threshold for the 5-year average GrIS SMB to become negative. To this end, a 1950-2100 climate change run with the global model CESM2 (app. 100 km resolution) was dynamically downscaled using the regional climate model RACMO2 (app. 11 km), which in turn was statistically downscaled to 1 km resolution. The result is a threshold regional Greenland warming of close to 4 degrees. We then use a range of CMIP5 and CMIP6 global climate models to translate the regional value into a global warming threshold for various warming scenarios, including its timing this century. We find substantial differences, ranging from stabilization before the threshold is reached in the RCP/SSP2.6 scenarios with a limited but still significant sea-level rise contribution (< 5 cm by 2100) to an imminent crossing of the warming threshold for the RCP/SSP8.5 scenarios with substantial and ever-growing contributions to sea level rise (> 10 cm by 2100). These results stress the need for strong mitigation to avoid irreversible GrIS mass loss. We finish by discussing the caveats and uncertainties of our approach.

How to cite: van den Broeke, M., Noël, B., van Kampenhout, L., and van de Berg, W.-J.: A regional atmospheric warming threshold for irreversible Greenland ice sheet mass loss, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19032, https://doi.org/10.5194/egusphere-egu2020-19032, 2020.

Future climate projections show a marked increase in Greenland Ice Sheet (GrIS) runoff
during the 21st century, a direct consequence of the Polar Amplification signal. Regional
climate models (RCMs) are a widely used tool to downscale ensembles of projections from
global climate models (GCMs) to assess the impact of global warming on GrIS melt and
sea level rise contribution. Initial results of the CMIP6 GCM model intercomparison
project have revealed a greater 21st century temperature rise than in CMIP5 models.
However, so far very little is known about the subsequent impacts on the future GrIS
surface melt and therefore sea level rise contribution. Here, we show that the total GrIS
melt during the 21st century almost doubles when using CMIP6 forcing compared to the
previous CMIP5 model ensemble, despite an equal global radiative forcing of +8.5 W/m2
in 2100 in both RCP8.5 and SSP58.5 scenarios. The total GrIS sea level rise contribution
from surface melt in our high-resolution (15 km) projections is 17.8 cm in SSP58.5, 7.9 cm
more than in our RCP8.5 simulations, despite the same radiative forcing. We identify a
+1.7°C greater Arctic amplification in the CMIP6 ensemble as the main driver behind the
presented doubling of future GrIS sea level rise contribution

How to cite: Hofer, S., Lang, C., Amory, C., Kittel, C., Delhasse, A., Tedstone, A., Alexander, P., Smith, R., and Fettweis, X.: Doubling of future Greenland Ice Sheet surface melt revealed by the new CMIP6 high-emission scenario, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19502, https://doi.org/10.5194/egusphere-egu2020-19502, 2020.

How to cite: Seroussi, H., Goelzer, H., and Morlighem, M. and the ISMIP6 Team: ISMIP6 Antarctica: a multi-model ensemble of the Antarctic ice sheet evolution over the 21st century, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6309, https://doi.org/10.5194/egusphere-egu2020-6309, 2020.

Totten glacier is draining 68% of the Aurora basin, East Antarctica, - an equivalent to 3.5m global sea level rise. Further, Totten’s thickness and velocity have been fluctuating during the last decades showing periodic speed-ups and thinning.

We investigate the effect of different ocean forcing on Totten glacier using the state-of-the-art ice sheet model BISICLES and based on the high-resolution data sets BedMachine Antarctica and REMA (Morlighem et al., 2019; Howat et al., 2019). Our simulations (2015-2100) are following the ISMIP6 setup and are based on CMIP5 & CMIP6 AOGCM outputs under RCP8.5 and RCP2.6. The contribution to sea level at 2100 varies between plus and minus 6mm. For all scenarios, we see thinning at the sides of Totten glacier in the slower flowing areas, but only climate models with sub-shelf melt rates that are at least 8m/a above the reference melt rates (1995 – 2017) lead to thinning and acceleration across Totten's grounding line. In agreement with ISMIP6 results, non-local quadratic melt rates adjusted to present day conditions at Pine island glacier, West Antarctica, results in the highest sub-shelf melt rates for all AOGCMs (up to 80m/a locally).

The ISMIP6 ocean melt scheme is based on a feedback given the simulated ice draft change: the thermal forcing of the ocean model is taken from the ocean layer closest to the bottom of the ice shelf at the current simulation step. Simulations not including this feedback lead to higher mass loss than the standard ISMIP6 scenario including the feedback.

How to cite: Haubner, K., Sun, S., Zipf, L., and Pattyn, F.: Changes on Totten glacier dependent on oceanic forcing based on ISMIP6, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9208, https://doi.org/10.5194/egusphere-egu2020-9208, 2020.

Ice sheets constitute the largest and most uncertain potential source of future sea-level rise. The Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) brings together a consortium of international ice sheet and climate models to explore the contribution from the Greenland and Antarctic ice sheets to future sea-level rise.

We use the Parallel Ice Sheet Model (PISM, pism-docs.org) to carry out spinup and projection simulations for the Antarctic Ice Sheet. Our treatment of the ice-ocean boundary condition previously based on 3D ocean temperatures (initMIP-Antarctica) has been adopted to use the ISMIP6 parameterisation and 3D ocean forcing fields (temperature and salinity) according to the ISMIP6 protocol.

In this study, we analyse the impact of the choices made during the model initialisation procedure on the initial state. We present the AWI PISM results of the ISMIP6 projection simulations and investigate the ice sheet response for individual basins. In the analysis, we distinguish between the local and non-local ice shelf basal melt parameterisation.

How to cite: Kleiner, T., Schmiedel, J., and Humbert, A.: ISMIP6 Future Projections for Antarctica performed using the AWI PISM ice sheet model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16948, https://doi.org/10.5194/egusphere-egu2020-16948, 2020.

The distribution of relict moraines in the Transantarctic Mountains affords geologic constraint of past ice-marginal positions of the East Antarctic Ice Sheet (EAIS). We describe the directly dated glacial-geologic record from Roberts Massif, an ice-free area in the central Transantarctic Mountains, to provide a comprehensive record of ice sheet change at this site since the Miocene and to capture ice sheet response to warmer-than-present climate conditions. The record is constrained by cosmogenic ³He, ¹⁰Be, ²¹Ne, and ²⁶Al surface-exposure ages from > 160 dolerite and sandstone erratics on well-preserved moraines and drift units. Our data set indicates that a cold-based EAIS was present, and similar to its current configuration, for long periods over the last ~14.5 Myr, including the mid-Miocene, Late Pliocene, and early-to-mid Pleistocene, with moraine ages increasing with distance from and elevation above the modern ice margin. We also report extremely low erosion rates over the duration of our record, reflecting long-term polar desert conditions at Roberts Massif. The age-elevation distribution of moraines at Roberts Massif is consistent with a persistent EAIS extent during glacial maxima, accompanied by slow, isostatic uplift of the massif due to subglacial erosion. Although our data are not a direct measure of ice volume, the Roberts Massif glacial record indicates that the EAIS was present and of similar extent to today during periods when global temperature was believed to be warmer and/or atmospheric CO₂ concentrations were likely higher than today.

How to cite: Bromley, G., Balter, A., Balco, G., and Jackson, M.: A 14.5 million-year geologic record of East Antarctic Ice Sheet fluctuations in the central Transantarctic Mountains, constrained with multiple cosmogenic nuclides, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19999, https://doi.org/10.5194/egusphere-egu2020-19999, 2020.

The fast ice in the McMurdo Sound plays an important role in the coastal ecological systems and climate changes, but its seasonal and interannual variations are poorly understood. In this study, the fast ice phenology and extent variation are investigated using Sentinel-1 Synthetic Aperture Radar (SAR) images from 2017 to 2019, and the factors controlling the fast ice development are explored. The results showed that the fast ice edge presented obvious seasonal change. In 2017/2018 and 2018/2019 years it arrived at northernmost during May – July, and keeps north until the end of December or January, and then moves south, arriving at most south on February or March. However, there are some difference between these two years. The date the fast ice edge arrived at northernmost in 2018 was about two months later than in 2017, but the ending time at the northern edge was about one month earlier (31 Dec 2018 vs 30 Jan 2018). The time when it retreated to the southernmost in 2019 was about one month before that in 2017 or 2018. It seems the longer the edge stays in the northernmost, the later it retreats to the southernmost, and it may not completely disappear; the shorter the edge stays in the northernmost, the earlier it retreats to the southernmost, and it may completely disappear. The dominant factor controlling the beginning and end dates are air temperature. This statement still needs to be confirmed when more data will be processed and analyzed in near future.

How to cite: Dai, L.:  Seasonal variability of fast ice edge in the McMurdo Sound between 2017 and 2019 based on Sentinel-1 SAR, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2857, https://doi.org/10.5194/egusphere-egu2020-2857, 2020.

The surface mass balance (SMB) of the Antarctic ice sheet is often considered as a negative contributor to the sea level rise as present snowfall accumulation largely compensates for ablation through wind erosion, sublimation and runoff. The latter is even almost negligible since current Antarctic surface melting is limited to relatively scarce events over generally peripheral areas and refreezes almost entirely into the snowpack. However, melting can significantly affect the stability of ice shelves through hydrofracturing, potentially leading to their disintegration, acceleration of grounded ice and increased sea level rise. Although a large increase in snowfall is expected in a warmer climate, more numerous and stronger melting events could conversely lead to a larger risk of ice shelf collapse. In this study, we provide an estimation of the SMB of the Antarctic ice sheet for the end of the 21st century by forcing the state-of-the-art regional climate model MAR with three different global climate models. We chose the models (from both the Coupled Model Intercomparison Project Phase 5 and 6 - CMIP5 and CMIP6) providing the best metrics for representing the current Antarctic climate. While the increase in snowfall largely compensates snow ablation through runoff in CMIP5-forced projections, CMIP6-forced simulations reveal that runoff cannot be neglected in the future as it accounts for a maximum of 50% of snowfall and becomes the main ablation component over the ice sheet. Furthermore, we identify a tipping point (ie., a warming of 4°C) at which the Antarctic SMB starts to decrease as a result of enhanced runoff particularly over ice shelves. Our results highlight the importance of taking into account meltwater production and runoff and indicate that previous model studies neglecting these processes yield overestimated SMB estimates, ultimately leading to underestimated Antarctic contribution to sea level rise. Finally, melt rates over each ice shelf are higher than those that led to the collapse of the Larsen A and B ice shelves, suggesting a high probability of ice shelf collapses all over peripheral Antarctica by 2100.

How to cite: Kittel, C., Amory, C., Agosta, C., Jourdain, N., Hofer, S., Delhasse, A., and Fettweis, X.: Decreasing Antarctic surface mass balance due to runoff-dominated ablation by 2100, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17355, https://doi.org/10.5194/egusphere-egu2020-17355, 2020.

The Antarctic ice sheet is a deeply uncertain component of future sea level under anthropogenic climate change. To shed light on the ice sheets response to warmer climates in the past and its’ response to future warming, periods in Earth’s geological record can serve as instructive modelling targets. The mid-Pliocene warm period (3.3 – 3.0 Ma) is characterised by global mean surface temperatures ~2.7-4^oC above pre-industrial, atmospheric CO₂ concentrations of ~400ppm and eustatic sea level rise on the order of ~10-30m above modern. The mid-Pliocene sea level record is subject to large uncertainties. The upper end of this record implies a significant contribution from Antarctica and possible collapse of regions of the ice sheet, driven by marine ice sheet instabilities.

We present a suite of BISICLES ice sheet model simulations, forced with a subset of Pliocene Modelling Intercomparison Project (PlioMIP phase 1) coupled atmosphere-ocean climate models, that represent the Pliocene Antarctic ice sheet. This ensemble captures a range of possible ice sheet model responses to a warm Pliocene-like climate under different parameter choices, sampled in a Latin hypercube design. Modelled Antarctic sea level contribution is compared to reconstructions of Pliocene sea level, to explore the extent to which available data with large uncertainties can constrain the model parameter values.

Our aim with this work is to provide insights on Antarctic contribution to sea level in the warm mid-Pliocene. We seek to characterise the role of ice-ocean, ice-atmosphere and ice-bedrock parameter uncertainty in BISICLES on the ice sheet sea level contribution range, and whether cliff instability processes are necessary in reproduce high Pliocene sea levels in this ice sheet model.

How to cite: O'Neill, J., Edwards, T., Gregoire, L., Gandy, N., Dolan, A., Wernecke, A., Cornford, S., de Boer, B., Kelman, I., and van de Flierdt, T.: Modelling the Antarctic Ice Sheet in the warm Mid-Pliocene, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19348, https://doi.org/10.5194/egusphere-egu2020-19348, 2020.

A warming planet, and particularly the warming Pacific Ocean, has led to major changes in the Larsen-Weddell System. While somewhat less significant than those in the adjacent Amundsen-Bellingshausen Sea and its coastal ice, the changes are nonetheless dramatic indicators of a closely interconnected system, driven by increased westerly winds and their impact on surface melting and ice drift. The system very likely will see further major changes if warming continues through the 22^nd Century.

A warming trend in the central Pacific over the past ~80 years has induced air circulation changes over the Southern Ocean and Antarctic Peninsula. A rise in the mean speed of westerly-northwesterly winds across the northern Peninsula led to more frequent foehn events, which in turn increased surface melting on the eastern Peninsula ice shelves, and were responsible for reduced sea ice cover and more frequent shore leads on the eastern edges of the ice shelves. This likely led to greater sub-ice-shelf circulation, possibly including solar-warmed surface water (in summer) and modified Weddell Deep Water (mWDW). Around 1986, structural evidence in the form of more disrupted shear zones and increased rifting suggests that the Larsen A and B ice shelves began to thin and weaken. At this progressed, a combination of increased surface ponding and reduced backstress on the iceshelves led to a series of catastrophic break-ups due to hydrofracture, in 1995 (Larsen A shelf) and 2002 (Larsen B).  More recently, thinning detected by altimetry on the northern Larsen C may have contributed to new fracturing and calving of a large iceberg there in 2016 (iceberg A-68), setting the ice shelf front significantly farther to the west than has previously been observed (since 1898).

Looking forward, if the trend in increased westerly winds and Southern Annular Mode index continues, it is anticipated (modelled) that the large clockwise Weddell Gyre will increase in mean flow speed, and that warm deep water entrained from the Antarctic Circumpolar Current will more frequently mix with the mid- to deep ocean layers in the Weddell Gyre. One outcome of this is likely to be advection of warm deep water into the Ronne Ice Shelf cavity, dramatically increasing the heat available for sub-ice-shelf melting there and potentially changing ice sheet flux from the outlet glaciers significantly.

How to cite: Scambos, T.: Recent Changes in the Larsen-Weddell System , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-603, https://doi.org/10.5194/egusphere-egu2020-603, 2020.

The presence of polynyas has a large effect on air-sea fluxes and deep water production, therefore impacting climate-relevant properties such as heat and carbon exchange between the atmosphere and ocean interior. One of the key areas of deep water formation is in the Weddell Sea, where much attention has recently been placed in the reoccurance of the open ocean Maud Rise polynya. In this study, two methods are presented to track the number, area and spatial distribution of polynyas with a focus on the Weddell Sea. The analysis is applied to a set of 10 Coupled Model Intercomparison Project phase 6 (CMIP6) models and to satellite sea ice concentration data. The first approach is a sea ice threshold method applied to the CMIP6 sea ice data at the original model grid. Open water areas surrounded by sea ice are classified as polynyas. Without requiring any remapping or interpolation, this method preserves the area information of all grid cells and is well suited to compute the combined area of the polynyas in the Weddell Sea. The second approach makes use of an image analysis technique to outline areas with low sea ice concentration. This method is preferable for counting the absolute number of polynyas and obtaining statistical information about their position. Satellite sea ice concentration is used as a reference to compare the performance of the models representing polynya area and to indicate model biases in the location of polynyas. All analyzed CMIP6 models show coastal polynyas, while only about half of the models regularly form open water polynyas. The resolution (about one degree for most models) sets a limit for the number of the polynyas in the numerical models.

How to cite: Mohrmann, M., Heuzé, C., and Swart, S.: Polynya area and frequency in the Weddell Sea in CMIP6 climate models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19812, https://doi.org/10.5194/egusphere-egu2020-19812, 2020.

The western Weddell Sea along the northward branch of the Weddell Gyre is a region of major outflow of various water masses, thick sea ice, and biogeochemical matter, linking the Antarctic continent to the world oceans. It features a deep shelf and the second largest ice shelf (Larsen C) in the WS, and its perennial sea ice cover is among the thickest on earth. This region is undergoing dramatic changes due to the breakup of ice shelves along the Antarctic Peninsula, which results in oceanographic conditions unprecedented in the past 10,000 years. Since this region is difficult to access, comprehensive physical and biogeochemical information is still lacking. During the interdisciplinary Weddell Sea Ice (WedIce) expedition to the northwestern Weddell Sea on board the German icebreaker RV Polarstern in spring 2019, oceanographic and biogeochemical studies were conducted together with in-situ snow and ice sampling. Most stations visited contained second- and third-year ice. Additional airborne ice-thickness surveys revealed a mean ice thicknesses between 2.6 and 5.4 m, increasing from the Antarctic Sound towards the Larsen B region. Usually rotten ice was present below a solid, ~30 cm thick surface-ice layer, however, pronounced gap layers, typical for late summer ice in the marginal ice zone, were rare. The associated high algal biomass was only found north of the Antarctic Sound. Nevertheless, diatom-dominated standing stocks of integrated sea ice algae biomass were among the highest, previously described in Antarctic waters. In contrast, despite overall high macro-nutrient concentrations in the water, the biomass of the flagellate dominated phytoplankton was negligible for primary production in the entire region. Overall, it seems that despite changing light conditions for the phytoplankton due to the loss of ice shelves, the sea ice-derived carbon represents an important control variable for higher trophic levels in the western Weddell Sea.

How to cite: Peeken, I., Arndt, S., Janout, M., Krumpen, T., and Haas, C.: The importance of sea ice biota for the ecosystem in the northwestern Weddell Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20152, https://doi.org/10.5194/egusphere-egu2020-20152, 2020.

Pine Island Glacier (PIG) has contributed more to sea level rise over the last four decades than any other glacier in Antarctica. Model projections indicate that this will continue in the future but at conflicting rates, depending partly on the model initialisation. Some models suggest that mass loss could dramatically increase over the next few decades, resulting in a rapidly growing contribution to sea level, and fast retreat of the grounding line. Other models indicate more moderate losses. Resolving this contrasting behaviour is important for sea level rise projections as PIG and the Amndsen Sea Sector have been used as calibration for plausibility, probabilistic and deterministic approaches. Here, we use high resolution satellite observations of elevation change since 2010 from CryoSat-2 swath data to show that thinning rates are now highest along the slow-flow margins of the glacier and that the present-day amplitude and pattern of elevation change is inconsistent with fast grounding line migration and the associated rapid increase in mass loss over the next few decades. Instead, our results support model simulations that imply only modest changes in grounding line location over that timescale. We demonstrate how the pattern of thinning is evolving in complex ways both in space and time and how rates in the fast-flowing central trunk have decreased by about a factor five since 2007. We also consider how the complex pattern of mass loss affects the interpretation of vertical land motion from GPS data and inferences made from these data for mantle viscosity and solid Earth response times.

How to cite: Bamber, J. and Dawson, G.: Complex, evolving patterns of mass loss from Antarctica’s largest glacier, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1567, https://doi.org/10.5194/egusphere-egu2020-1567, 2020.

Thwaites Glacier (TG), West Antarctica, is losing mass in response to oceanic forcing. Future evolution could lead to deglaciation of the marine basins of the West Antarctic ice sheet, depending on ongoing and future climate forcings, but also on basal topography/bathymetry, basal properties, and physical processes operating within the grounding zone. Hence, it is important to know the actual distribution of bed types of TG’s interior and grounding zone, and to incorporate them accurately in models to improve estimates of retreat rates and stability. Here we determine bed reflectivity and acoustic impedance via amplitude analysis of reflection seismic data. We report on the results from two lines – a longitudinal (L-Line) and a transverse (N-Line) – on a central flowline of TG 100 km inland from the grounding zone. There is considerable variability in bed forms and properties, both within this dataset and in-comparison with nearby work. Notably, we find the same hard (bedrock) stoss and soft (till) lee pattern observed elsewhere on TG in prior work. Physical understanding indicates the basal flow law describing motion over different regions of TG’s bed likely varies from nearly viscous over the hard bedrock regions to nearly plastic over soft till regions, providing a template for modeling.

How to cite: Clyne, E., Anandakrishnan, S., Muto, A., Alley, R., Voight, D., and Riverman, K.: Reflection Seismic Interpretation of Topography and Acoustic Impedance beneath Thwaites Glacier, West Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1304, https://doi.org/10.5194/egusphere-egu2020-1304, 2020.

Multibeam echo-sounders were deployed from Autonomous Underwater Vehicles (AUVs) flying close to the seafloor of the Weddell Sea shelf in order to investiagte the glacial landforms there with a view to understanding processes and patterns associated with deglaciation from the Last Glacial Maximum on the eastern side of the Antarctic Peninsula. A horizontal resolution of 0.5 m (using conventional mulitbeam systems), and in some cases 0.05 m (using interferometric multibeam equipment), allowed delicate seafloor landforms to be mapped in several areas of the shelf beyond the Larsen C and former Larsen A and B ice shelves. A number of glacial landform assemblages were observed, including suites of delicate ridges associated with grounding-zone wedges and the grounding of icebergs on the shelf. These landforms are probably related to the action of tides moving the ice up and down through a series of tidal cycles. At the highest spatial resolution, individual dropstones derived from rain-out during the melting of floating ice were imaged clearly. Imaging the seafloor at such high resolution allows both very detailed descriptions of submarine landform morphology and also the complexity of such landforms and landform assemblages to be better understood, aiding the interpretation of the glacial and related processes that led to their formation.

How to cite: Dowdeswell, J., Batchelor, C., Montelli, S., Ottesen, D., Dowdeswell, E., and Evans, J.: Submarine landforms on the Weddell Sea shelf imaged at high resolution using AUVs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1603, https://doi.org/10.5194/egusphere-egu2020-1603, 2020.

Icefin performed the first long range robotic exploration of the grounding zone of Thwaites Glacier from January 9-12 2020. Icefin was part of the MELT project of the International Thwaites Glacier Collaboration deployed to the grounding zone of Thwaites Glacier, West Antarctica over the period December 2019-February 2020.  MELT is an interdisciplinary project to explore rapid change across the grounding zone, and in particular basal melting.  The subglacial cavity ~2km north of the grounding zone was accessed via hot water drilling on January 7-8, 2020.  Icefin, a hybrid autonomous and remotely operated underwater vehicle designed for sub-ice and borehole operations, conducted over 15km of round-trip data collection under the ice along a section of the glacier from the grounding zone extending to a point 4 km oceanward.   The vehicle collected data with ten different science sensors including cameras, sonars, conductivity/temperature and dissolved oxygen.  Overall, the water column ranged from ~100m downstream that narrowed quickly to an average of 50m that spanned over 2km, to a long segment of ~30m thickness before quickly narrowing over 500m towards the grounding zone. The seafloor structures run roughly parallel to ice flow direction, consisting of furrows, ridges, and grooves in some cases mirrored by the ice structure. The Icefin dives revealed a diverse set of basal ice conditions, with complex geometry, including a range of terraced features, smooth ablated surfaces, crevassing, sediment rich layers of varying kinds, as well as interspersed clear, potentially accreted freshwater ice.  The ocean directly beneath the ice varies spatially, from moderately well-mixed near the grounding zone to highly stratified within and below concavities in the ice downstream.  Sediments along the sea floor range from fine grained downstream to course angular gravel near the grounding zone distributed between larger boulders.  We observed rocky material in the ice that ranged from fine grained layers compressed within the ice to small angular particles volumetrically distributed within ice, to gravel and cobbles, as well as trapped boulders up to meter scale. In addition to the oceanographic, glaciological and sea floor conditions, we also catalogued communities of organisms along the seafloor and ice-ocean interface. We will report the highlights and initial conclusions from Icefin’s in situ data collection, and offer perspectives on change at the grounding zone.

How to cite: Schmidt, B., Nicholls, K., Davis, P., Smith, J., Riverman, K., Holland, D., Dichek, D., Mullen, A., Lawrence, J., Washam, P., Basinski-ferris, A., Anker, P., Meister, M., Spears, A., Hurwitz, B., Quartini, E., Clyne, E., Thomas, C., Wake, J., and Vaughn, D. and the ITGC Team (MELT & Thwaites Glacier Grounding Zone Downstream teams): The Grounding Zone of Thwaites Glacier Explored by Icefin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20512, https://doi.org/10.5194/egusphere-egu2020-20512, 2020.

The distribution and concentration of sea ice presents a significant challenge to shipping and scientific expeditions in high-latitude regions. In addition to achieving safe navigation, information about likely sea ice conditions is needed for expedition planning, and the deployment and retrieval of scientific instruments and their data. In areas where time series of passive microwave data exist, broad-scale analysis of sea ice concentration can be readily achieved. However, the spatial resolution of these data does not permit detailed investigations of sea ice conditions, including near-shore lead development.

Here we present a new methodology for processing moderate resolution multispectral and thermal satellite imagery to summarise inter-annual differences in the probability of sea ice observation. By using multiple daily imagery sources (Terra and Aqua MODIS; Suomi-NPP VIIRS), and averaging resultant concentration maps over longer time periods, we reduce the impediment of cloud cover to characterising sea ice using this type of imagery. Our processing provides a higher-resolution depiction of sea ice conditions and their variability than that afforded by passive microwave data. By estimating a sub-pixel concentration for all pixels identified as ‘Ice’, we capture further nuances of narrower water/thin ice inclusions within the ice cover.

The utility of this new methodology to support operational ship survey in polar regions is demonstrated using examples from the Weddell Sea, Antarctica. Our description of sea ice cover agrees well with that derived from very high-resolution imagery from the Operation Ice Bridge DMS camera system, and with experience of the actual sea ice conditions encountered during the Weddell Sea Expedition in early 2019.

How to cite: Benham, T., Christie, F., and Dowdeswell, J.: Sea ice in the Weddell Sea: use of moderate resolution imagery to summarize inter-annual variation in conditions and support operational ship survey, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21821, https://doi.org/10.5194/egusphere-egu2020-21821, 2020.

The Weddell Sea Expedition 2019 (WSE) was conceived with dual aims: (i) to undertake a comprehensive international inter-disciplinary programme of science centred in the waters around Larsen C Ice Shelf, western Weddell Sea; and (ii) to search for, survey and image the wreck of Sir Ernest Shackleton’s Endurance, which sank in the Weddell Sea in 1915. 

The 6-week long expedition, funded by the Flotilla Foundation, required the use of a substantial ice-strengthened vessel given the very difficult sea-ice conditions encountered in the Weddell Sea, and especially in its central and western parts. The South African ship SA Agulhas II was chartered for its Polar Class 5 icebreaking capability and design as a scientific research vessel. The expedition was equipped with state-of-the-art Autonomous Underwater Vehicles (AUVs) and a Remotely Operated Vehicle (ROV) which were capable of deployment to waters more than 3,000 m deep, thus making the Larsen C continental shelf and slope, and the Endurance wreck site, accessible. During the expedition, a suite of passive and active remote-sensing data, including TerraSAR-X radar images delivered in near real-time, was provided to the ice-pilot onboard the SA Agulhas II. These data were instrumental for safe vessel navigation in sea ice and the detection and tracking of icebergs and ice floes of scientific interest.

The scientific programme undertaken by the WSE was very successful and produced many new geological, geophysical, marine biological and oceanographic observations from a part of the Weddell Sea that has been little studied previously, particularly the area east of Larsen C Ice Shelf. The expedition also reached the sinking location of Shackleton’s Endurance, where the presence of open-water sea ice leads allowed the deployment of an AUV to the ocean floor to try and locate and survey the wreck. Unfortunately, SA Agulhas II later lost communication with the AUV, and deteriorating weather and sea ice conditions meant that the search had to be called off.

How to cite: Shears, J., Dowdeswell, J., Ligthelm, F., and Wachter, P.: The Weddell Sea Expedition 2019, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21896, https://doi.org/10.5194/egusphere-egu2020-21896, 2020.

The 2019 Weddell Sea expedition provided a unique opportunity for geophysical and glaciological sea ice measurements in one of the least accessible regions of the Southern Ocean. Although the extent and area of sea ice is well known based on satellite measurements, the limited information on thickness does still hinder the calculation of trends trends in volume and mass. Sea ice thickness is therefore one of the missing key variables in the global cryosphere mass balance, and difficult logistics are a challenge for near synchronous satellite validation measurements. Another key variable in this context is snow on sea ice, as knowledge of snow is required to convert satellite-derived freeboard to thickness.

We measured the sea-ice morphology by a combination of on ice and remote sensing methods: near-synchronous temporal and spatial measurements from a drone equipped with a radar sensor and camera, manually-derived on-ice surveys and samples such as snow pits, snow-depth transects and drill holes, and a AUV with upward-looking multibeam sonars. We also deployed ice-drifter buoys on several ice floes which we used to provide floe drift over an extended period of time.

In this contribution we present the results of our observations in conjunction with a close sequence of high resolution satellite radar images (TerraSAR-X, Sentinel-1) and altimeter data (ICESat-2 and CryoSat-2) to characterise the sea ice conditions in the western Weddell Sea. We found a mixture of fragments of deformed first-year and multi-year sea-ice which was consolidated in larger ice floes. A thick snow cover frequently depressed the ice cover of the thinner first year ice below sea level. Satellite data allow to extend our findings in time to a larger area and to improve our information on sea ice over a larger region.

How to cite: Rack, W., Christie, F., Dowdeswell, E., Dowdeswell, J., Wachter, P., Benham, T., Haas, C., and Bealing, P.: Sea ice characteristics during the Weddell Sea expedition explored by geophysical and remote sensing methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20610, https://doi.org/10.5194/egusphere-egu2020-20610, 2020.

Crane and Hektoria glaciers, the major tributaries of the former Larsen B Ice Shelf, underwent major structural and ice flow changes in the aftermath of the ice shelf’s disintegration in March, 2002. In addition to the widely reported initial acceleration (leading to speeds 3 to 6 times the pre-disintegration rate), the continued retreat led to the formation of significant ice cliffs. For Hektoria, this occurred as a seamless transition from ice shelf disintegration. Crane Glacier had a two-stage acceleration, first increasing in speed by 3x in the first few months after disintegration, then slowing through September 2004, and then a rapid additional acceleration in 2005-2006. Both glaciers developed significant ice cliffs during retreat, with peak ice-front heights of 105 m for Crane and 85 m for Hektoria.

How to cite: Scambos, T., Bohlander, J., and Alley, K.: Post-disintegration evolution of the largest Larsen B tributary glaciers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1330, https://doi.org/10.5194/egusphere-egu2020-1330, 2020.

Linear to curvilinear depressions interpreted as iceberg ploughmarks are identified on the continental shelf beyond Larsen C Ice Shelf in about 350 m water depth using multibeam echo-sounding at sub-metre horizontal resolution. Detailed imaging of ploughmark morphology demonstrates the presence of irregularly spaced ridges extending across the full ploughmark width. These ridges have an arcuate shape in plan-view, are up to 2 m high, 20-40 m wide, show occasional presence of subdued debris-flow lobes on their distal side and have an asymmetric cross-profile in which the seafloor deepens beyond their slightly steeper side. The ridges are interpreted to have been produced when the iceberg moved backwards under the falling tide, which pushed up a ridge of sediment behind the iceberg keel, before it continued on its original trajectory under the rising tide. Similar features, which we term ‘iceberg tidal ridges’, can be identified at lower resolution on bathymetric and three-dimensional seismic data from the mid-Norwegian margin, suggesting the broader implications of the interpretations presented here. For example, the mapping of delicate ridges preserved within iceberg ploughmarks can be used to reconstruct past oceanic circulation including the former direction and strength of ocean currents.

How to cite: Montelli, A., Batchelor, C., Ottesen, D., Dowdeswell, J., Evans, J., and Dowdeswell, E.: Tidally influenced iceberg motion: sub-metre resolution imaging of iceberg ploughmarks using autonomous underwater vehicles in the Weddell Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2726, https://doi.org/10.5194/egusphere-egu2020-2726, 2020.

Ice shelf buttressing plays a critical role in the long-term stability of ice sheets. The underlying bathymetry and cavity thickness therefore is a key to accurate models of future ice sheet evolution. However, direct observation of sub-ice shelf bathymetry is time consuming, logistically risky, and in some areas simply not possible, meaning there is a blind-spot in our understanding of this key system. Here we use airborne gravity anomaly data to provide new estimates of sub-ice shelf bathymetry outboard of the rapidly changing West Antarctic Thwaites Glacier, and beneath the adjacent Dotson and Crosson Ice Shelves. These regions are of especial interest as the low-lying inland reverse slope of the Thwaites glacier system makes it vulnerable to collapse through marine ice sheet instability, with rapid grounding-line retreat observed since 1993 suggesting this process may be underway. Our results confirm a major marine channel > 800 m deep extends to the front of Thwaites Glacier, while the adjacent ice shelves are underlain by more complex bathymetry. Comparison of our new bathymetry with ice shelf draft reveals that ice shelves formed since 1993 comprise a distinct population where the draft conforms closely to the underlying bathymetry, unlike the older ice shelves which show a more uniform depth of the ice base. This indicates that despite rapid basal melting in some areas, these “new” ice shelves are not yet in equilibrium with the underlying ocean system. We propose qualitative models of how this transient ice-shelf configuration may have developed, but further investigation is required to constrain the longevity and full impact of these newly recognised systems.

How to cite: Jordan, T., Porter, D., Tinto, K., Millan, R., Muto, A., Hogan, K., Larter, R., Graham, A., and Paden, J.: Two ice shelf populations revealed in new gravity- derived bathymetry for the Thwaites, Crosson and Dotson ice shelves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19603, https://doi.org/10.5194/egusphere-egu2020-19603, 2020.

As part of the International Thwaites Glacier Collaboration (ITGC) field activity in West Antarctica for the 2019-2020 season, the Thwaites-Amundsen Regional Survey and Network (TARSAN) team drilled boreholes using hot water, deployed long-term instruments, and gathered several ground-based geophysical data sets to assess the ice-shelf stability and evolution.

The Thwaites Eastern Ice Shelf is an important buttress for a broad (25 km) section of Thwaites Glacier outflow and is restrained at present by a few pinning points at the northwestern edge of the shelf. The grounding line of this buttress has retreated within the last 5 years indicating instability. Recent imagery shows major new rifting and shearing within the ice shelf.

In the Dotson-Crosson Ice Shelf (a single ice shelf with a rapidly evolving central region that has thinned and ungrounded over the past 80 years), satellite data show significant ice flow speed and direction changes, as well as retreating grounding lines where tributary glaciers start to float and where ice flows over and around isolated bedrock pinning points. A complex geometry of deep seafloor troughs underlie the central ice-shelf area which lies at the convergence of the two major troughs that extend to the continental shelf edge at two widely separated locations (roughly 103°W and 117°W longitude along the continental shelf break).

We surveyed the central Thwaites Eastern Ice Shelf (‘Cavity Camp’, 75.05°S, 105.58°W) and central Dotson-Crosson Ice Shelf (`Upper Dotson’, 74.87°S, 112.20°W) to the extent possible considering site safety and scientific interest. Cavity Camp is located approximately 17 km down-flow of the 2011 Thwaites Glacier grounding line. Ground-penetrating radar data show the ice thickness near Cavity Camp to be 300m, which is ~200m thinner than in 2007 estimated from hydrostatic assumption using altimetry analysis by other researchers. The seafloor below Cavity Camp is 816m, based on pressure from a CTD profile (a ~540 m water column and ~40m of firn).   

Across the central Dotson-Crosson Ice Shelf, a network of basal channels creates variable thinning rates from near-zero to over 30 m/yr (estimated in several previous remote-sensing-based studies). Ice thickness near our camp over a subglacial channel is 390m and the ice has been thinning at ~25 m/yr estimated from satellite data. Seafloor elevation at the Dotson site is estimated at -570 m, but seismic surveys suggest that the seabed topography varies considerably beneath Dotson. 

On each ice shelf, we conducted ~200 km of multi-frequency ground-penetrating radar profiles. We also conducted 46 (Thwaites) and 17 (Dotson) autonomous phase-tracking radio echo-sounding (ApRES) repeat point measurements, as well as 37 (Thwaites) and more than 20 (Dotson) active-seismic spot soundings to characterize the sub-ice-shelf cavity shape, thinning rates, basal ice structures, and ocean circulation. We deployed two Automated Meteorology Ice Geophysics Ocean observation Systems (AMIGOS-III stations) on the Thwaites Ice Shelf that include a suite of surface sensors, a fiber-optic-based thermal profiler, and an ocean mooring. Additionally, we deployed four long-term ApRES on the two ice shelves to monitor temporal variability in ice melt.

How to cite: Pettit, E., Muto, A., Wild, C., Alley, K., Scambos, T., Wallin, B., Truffer, M., and Pomraning, D.: Thwaites and Dotson Ice Shelves: Field Site Selection and Early Results of Field Measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1331, https://doi.org/10.5194/egusphere-egu2020-1331, 2020.

High snowfall events on Thwaites Glacier are a key influencer of its ice mass change. In this study, we diagnose the mechanisms for orographic precipitation on Thwaites Glacier by analyzing the atmospheric conditions that lead to high snowfall events. A high-resolution regional climate model, RACMO2, is used in conjunction with MERRA-2 and ERA5 reanalysis to map snowfall and associated atmospheric conditions over the Amundsen Sea Embayment. We examine these conditions during high snowfall events over Thwaites Glacier to characterize the drivers of the precipitation and their spatial and temporal variability. Then we examine the seasonal differences in the associated weather patterns and their correlations with El Nino Southern Oscillation and the Southern Annular Mode. Understanding the large-scale atmospheric drivers of snowfall events allows us to recognize how these atmospheric drivers and consequent snowfall climatology will change in the future, which will ultimately improve predictions of accumulation on Thwaites Glacier.

How to cite: Maclennan, M. and Lenaerts, J.: Large-Scale Atmospheric Drivers of Snowfall on Thwaites Glacier, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12441, https://doi.org/10.5194/egusphere-egu2020-12441, 2020.

Permafrost thaw is altering northern ecosystems and the services they provide at scales ranging from local subsidence to global climate feedbacks. In organic-rich peatlands, thermokarst initiation and spread rates are increasing with rising mean annual air temperatures, changes in wildfire, and human land use. This presentation will outline empirical and modeling approaches to better understand the consequences of thermokarst in peatlands as well as other types of northern terrains on carbon cycling, wildlife, and other aspects of ecosystem services.  We are using fine scale datasets and remote sensing to map thermokarst coverage and expansion in both the Northwest Territories, Canada and interior Alaska. Using chronosequences and regional gradients, we are studying thermokarst impacts along gradients of time-since-thaw. Through a Permafrost Carbon Network synthesis, we developed conceptual and numerical models to understand how thermokarst development (formation, stabilization, re-accumulation of permafrost in some conditions) affects carbon storage and release. We are using a combination of empirical and modelled data to test hypotheses about climatic, ecological, and Quaternary controls on thermokarst rates and subsequent impacts on ecosystem services. We demonstrate that thermokarst in peat-rich landscapes are hotspots for permafrost carbon release primarily through methane emissions, have the potential to impact hunter movement and safety, and affect caribou habitat quality.

How to cite: Turetsky, M., Gibson, C., and Dieleman, C.: Impacts of thermokarst on permafrost carbon losses and ecosystem services, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22219, https://doi.org/10.5194/egusphere-egu2020-22219, 2020.

Almost 65% of Siberian forests and 23% of tundra vegetation grow in permafrost zone. According to our estimate, carbon stocks in the soils of forest and tundra ecosystems of Yakutia (Eastern Siberia, Russia) amount to 17 billion tons (125.5 million hectares of forest and 37 million hectares of tundra in total) that is about 25% of total carbon resource in the forest soils of the Russian Federation.
This presentation is compiled from the results of many years time series investigations conducted on the study of carbon cycle in permafrost-dominated forests with different productivity and typical tundra and along Great Lena river basin including Aldan and Viluy tributaries. 
Seasonal photosynthesis maximum of forest canopy vegetation in dry years falls into June, and in humid ones – into July. During the growing season the woody plants of Yakutia uptake from 1.5 to 4.0 t C ha^-1 season^-1 depending on water provision. Night respiration is higher in dry and extremely dry years (10.9 and 16.1% respectively). The productive process of tree species in Eastern Siberia is limited by endogenous (stomatal conductance) and exogenous (provision with moisture and nutrients, nitrogen specifically) factors. The increase of an atmospheric precipitation after long 2-3 annual droughts accompanied with strong surge in photosynthetic activity of forest plants is almost 2.5 times. 
The temperature of soil is a key factor influencing soil respiration in the larch forests. Average soil respiration for the growing season comes to 6.9 kg C ha^-1 day^-1, which is a characteristic of Siberian forests. Annual average soil emission is 4.5±0.6 t C ha^-1 yr^-1.
As our multi-year studies showed, there is significant interannual NEE variation in the Central Yakutia larch forest, while in the Southern Yakutia  larch forest and tundra ecosystem variation is more smooth, because the climatic conditions in these zones (close to the mountain and sea)  are less changeable than in sharply continental Central Yakutia. 
According to our long-term eddy-correlation data, the annual uptake of carbon flux (NEE) in the high productivity larch forest of South eastern Yakutia, 60N – 2.43±0.23 t C ha^-1 yr^-1, in the moderate productivity larch forest of the Central Yakutia, 62N makes 2.12±0.34 t C ha^-1 yr^-1 and in the tundra zone, 70N – 0.75±0.14 t C ha^-1 yr^-1.
Interannual variation of carbon fluxes in permafrost forests in Northeastern Russia (Yakutia) makes 1.7-2.4 t C ha^-1 yr^-1 that results in the upper limit of annual sequestering capacity of 450-617 Mt C yr^-1. In connection with climate warming there is a tendency of an increase in the volume of carbon sequestration by tundra and as opposed to decrease by forest ecosystem in the result of prolongation of the growing season and changing of plant successions.  This is also supported by changes in land use as well as by CO₂ sequestration in the form of fertilizer. 
According our biogeochemical investigation annual flux of carbon from main in Eastern Siberia Lena river hydrological basin is almost 6.2 Mt C yr^-1 including 28% at Aldan and 14% at Viluy rivers.

How to cite: Maximov, T., Dolman, H., Kotani, A., Anderson, P., Maksimov, A., and Petrov, R.: Carbon cycle of permafrost transect: main terrestrial and hydrological ecosystems of Eastern Siberia , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4322, https://doi.org/10.5194/egusphere-egu2020-4322, 2020.

Permafrost-affected river plains are highly diverse in discharge regime, floodplain morphology, channel forms, channel mobility and ecosystems. Frozen floodplains range from almost barren systems with high channel mobility, to extensive wetland areas with low channel mobility, abundant abandoned channels, back-swamps and shallow floodplain lakes. Floodplain processes are increasingly affected by climate-induced changes in river discharge and temperature regime: changes in the dates of freeze-up, break-up and spring floods, and changes in the discharge distribution throughout the year.

In permafrost floodplains, changes in flooding frequency, flood height and water temperature affect active layer thickness, subsidence and erosion processes. Data from the Northeast Siberian Berelegh river floodplain (a tributary to the Indigirka river) demonstrate that increasing spring flood height potentially causes permafrost thaw, soil subsidence and increase of the floodplain area. INSAR (interferometric synthetic aperture radar) data indicate that poorly drained areas in this region are affected by soil subsidence. Morphological evidence for subsidence of the river floodplain is abundant, and river-connected lakes show expansion features also seen in thaw lakes.

These floodplain wetland ecosystems are also affected by changes in the discharge regime and permafrost. On the one hand, floodplains are sites of active sedimentation of organic matter-rich sediments and sequestration of carbon. This carbon is derived from upstream erosion of permafrost and vegetation, and from autochthonous primary production. Nutrient supply by flood waters supports highly productive ecosystems with a comparatively large biomass.

On the other hand, these ecosystems also emit high amounts of CH₄, which may be affected by flooding regime. In the example presented here, the CH₄ emission from floodplain wetlands is about seven times higher that the emission from similar tundra wetlands outside the floodplain.

The dynamic nature of floodplains hinders carbon and greenhouse gas flux measurements. Better quantification of greenhouse gas fluxes from these floodplains, and their relation with river regime changes, is highly important to understand future emissions from thawing permafrost. Given the difficulties of surface greenhouse gas flux measurements, recent remote sensing material could play an important role here.

How to cite: van Huissteden, K., Teshebaeva, K., Cheung, Y., Noorbergen, H., and van Persie, M.: Climate change and the carbon cycle of frozen floodplains., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1445, https://doi.org/10.5194/egusphere-egu2020-1445, 2020.

Due to the climate warming, permafrost on the Qinghai-Tibet Plateau (QTP) was degradating in the past decades. Since its impacts on East Asian monsoon, and even on the global climate system, it is fundamental to reveal permafrost status, changes and its physical processes. Based on previous research results and new observation data, this paper reviews the characteristics of the status of permafrost on the QTP, including the active layer thickness (ALT), the spatial distribution of permafrost, permafrost temperature and thickness, as well as the ground ice and soil carbon storage in permafrost region.

The results showed that the permafrost and seasonally frozen ground area (excluding glaciers and lakes) is 1.06 million square kilometters and 1.45 million square kilometters on the QTP. The permafrost thickness varies greatly among topography, with the maximum value in mountainous areas, which could be deeper than 200 m, while the minimum value in the flat areas and mountain valleys, which could be less than 60 m. The mean value of active layer thickness is about 2.3 m. Soil temperature at 0~10 cm, 10~40 cm, 40~100 cm, 100~200 cm increased at a rate of 0.439, 0.449, 0.396, and 0.259°C/10a, respectively, from 1980 to 2015. The increasing rate of the soil temperature at the bottom of active layer was 0.486 oC/10a from 2004 to 2018.

The volume of ground ice contained in permafrost on QTP is estimated up to 1.27×10⁴ km³ (liquid water equivalent). The soil organic carbon staored in the upper 2 m of soils within the permafrost region is about 17 Pg. Most of the research results showed that the permafrost ecosystem is still a carbon sink at the present, but it might be shifted to a carbon source due to the loss of soil organic carbon along with permafrost degradation.

Overall, the plateau permafrost has undergone remarkable degradation during past decades, which are clearly proven by the increasing ALTs and ground temperature. Most of the permafrost on the QTP belongs to the unstable permafrost, meaning that permafrost over TPQ is very sensitive to climate warming. The permafrost interacts closely with water, soil, greenhouse gases emission and biosphere. Therefore, the permafrost degradation greatly affects the regional hydrology, ecology and even the global climate system.

How to cite: Zhao, L., Hu, G., Zou, D., Li, R., Sheng, Y., and Pang, Q.: Status, Changes and Impacts of Permafrost on Qinghai-Tibet Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6246, https://doi.org/10.5194/egusphere-egu2020-6246, 2020.

Mean annual temperatures in interior Alaska, currently -1°C, are projected to increase as much as 5°C by 2100. An increase in mean annual temperatures is expected to degrade permafrost and alter hydrogeology, soils, vegetation, and microbial communities. Ice and carbon rich “yedoma type” permafrost in the area is ecosystem protected against thaw by a cover of thick organic soils and mosses. As such, interactions between vegetation, permafrost ice content, the snow pack, and the soil thermal regime are critical in maintaining permafrost. We studied how and where vegetation and soil surface characteristics can be used to identify subsurface permafrost composition. Of particular interest were potential relationships between permafrost ice content, the soil thermal regime, and vegetation cover. We worked along 400-500 m transects at sites that represent the variety of ecotypes common in interior Alaska. Airborne LiDAR imagery was collected from May 9-11, 2014 with a spatial resolution of 0.25 m. During the winters from 2013-2019 snow pack depths have been made at roughly 1 m intervals along site transects using a snow depth datalogger coupled with a GPS. In late summer from 2013-2019 maximum seasonal thaw depths have been measured at 4 m intervals along each transect. Electrical resistivity tomography measurements were collected across the site transects. A variety of machine learning geospatial analysis approaches were also used to identify relationships between ecosystem characteristics, seasonal thaw, and permafrost soil and ice composition. Wintertime measurements show a clear relationship between vegetation cover and snow depth. Interception (and shallow snow) was evident in the birch and white spruce forests and where dense shrubs are present while the open tussock and intermittent shrub regions yield the greatest snow depths. Results from repeat seasonal thaw depth measurements also show a strong relationship with vegetation where mixed birch and spruce forest is associated with the deepest seasonal thaw. The tussock/shrub and spruce forest zones consistently exhibited the shallowest seasonal thaw. Roughly 60% of the seasonal thaw along the transects occurred by mid-July and downward movement of the thaw front had mostly ceased by late August with little additional thaw between August 20 and early October. Summer precipitation shows a relationship with seasonal thaw depth with the wettest summers associated with the deepest thaw. Results from this study identify clear relationships between ecotype, permafrost composition, and seasonal thaw dynamics that can help identify how and where permafrost degradation can be expected in a warmer future arctic.

How to cite: Douglas, T., Hiemstra, C., Anderson, J., and Zhang, C.: Identifying vegetation-geomorphology relationships in permafrost with airborne LiDAR, electrical resistivity tomography, seasonal thaw depth measurements, and machine learning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1900, https://doi.org/10.5194/egusphere-egu2020-1900, 2020.

The release of vast amounts of organic carbon during thawing of high-latitude permafrost is an urgent issue of global concern, yet it is unclear what controls how much carbon will be released and how fast it will be subsequently metabolized and emitted as greenhouse gases. Binding of organic carbon by iron(III) oxyhydroxide minerals can prevent carbon mobilization and degradation. This “rusty carbon sink” has already been suggested to protect organic carbon in soils overlying intact permafrost. However, the extent to which iron-bound carbon will be mobilized during permafrost thaw is entirely unknown. We have followed the dynamic interactions between iron and carbon across a thaw gradient in Abisko (Sweden), where wetlands are expanding rapidly due to permafrost retreat. Using both bulk (selective extractions, EXAFS) and nanoscale analysis (correlative SEM and nanoSIMS), we found that up to 19.4±0.7% of total organic carbon is associated with reactive iron minerals in palsa underlain by intact permafrost. However, during permafrost collapse, the rusty carbon sink is lost due to more reduced conditions which favour microbial Fe(III) mineral dissolution. This leads to high dissolved Fe(II) (2.93±0.42 mM) and organic carbon concentrations (480.06±34.10 mg/L) in the porewater at the transition of desiccating palsa to waterlogged bog. Additionally, by combining FT-ICR-MS and greenhouse gas analysis both in the field and in laboratory microcosm experiments, we are currently determining the fate of the mobilized organic carbon directly after permafrost collapse. Our findings will improve our understanding of the processes controlling organic carbon turnover in thawing permafrost soils and help to better predict future greenhouse gas emissions.

How to cite: Patzner, M. S., Logan, M., Mueller, C. W., Joss, H., Anthony, S. E., Scholten, T., Byrne, J. M., Borch, T., Kappler, A., and Bryce, C.: Organic carbon sorbed to reactive iron minerals released during permafrost collapse , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1465, https://doi.org/10.5194/egusphere-egu2020-1465, 2020.

Current global climate changes represent a threat for the stability of the polar regions and may result in cascading broad impacts. Studies conducted on permafrost in the Arctic regions indicate that these areas may store almost twice the carbon currently present in the atmosphere. Therefore, permafrost thawing may potentially cause a significant increase of greenhouse gases concentrations in the atmosphere, exponentially rising the global warming effect. Although several studies have been carried out in the Arctic regions, there is a paucity of data available from the Southern Hemisphere. The Seneca project aims to fill this gap and to provide a first degree of evaluations of gas concentrations and emissions from permafrost and/or thawed shallow strata of the Dry Valleys in Antarctica. The Taylor and Wright Dry Valleys represent one of the few Antarctic areas that are not covered by ice and therefore represent an ideal target for permafrost investigations.

Here we present the preliminary results of a multidisciplinary field expedition conducted during the Antarctic summer in the Dry Valleys, aimed to collect and analyse soil gas and water samples, to measure CO₂ and CH₄ flux exhalation, to investigate the petrological soil properties, and to acquire geoelectrical profiles. The obtained data are used to 1) derive a first total emission estimate for methane and carbon dioxide in this part of the Southern Polar Hemisphere, 2) locate the potential presence of geological discontinuities that can act as preferential gas pathways for fluids release, and 3) investigate the mechanisms of gas migration through the shallow sediments. These results represent a benchmark for measurements in these climate sensitive regions where little or no data are today available.

How to cite: Ruggiero, L., Sciarra, A., Mazzini, A., Mazzoli, C., Romano, V., Tartarello, M. C., Florindo, F., Ascani, M., Wilson, G., Dagg, B., Hardie, R., Anderson, J., Worthington, R., Lupi, M., Bigi, S., Ciotoli, G., Graziani, S., Fischanger, F., and Sassi, R.: SourcE and impact of greeNhousE gasses in AntarctiCA: the Seneca project , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1431, https://doi.org/10.5194/egusphere-egu2020-1431, 2020.

Ice wedge (IW) polygons form through thermal contraction induced by winter cooling of ice-rich permafrost which results in the formation of cracks. Hoar frost develops in the cracks in winter and meltwater infills the cracks during spring and freezes. As the cracking and infilling occurs repeatedly, IWs grow, leading to characteristic surface morphology with depressions or troughs aligned on the axis of the IW and raised rims or ridges on either side. Surface expression of IW is either characterized as low-centered polygons or high-centered polygons, the former being associated with the first stages of IW development, and the latter with IW degradation. Because IWs represent important excess ice close to the surface, considerable local subsidence and related effects on landscape parameters, such as vegetation and moisture, are likely to occur upon degradation.

IW polygons distribution, morphometry and state were characterized in the Tombstone Territorial Park (Central Yukon, Canada) using semi-automated remote sensing techniques, field observations and laboratory analyses. The data is used to define determining landscape factors for IW polygons occurrence, to characterise the stages of the IWs development and/or degradation and to estimate the volume of buried ice in the region. Results show that elevation, slope and material are important elements defining IW polygons distribution. The relationship between landscape factors and stages of development is not as clear, and, despite climate changes being homogenous in the area, IW development and degradation is very heterogenous, as shown by the differing moisture, greenness and brightness signals across the polygonal terrain.

How to cite: Frappier, R. and Lacelle, D.: Ice-wedge polygons distribution, morphometry and state in the Tombstone Territorial Park, Central Yukon, Canada, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12044, https://doi.org/10.5194/egusphere-egu2020-12044, 2020.

At present, the degradation of permafrost caused by climate warming raises serious concerns of scientists and the public around the world. As a result of degradation of permafrost containing a huge amount of organic material and the decomposition of this organic material, the greenhouse effect can increase significantly. Some scientists estimate that the amount of carbon in the permafrost is more than two times than there is in atmospheric carbon dioxide (Schuur E. A. G.et al., 2015).  Besides, a large amount of greenhouse gasses, mostly methane, is already contained in watery glacier bottoms, where these gasses build up through anaerobic organic decomposition (Burns R., 2018). Therefore, there are concerns that permafrost thaw and glacier retreat as the Earth warms will lead to new greenhouse gasses being released into the atmosphere, thus further accelerating the global warming process. 
Our research devoted to this problem was carried out at the archaeological Upper Paleolithic site Divnogorie 9 (50.9649° N, 39.3031° E) in the National Park “Divnogorie”. Our study area occupies the southern part of the Middle Russian Upland (the East European Plain). It has experienced several Quaternary glaciations: the Don, Dnepr, Moscow, and Valdai Glaciations. The facts of the presence of permafrost and its degradation during the late Pleistocene and Holocene are established here as well. The site is located at the right bank of the Tikhaya Sosna River, a right tributary of the Don River. The Don River basin is a world known area because of high concentration of the Upper Paleolithic archaeological sites here - Kostenki-Borshevo district (51°23'40'' N, 39º30'31''E) which contains 26 open-air mammoth remnant sites (38-18 ka BP). 
Divnogorie 9 is an unique site in Europe which is well-known for numerous findings of fossilized equestrian remains of wild horses - more than eight thousands samples. Our most detailed study of the Quaternary deposits was carried out at a 18-m thick section. Bones are concentrated in seven layers (levels). This section exposes several paleosol layers, as well. Estimates of the radiocarbon age of the fossils and paleosol layers here yielded 14-12 ka BP. We studied the organic carbon from paleo-soils of Divnogorie 9. The abundant presence of such large grazers as horses and especially mammoths during the Late Pleistocene supports the widespread existence of high productivity grasslands and organic-rich soils. 
However, the results of our analysis do not show a significant amount of organic carbon in these paleo-soils at the present. It may possibly be an indication that the originally carbon rich permafrost and subglacial deposits lost their carbon upon permafrost thaw and glacial retreat during the transition from the last glaciation to the Holocene. This ancient carbon was massively released into the atmosphere and to the aquatic systems during that time. At the same time, there were not widespread catastrophic consequences to the Earth’s environment except possibly for the extinction of mammoths and other large fauna in the arctic and subarctic. These results provide some cautious optimism about the severity in current amount of changes and consequences thereof. 

How to cite: Romanovskaya, M., Romanovsky, V., and Kuznetsova, T.: Global climate warming: permafrost degradation and expected consequences, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3859, https://doi.org/10.5194/egusphere-egu2020-3859, 2020.

Boreal forests are located at latitudes that are predicted to experience some of the greatest warming on the planet. Forests growing on permafrost may be particularly vulnerable, with accelerated soil warming and permafrost degradation linked to changing patterns of tree growth and longevity. Many have speculated that thawing permafrost, through its effects on soil water content and ground stability, will increase forest mortality across the boreal region. However, recent evidence indicates mixed forest responses to permafrost thaw. In some areas, the onset of thaw is followed by increased tree growth and increased forest cover area. In other sites, thaw has been linked to decreased growth and forest cover loss. It is currently poorly understood what determines these contrasting responses, and the roles that different environmental and climatic factors may play. This leads to two major issues: (1) uncertainties in predicting the effects of future permafrost thaw on carbon dynamics in northern ecosystems, and (2) poor understanding of where scientific and conservation efforts should be focused. Here, we present a review of the recent evidence of permafrost thaw effects on boreal forest dynamics and propose an explanation for the differing responses across sites. We argue that the outcome is controlled by a set of factors that influence two major pathways and the interactions between them: (1) permafrost-soil water content and (2) soil water content-plant growth. We present a series of conceptual models explaining these interactions and highlight the largest sources of uncertainties. Based on these, we propose a set of hypotheses and methodologies to guide future research in this area.

How to cite: Kulawska, A., Pugh, T. A. M., Kettridge, N., MacKenzie, R., and Ullah, S.: What governs the effects of permafrost thaw on boreal forest dynamics? , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5404, https://doi.org/10.5194/egusphere-egu2020-5404, 2020.

The Arctic is warming at twice the rate of the rest of the world, causing precipitation to shift from snowfall to rainfall, permafrost to thaw, longer snow-free land and ice-free lakes, and increased evaporation. Thermokarst lakes across the Arctic have experienced different changes over the past decades: in some regions, lakes are expanding through thawing adjacent permafrost, while in other regions they are drying up and shrinking, or not changing at all. It is important to understand what governs lake water balance as it affects lake ecosystems that support large populations of migratory birds and fish; are important to local communities for food and recreation; and control the flux of carbon and other nutrients from thawing permafrost into lakes. For example, lake inflow, evaporation and water residence time affect the concentration of nutrients within lakes, ultimately affecting the aquatic ecosystem and greenhouse gas release. Previous research has focused on quantifying the water inputs and outputs of individual lakes, but a better understanding of the drivers and processes controlling lake water balances is required to understand how they will respond to a changing climate.

We measured lake water flux components at multiple spatial and temporal scales across the 5000 km² boreal – tundra transition zone between Inuvik and Tuktoyaktuk, Northwest Territories, Canada. Lake water flux components were measured at two adjacent thermokarst lakes with different ratios of lake area to catchment area (LACA), from 2017 – 2019. Also, water isotope samples were collected from March – September 2018 from ~100 lakes across 2000 km². From these water isotope compositions we estimated the ratio of evaporation to inflow, residence time, and the mixture of snowmelt and rainfall runoff in each lake. Catchments of all 7500 lakes in the region were delineated using a high-resolution digital elevation model in order to estimate their LACA, and evaluate connectivity between lakes.

Paired lake water balance measurements showed that the lake with a larger LACA had a residence time an order of magnitude shorter than the larger lake, and displayed larger fluctuations in water level. Also, the ratio of evaporation to inflow was significantly larger in lakes with smaller LACA. Water isotope compositions showed that only 10-50% of a lake’s water is replaced by snowmelt in spring, as the majority of snowmelt runoff flowed overtop of lake ice and through the lake outlet. Deeper lakes had significantly less snowmelt mixing, as the volume of water for the snowmelt to mix with was greater than in shallower lakes. These results show that lake water balance can be characterized using lake and catchment properties, allowing future research to more easily characterize lake hydrology and build further understanding about how lake water balance is connected to other aspects of the permafrost environment.

How to cite: Wilcox, E. J., Walker, B., Hould - Gosselin, G., Sonnentag, O., Wolfe, B. B., and Marsh, P.: Landscape Controls on the Hydrological Variability of Thermokarst Lakes between Inuvik and Tuktoyaktuk, NWT , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9279, https://doi.org/10.5194/egusphere-egu2020-9279, 2020.

    Circumpolar permafrost is degrading under anthropogenic global warming, thus the large amount of soil organic carbon in it would be vulnerable to microbial decomposition and further aggravating future warming. However, solar radiation modification (SRM), as a theoretical approach to reducing some of the impacts of anthropogenic climate change, hopefully could mitigate the permafrost degradation and slow down permafrost carbon loss. Here we use two solar geoengineering experiments came up in CMIP6/GeoMIP6 -- G6solar and G6sulfur, to explore changes in circumpolar permafrost carbon under solar radiation modification scenarios. Earth system models' simulations show that under G6 scenarios, annual mean surface air temperature in circumpolar permafrost region is about 5℃ lower relative to the high forcing scenario SSP5-8.5 by year 2100, with a growing trend but remains below 0℃ from 2015 to 2100, which is close to that in the medium forcing scenario SSP2-4.5. The lower temperature causes lower degradation rate of permafrost area. In SSP5-8.5 scenario, almost all the permafrost thaws by year 2100, but up to half of it remains frozen in SSP2-4.5 and G6 scenarios compared to year 2015. The lower temperature also results in less carbon assimilation in this area, thus the lower vegetation carbon accumulation. By 2100, a maximum soil carbon loss of 18.09 PgC under SSP5-8.5 scenario regarding to different model constructions, while in G6 the soil carbon loss could be reduce to 3.70 PgC, even less than that of 5.29 PgC in SSP2-4.5 scenario.

How to cite: Chen, Y. and Ji, D.: Solar Radiation Modification Slows Down Permafrost Carbon Loss , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1740, https://doi.org/10.5194/egusphere-egu2020-1740, 2020.

Permafrost thaw, as a consequence of climate warming, liberates large quantities of frozen organic carbon in the Arctic regions. The response of gaseous carbon release upon permafrost thaw might play a crucial role in the future evolution of atmosphere-land fluxes of biogenic gases such as volatile organic compounds (VOCs), a group of reactive gases and the dominant modulator of tropospheric oxidation capacities. Here, we examine the response of volatile release from Finnish Lapland permafrost soils to temperature increase in a series of laboratory incubation experiments. The experiments show that when the temperature rises from 0 °C to 15 °C, various VOC species are significantly emitted from the gradually thawing soils. The VOC fluxes from thawing permafrost are on average four times as high as those from active layer. Acetic acid and acetone dominate the total volatile emissions from both permafrost and active layer, with significant amounts of aromatics and terpenes detected as well. The emission rate and the composition of volatile release from thawing soils are highly responsive to temperature variations. As temperature increases, more less volatile compounds are released, i.e., sesquiterpenes and diterpenes. Collectively, these results demonstrate the highly overlooked volatile production from thawing permafrost, which will create a stronger permafrost carbon-climate feedback.

How to cite: Li, H., Mäki, M., Kohl, L., Väliranta, M., Bäck, J., and Bianchi, F.: Overlooked volatile production from Arctic permafrost triggered by global warming, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5988, https://doi.org/10.5194/egusphere-egu2020-5988, 2020.

The Arctic permafrost soils are very diverse in regard to parent material, geobiological composition and genesis. There is sparse knowledge about nutrient availability in Arctic soil and it was found that the permafrost layer differs in nutrient availability compared to the active layer. Recently, it was shown that elements like Si, Ca and P are potentially affecting the greenhouse gas from Arctic soil. However, it is not known how those elements are distributed in Arctic soils for a larger dataset. Furthermore, it is unclear whether regional differences in the availability of those elements or a change in availability due to permafrost thaw is changing microbial decomposer community. Therefore, we analyzed 445 soil depth profiles around the Arctic regarding different element availabilities.

Furthermore, we conducted an incubation experiment to measure the effect of different Si, Ca and P availabilities on the structure of the microbial decomposer community. We found large differences in the availability of Si, Ca, Al, Fe and P in the layers of the panarctic permafrost soils from Canada, Alaska, Russia, Scandinavia, Greenland and Svalbard. There are differences in the distribution of Ca and Si pools over the panarctic permafrost soils. Especially the availability of P is directly linked to the concentration of Ca and Si and the presence of Al and Fe based minerals. With rising temperatures, the thaw depth of the upper horizon may increase and elements stored in deeper layers become potentially mobilized. These processes modify the nutrient availability for microorganisms and by this the production of greenhouse gases like CO₂ and CH₄.

The community structure of bacteria and fungi is related to the availability of Ca and Si. With modified availabilities of Si and Ca, we found direct linear correlations in the changes of the microbial structure at the phylum level for Greenlandic soils. These changes depend on the origin of the soil and the original availability of Ca and Si. We found direct links between the share of gram-positive bacteria and the Ca concentration in both soils and the production of greenhouse gases. The availabilities of these elements may be helpful for better predicting greenhouse gases fluxes in the Arctic as well as element transfer to marine systems.

How to cite: Stimmler, P.: Future Arctic soil nutrient availability and microbial community structure, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6870, https://doi.org/10.5194/egusphere-egu2020-6870, 2020.

The Arctic is warming at twice the rate of the rest of the globe. While it has been increasingly highlighted that thawing permafrost accelerates soil organic matter decomposition, research on biogeochemical N cycling is still underrepresented. Arctic nitrous oxide (N₂O) emissions have long been assumed to have a negligible climatic impact but recently increasing evidence has emerged of N₂O hotspots in the Arctic. Even in small amounts, N₂O has the potential to contribute to climate change due to it being nearly 300 times more potent at radiative forcing than CO₂. Therefore, the ‘NOCA’ project aims to establish the first circumarctic N₂O budget. Following intensive N₂O flux sampling campaigns at primary sites within Northern Russia and soil N₂O concentration measurements from secondary sites across the Arctic, we are now entering the phase of spatial extrapolation. Challenges to overcome are the small-scale heterogeneity of the landscape and incorporating small features that can function as N₂O hotspots. Therefore, as a first step in upscaling the N₂O fluxes, high resolution imagery is needed. We show here novel high-resolution 3D imagery from an unmanned aerial vehicle (UAV), which will be used to upscale N₂O fluxes from plot to landscape scale by linking ground-truth N₂O measurements to vegetation maps. This approach will first be applied to the East cliff of Kurungnakh Island in the Lena River Delta of North Siberia and is based on 2019 sampling campaign data. Kurungnakh Island is characterized by ice- and organic-rich Yedoma permafrost that is thawed by fluvial thermo-erosion forming retrogressive thaw slumps in various stages of activity. Overall, 20 sites were sampled along the cliff and inland, covering the significant topographic and vegetative characteristics of the landscape. The data from this scale will provide the basis for extrapolating, by using a stepwise upscaling approach, to the regional and finally circumarctic scale, allowing a first rough estimate of the current climate impact of N₂O emissions from permafrost affected soils. Available international circumarctic data from this and past projects will be synthesized with an Arctic N₂O database under development for use in future ecosystem and process-based climate model simulations.   

How to cite: van Delden, L., Marushchak, M., Voigt, C., Grosse, G., Faguet, A., Lashchinskiy, N., Kerttula, J., and Biasi, C.: Towards the first circumarctic N2O budget – Extrapolating to the landscape scale, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14347, https://doi.org/10.5194/egusphere-egu2020-14347, 2020.

Airborne radar sounding is the primary geophysical method for directly observing conditions beneath ice sheet and glaciers at the catchment to continent scale. From single flow-lines to regional surveys to ice-sheet wide gridded topographic datasets, radar sounding profiles provide information-rich constraints on the englacial and subglacial environment. This can include roughness, lithology, hydrology, thermal state, melt, fabric, and structure for both grounded and floating ice. However, the snap-shot view provided by one-time soundings fails to capture subsurface processes across the time-scales over which they evolve and control ice flow.  Doing so requires advancing multi-temporal radar sounding instruments, platforms, and data analysis. For example, point-measurements by ground-based or stationary sounder can be used produce local time-series observations of englacial and subglacial conditions. However, low-cost, low-power active and/or passive radar-sounder networks can dramatically extend the reach and scope of such measurements. Further, repeat surveys by sled-drawn or airborne sounders can capture seasonal and interannual subsurface variations. However, digitization of archival radar film are extending the temporal baseline for such comparison by decades, making multi-decadal studies of subsurface changes possible. Finally, the development of autonomous rover, drone, and satellite sounding platforms and systems promise to enable pervasive, stable, and frequent monitoring of subglacial conditions. Here, we discuss the advances, challenges, and the path forward to observing subsurface conditions across the full range spatial and temporal scales at which they occur.

How to cite: Schroeder, D.: Observing Evolving Subglacial Conditions with Mutitemporal Radar Sounding , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-179, https://doi.org/10.5194/egusphere-egu2020-179, 2020.

Radio echo-sounding is a powerful technique for investigating the subsurface of the glaciers. However, physics underlying the formation of the reflected signal is sometimes oversimplified  in the geophysical glacier studies, leading to wrong results. Various remote sensing techniques use different wavelengths (e.g., 13.575 GHz for CryoSat and 20-25/200-600 MHz for ground-penetrating radar), but it is still not clear which particular wavelengths are the best to detect different characteristics of the ice. Possibly, the results gained using different wavelengths may not coincide but rather complement each other due to frequency dependence of the dielectric permittivity and conductivity of snow, ice and especially water.

Here we attempt to construct an electrophysical model of a cold glacier. This mathematical model considers the variability of the depth profile of the complex dielectric permittivity depending on the frequency of the probing radio signal and the surface temperature. A series of calculations of the reflection coefficients of radio waves from the modelled glacier show that at low temperatures for frequencies above 1 MHz the real part of the dielectric constant of the glacier does not change with frequency and surface temperature, but depends on the glacier structure, while the depth profile of the loss tangent is constant throughout the glacier.  As wavelength decreases, the absorption of radio-waves by the glacier decreases and the frequency dependence of the reflection coefficient becomes a periodic function, its period and amplitude depend on the glacier thickness, the dielectric constant of the bedrock and ice on the surface.

The range of radio-waves from 0.1 to 1 MHz is not optimal for sounding cold glaciers: the absorption of radio-waves by ice is large for studying thick layers of the glacier, and the wavelength does not allow studying thin layers. Hence, reflection from the glacier surface prevails upon reflection of the signal. The small absorption of short radio waves by ice leads to the fact that the frequency dependence of the reflection coefficient of short radio-waves is practically the sum of the partial reflections of radio-waves from the surface and internal snow/firn and firn/ice boundaries. Period and amplitude of oscillations of the function  depend on the depth of the internal boundaries and the gradient of dielectric characteristics of ice, snow, firn and bedrock.

Changes in surface temperature, leading to a change in the loss tangent of the upper glacier layers, are manifested in the phase magnitude of the reflection coefficient of radio-waves:it grows with the temperature. Theoretically, the high-frequency signal reflected from the glacier contains information about the structure of the cold glacier and the depth distribution of the dielectric constant, but to restore the electrophysical parameters of the glaciers, it is necessary to use a broadband signal with smooth spectrum and high digitization speed.

The reported study was funded by RFBR, project number 18-05-60080 (“Dangerous nival-glacial and cryogenic processes and their impact on infrastructure in the Arctic”).

How to cite: Yushkova, O., Dymova, T., and Popovnin, V.: Radio-wave reflectivity from cold glaciers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21606, https://doi.org/10.5194/egusphere-egu2020-21606, 2020.

Part of the movement that occurs on all glaciers in Antarctica is a continuous and stable movement that unloads the ice into the sea. The Whillans Ice Plain (WIP) is a portion of the Whillans ice stream that measures 8000 km² for an ice thickness of 800 meters. This glacier has a unique characteristic of moving thanks to tidally modulated stick-slip events twice a day. The slip speed varies laterally across the glacier.  We measured surface wave velocity variations computed from ambient seismic noise cross-correlation. The cross-correlations make it possible to monitor temporally and spatially the seismic velocities at the bed of the glacier, associated with changes in poro-elastic parameters and frictional properties of the glacial till. We averaged our observations for the 78 stick-slip events of our dataset and managed to achieve a 5 min temporal resolution along the 45 min long slip events. The results show a decrease in velocity of about 9% of the S-wave velocity in the subglacial sediment layer about 30 minutes after the initiation of the slip. This velocity drop mainly affects the central part of the glacier. A 10% increase in porosity could induce this velocity decrease due to dilatancy. Dilatant strengthening results from this porosity increase, which in turn keeps the glacier in a slow-sliding regime. The high rate of seismic cycles on such a large scale makes the Whillans ice stream a unique laboratory to study transient aseismic slips in glacial context but also in active tectonic faults one. 

How to cite: Mordret, A., Guerin, G., Rivet, D., Lipovsky, B., and Minchew, B.: Imaging the poro-elastic properties of glacier beds using ambient seismic noise monitoring : application to Whillans ice stream, Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11594, https://doi.org/10.5194/egusphere-egu2020-11594, 2020.

Basal slip is an important mechanism by which glaciers and ice-sheets flow, and is a major source of uncertainty in simulations of ice-mass loss and sea level rise from the Greenland Ice Sheet (GrIS). Sub-ice geology is a dominant control on ice flow velocity with fast flow often coinciding with the presence of deformable subglacial till eroded from underlying sedimentary rocks. The subglacial geology of Greenland has received relatively little attention thus far and its control on ice flow is poorly understood. Seismic studies of the crust beneath the GrIS have been limited due to a lack of seismic stations and the reliance on earthquake event data. However, in the past decade, there has been a rapid increase in the number of both permanent and temporary seismic stations deployed in Greenland as well developments in ambient noise methods, allowing for improved spatial resolution of crustal geology.

Ellipticity measurements, the ratio of the horizontal to vertical component of a Rayleigh wave, have been shown to be particularly sensitive to the geological structure directly beneath the station. Ambient noise H/V measurements have been used for decades in geotechnical and civil engineering for site characterisation, making them a well-suited technique to determine the subglacial geology of the GrIS. Using all available broadband stations deployed on Greenland from 2012 to 2018 we extract Rayleigh wave ellipticity measurement from ambient noise data using the degree-of-polarization (DOP) method where meaningful signals are defined as a waveform with an arbitrary polarization which remains stable for a given time window. We invert these ellipticity measurements in the period range of 4 – 9 s to generate V_s profiles of the first 5 km beneath each station. Our inversions indicate that: (1) off-ice stations along the margins of the GrIS produce a good agreement with the litho1.0 model to within error and (2) an additional subglacial layer 1.0 - 2.0km thick with a V_s < 3.0km is necessary to match the data recorded at several of the on-ice stations. We attribute these observations to the widespread presence of sedimentary rocks beneath the GrIS, potentially capable of sustaining extensive subglacial till layers that can support enhanced basal slip.

How to cite: Jones, G., Kulessa, B., Ferreira, A., Schimmel, M., Berbellini, A., and Morelli, A.: Constraining subglacial geology using ambient noise Rayleigh wave ellipticity , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8274, https://doi.org/10.5194/egusphere-egu2020-8274, 2020.

Parameterization of glacier sliding-laws remains a large source of uncertainty in modeling glacier and ice-sheet flow, requiring validation with experimental and observational data. In the case of ice flowing over a till-free, step-shaped bed, theory predicts bed resistance is independent of glacier sliding speed – a “rate neutral” sliding-law (e.g., Iken, 1981). However, experimental simulation of this system resulted in a notable anti-correlation between sliding speeds and bed resistance – a “rate weakening” sliding-law – that may give rise to basal seismicity (Zoet & Iverson, 2016). To investigate this discrepancy, we conducted a seismic field campaign on Saskatchewan Glacier, which is thought to have a stepped bed like those observed in adjacent glacier forefields. The campaign included a dense, 32 seismometer deployment during the middle of the 2019 melt season, complemented by continuous meteorologic, hydrologic, and GPS observations.

Visual and automated characterization of collected seismic data indicate abundant seismicity near the glacier’s bed. Basal seismicity clusters down-flow from an active moulin and a crevassed region likely connected to the bed. Rates of basal seismicity show a strong diurnal signal, consistently occurring 0.5-4 hours after peak surface melting and subglacial discharge, and continuous GPS data indicate temporary ice-flow acceleration during at least two diurnal seismic cycles. Spikes in seismic rate are also observed during most rain events with shorter response-times than diurnal cycles. The diurnal basal seismic cycle was interrupted by two periods of relative quiescence. The first lasted six days, initiating as mean air temperatures and peak daily subglacial discharge rose, and concluding after mean air temperatures and peak discharge declined. The second lasted one day following an abrupt drop in air temperature and was concurrent with reduced subglacial discharge.

We postulate that rapid surface water delivery to the bed strongly influences basal water pressure near delivery points, triggering bursts of seismicity on parts of Saskatchewan Glacier’s bed. Elevated rates of basal seismicity follow peak hydrologic flux through the subglacial drainage system, indicating that stick-slip motion likely occurs as water pressures fall from a transient. Some seismicity is accompanied by temporary acceleration of the glacier, consistent with results from Zoet & Iverson (2016). The six day period of relative quiescence may reflect reorganization of the subglacial hydrologic system into a more efficient drainage network in seismogenic regions, thus damping water pressure transients. Conversely, the one day quiescent period was likely the result of limited surface-water supply. We propose that temporary transitions from stable to stick-slip sliding occurred when basal water pressure exceed a critical threshold on parts of the bed, as modulated by surface-water supply and subglacial drainage efficiency.

Iken, A. (1981). The Effect of the Subglacial Water Pressure on the Sliding of a Glacier in an Idealized Numerical Model. Journal of Glaciology, 27(97).

Zoet, L. K., & Iverson, N. R. (2016). Rate-weakening drag during glacier sliding. Journal of Geophysical Research: Earth Surface, 121, 1328–1350. https://doi.org/10.1002/2016JF003909

How to cite: Stevens, N., Roland, C., Hansen, D., Schwans, E., and Zoet, L.: Basal Seismicity Forced by Surface-Water Supply on a Stepped-Bed Glacier: Saskatchewan Glacier, Alberta, Canada, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10952, https://doi.org/10.5194/egusphere-egu2020-10952, 2020.

The climatic conditions over ice sheets at the time of snow deposition and compaction imprint distinctive crystallographic properties to the resulting ice. As the snow gets buried, its macroscopic structure evolves due to vertical compression but retains traces of the climatic imprint that generate distinctive mechanical, thermal and optical properties. Because climate alternates between glaciar periods, that are colder and dustier, and interglacial periods, the ice sheets are composed from layers with alternating mechanical properties. Here we compare ice core dust content and crystal orientation fabrics, from the ice core records, with englacial vertical strain-rates, measured with a phase-sensitive radar (ApRES), at South Pole and EPICA Dome C ice cores. Similarly to previous observations, we show that ice deposited during glacial periods develops stronger crystal orientation fabrics. In addition, we show that ice deposited during glacial periods is harder to vertically compress and horizontally extend, up to about 3 times, but softer to shear. These variations in mechanical properties are typically ignored in ice-flow modelling but they could be critical to interpret ice core records. Also, we show that the changes in crystal orientation fabrics due to transitions from interglacial to glacial conditions can be detected by phase-sensitive radar. This information can be used to constrain age-depth in future ice-core locations.

How to cite: Martin, C., Conway, H., Koutnik, M., Ritz, C., Bauska, T., Drews, R., and Ershadi, M. R.: Climatic imprint in the mechanical properties of ice sheets and its effect on ice flow: Observations from South Pole and EPICA Dome C ice cores, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19541, https://doi.org/10.5194/egusphere-egu2020-19541, 2020.

Firn densification operates at the boundary between the atmosphere and ice sheets, impacting estimates of ice thickness change, paleoclimate reconstructions, and near-surface hydrology. Direct measurements of firn densification are scarce and firn densification models, which rely mostly on point measurements of density, disagree on the impact of environmental factors like surface temperature and accumulation rate. Phase-sensitive radar systems (pRES) can observe the movement of isochronal layers in firn and ice by tracking the relative two-way travel times (T) of radio waves. In this work, we demonstrate three methods for extracting measurements of densification velocities from pRES data. Method one uses independently measured firn densities to derive compaction velocities. Method two derives vertical velocities in the firn from an inversion that assumes a steady state and exponential density profile. Method three models changes in T using a semi-physical densification model and compares these changes to the pRES observations. We apply each method to radar data from three ice rises in the Weddell Sea Sector of West Antarctica. Results demonstrate how pRES can be used to explore the accumulation dependence of steady-state densification rates. Average accumulation rate is estimated from pRES measurements in areas that are approximately in steady state. Accumulation gradients can be seen across divides (Korff Ice Rise) and densification-rate differences are observed between relatively high (Fletcher Promontory) and low (Skytrain Ice Rise) accumulation environments. With minimal logistic requirements, new pRES systems like autonomous pRES could be inexpensively deployed to monitor firn densification. Furthermore, existing data may contain densification information even in cases when its deployment primarily targeted other processes.

How to cite: Case, E. and Kingslake, J.: Three methods for observing firn densification velocities with phase-sensitive radar, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5759, https://doi.org/10.5194/egusphere-egu2020-5759, 2020.

The transformation of snow into ice is a fundamental process in glaciology. The yearly accumulation of fresh snowfall increases the overburden pressure, changing the snow’s properties such that it transitions into firn and pure glacier ice thereafter. Additionally, periods of melt and variations in subsurface and surface conditions can lead to the presence of ice layers and firn aquifers within the firn column. Therefore, firn characteristics provide a tool for evaluating past and present climate conditions relating to the amount of snow accumulation, melt, temperature conditions and the subsequent preservation of the snow.

Due to the importance of relationships between firn and other glaciological processes (e.g., settling, sublimation, recrystallization and other deformation processes) it has not been possible to develop a theoretically-based model which accurately predicts firn properties with depth. Therefore, methods of measuring firn are either intrusive or rely on (potentially unreliable) empirical conversions. Full Waveform Inversion (FWI) may offer a new standard for glaciological seismic modelling, mitigating issues within current seismic modelling techniques and paving the way for the recovery of elastic properties, including density. Constraining firn properties also leads to improved corrections for deeper seismic responses, e.g. glacier bed reflectivity.

Using seismic datasets obtained from Norway’s Hardangerjøkulen Ice Cap (60.47°N, 7.49°W) along with varying synthetic firn column scenarios (introducing the presence of ice lenses and firn aquifers), we show how FWI can mitigate the dependence on intrusive techniques and empirical relationships. Furthermore, we compare the robustness of the FWI approaches versus traditional glaciological approaches to velocity model building (Herglotz-Wiechert inversion).

How to cite: Pearce, E., Booth, A., Sava, P., and Jones, I.: Seismic Full Waveform Inversion (FWI) to characterise the structure of firn, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5307, https://doi.org/10.5194/egusphere-egu2020-5307, 2020.

Geoelectrical methods are increasingly used for non-invasive characterization and monitoring of permafrost sites, since the electrical properties of the subsoil are sensitive to the phase change of liquid to frozen water. In this context, electrical subsurface parameters act as proxies for temperature and ice content.  However, it is still challenging to distinguish between air and ice in the pore space of the rock based on the resistivity method alone due to their similarly low electrical conductivity. This ambiguity in the subsurface conduction properties can be reduced by considering the spectral electrical polarization signature of ice using the Spectral Induced Polarization (SIP) method, in which the complex, frequency-dependent impedance is measured. These measurements are hypothesized to allowing for the quantification of ice content (and thus differentiation of ice and air), and for the improved thermal characterization of alpine permafrost sites.

In the present study, vertical SIP sounding measurements have been made at different alpine permafrost sites in a frequency range from 100 mHz to 45 kHz. From borehole temperature measurements, we know the thermal state of these sites during our SIP soundings, i.e., an active layer thickness of about 4 m at the Schilthorn field site. In order to understand and to calibrate ice and temperature relationships, the electrical impedance was likewise measured on water-saturated soil and rock samples from these field sites in a frequency range from 10 mHz to 45 kHz during controlled freeze-thaw cycles (+20°C to -40°C) in the laboratory.

For field and laboratory measurements, the resistance (impedance magnitude) shows a similar temperature dependence, with increasing resistance for decreasing temperatures. For each sample, the impedance phase spectra exhibit the well-known temperature-dependent relaxation behavior of ice at higher frequencies (1 kHz - 45 kHz), with an increasing polarization magnitude for lower temperatures or larger depths of investigation, respectively. At lower frequencies (1 Hz - 1 kHz), a polarization with a low frequency dependence is observed in the unfrozen state of the samples. We interpret this response as membrane polarization, considering that it decreases in magnitude with decreasing temperature (i.e., with ongoing freezing).

Using the independently measured borehole temperature data, a systematic comparison of the SIP laboratory and field measurements indicates the possibility of a thermal characterization of an alpine permafrost site using SIP.

How to cite: Limbrock, J. K., Weigand, M., and Kemna, A.: Improved thermal characterization of alpine permafrost sites by broadband SIP measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20081, https://doi.org/10.5194/egusphere-egu2020-20081, 2020.

Abstract. The climate change has aroused great concern on the stability and durability of the infrastructure installed on permafrost, especially for frozen saline clay with a large amount of unfrozen water content at subzero temperature. The joint electrical resistivity and acoustic velocity measurements are conducted for frozen saline sand and onsøy clay with 50% clay content and 20~40 g/L salinity in order to determine the unfrozen water content. A systematic program of tests involves the saline sand with different salinity, natural onsøy clay with the variable of temperature and freezing-thawing cycles and reconstituted onsøy clay with distinctive density and salinity. The data analysis of measurement results in combination with previous joint measurements for frozen soil resolves the effect of temperature, salinity, soil type and freezing-thawing cycles on the acoustic and electrical properties. An increase of temperature, fine content and salinity results in a decrease of both acoustic velocity and electrical resistivity. Electrical resistivity is sensitive to salinity, while acoustic velocity changes substantially near thawing temperature. We also find that both natural and reconstituted clay with similar water content and salinity show quite different acoustic velocity and electrical resistivity, which indicates that ice crystal structures are distinctive between natural and reconstituted samples. Besides, P-wave velocity is much more sensitive to the fabric change or induced cracks than electrical resistance during freezing-thawing cycles.  In the end, acoustic models like the weighted equation (Lee et al., 1996), Zimmerman and King’s model (King et al., 1988) and BGTL (Lee, 2002) are applied to the UWS estimates based on P-wave velocity and electrical models like Archine’s law are adopted based on electrical resistance. Both estimated UWS from different methods is not always consistent. The difference can be up to 20%.

Keywords: Frozen Saline Clay, Acoustic Velocity, Electrical Resistance, Unfrozen Water Saturation

References:

King, M. S., Zimmerman, R. W., & Corwin, R. F. (1988, May). Seismic and Electrical-Properties of Unconsolidated Permafrost. Geophysical Prospecting, 36(4), 349-364. https://doi.org/10.1111/j.1365-2478.1988.tb02168.x

Lee, J. S. (2002). Biot–Gassmann theory for velocities of gas hydrate-bearing sediments.

Lee, M. W., Hutchinson, D. R., Collett, T. S., & Dillon, W. P. (1996). Seismic velocities for hydrate-bearing sediments using weighted equation. Journal of Geophysical Research: Solid Earth, 101(B9), 20347-20358. https://doi.org/10.1029/96jb01886

How to cite: Lyu, C., Ingeman-Nielsen, T., Ali Ghoreishian Amiri, S., Reidar Eiksund, G., and Grimstad, G.: Joint Acoustic and Electrical Measurements for Unfrozen Water Saturation of Frozen Saline Soil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10334, https://doi.org/10.5194/egusphere-egu2020-10334, 2020.

Rock glaciers play an important hydrological role in the semiarid Andes (SA; 27º-35ºS). They cover about three times the area of uncovered glaciers and they are an important contribution to streamflow when water is needed most, especially during dry years and in the late summer months. Their characteristics such as their extension in depth and their ice content is poorly known. Here, we present a case study of one active rock glacier and periglacial inactive geoform in Estero Derecho (~30˚S), in the upper Elqui River catchment, Chile. Three geophysical methods (ground-penetrating radar and electrical resistivity and seismic refraction tomography) were combined to detect the presence of ice and understand the internal structure of the landform. The results suggest that the combination of electrical resistivity and seismic velocity provide relevant information on ice presence and their geometry. Radargrams shows diffraction linked to boulders presence but some information regarding electromagnetic velocity could be extracted. These results strongly suggest that such landforms contain ice, are therefore important to include in future inventories and should be considered when evaluating the hydrological importance of a particular region.

How to cite: valois, R., Schafer, N., De Pasquale, G., Navarro, G., and MacDonell, S.: Geophysical characterization of inactive/active rock glaciers in the semi-arid Andes using seismic, geoelectrics and GPR, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11808, https://doi.org/10.5194/egusphere-egu2020-11808, 2020.

The Antarctic Ice Sheet has undergone extensive retreat and re-advance in its glacial-interglacial history. With progressing anthropogenic climate change, the associated ice dynamics and feedbacks could further lead to persistent and potentially irreversible ice loss from Antarctic drainage basins in the future.

Process-based models, in combination with paleo and modern records, provide the tools to reconstruct the glacial-interglacial history of the Antarctic Ice Sheet, to improve our understanding of the involved processes and critical thresholds, and to better anticipate possible future pathways.

Here we present simulations of the Antarctic Ice Sheet over the past two glacial cycles using the Parallel Ice Sheet Model PISM. As the conditions in particular at the base of the ice sheet are weakly constrained, and proxy data for the climatic forcing over the last glacial cycles is sparse, we assess the sensitivity of the model response with respect to the choice of boundary conditions. We further conduct an ensemble analysis in order to systematically constrain uncertainties with respect to representative model parameters associated with ice dynamics, climatic forcing, basal sliding and bed deformation.

Based on the insights into the dynamic threshold behavior and estimates of the ice sheet’s contributions to global sea-level changes in the past, we investigate the long-term future stability of the Antarctic Ice Sheet under different levels of global warming. We show that the ice sheet exhibits a multitude of temperature thresholds beyond which ice loss into the ocean becomes irreversible. Each of these thresholds gives rise to hysteresis behavior, meaning that the currently observed ice-sheet configuration cannot be regained even if temperatures were to be reversed to their present-day levels.

How to cite: Winkelmann, R., Albrecht, T., Garbe, J., Donges, J., and Levermann, A.: Antarctic ice dynamics - from deep past to deep future, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19740, https://doi.org/10.5194/egusphere-egu2020-19740, 2020.

Glacial thermal processes exert a fundamental control on ice flow, governing viscosity and frozen-to-thawed transitions at the ice-bed interface. Across Antarctica, frozen bed regions characterized by numerical models and geophysical observations, can also reduce ice flow by increasing basal traction. Some frozen bed regions can separate or confine fast-flowing glaciers and ice streams. Others separate inland catchments with thawed beds from the grounding zone of marine ice-sheet sectors. If regions with frozen bed experienced thawing, such a transition may lead to ice-sheet acceleration, reconfiguration, or retreat. To investigate the potential impact of such a thermal transition, we use the JPL/UCI Ice Sheet System Model (ISSM) to identify vulnerable regions across Antarctica that are close to the basal melting point. We assess the impact of thawing these regions by quantifying resulting volume changes and surface expressions. This allows us to identify the areas of the ice sheet where the thermal regime at the ice-bed interface has the largest potential impact on ice-sheet stability and sea-level contribution. We also examine the potential basal temperature and thaw-propagation thresholds governing this process. We then compare the ISSM results to a selection of ice-penetrating radar sounding observations to refine our constraints of the configuration, distribution, and extent of these thermally critical areas.

How to cite: Dawson, E., Schroeder, D., Chu, W., Mantelli, E., and Seroussi, H.: Investigating basal thaw as a potential driver of ice flow acceleration in Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1296, https://doi.org/10.5194/egusphere-egu2020-1296, 2020.

Our knowledge of the past surface mass balance on Greenland depends on scarce paleoclimate reconstructions and uncertain climate simulations. However, reconstructions of the internal layering of the ice sheet can provide an independent dataset of accumulation. The thickness of isochronal layers is directly affected by accumulation, but modified over time by the flow of ice. Existing methods can disentangle these two effects only near the ice divide where assumptions of stationarity may be justified. To solve this problem and to obtain a spatially comprehensive reconstruction of accumulation, we use an ice sheet model with an isochronal grid. Thinning rates are calculated prognostically by the model and can be used to define an inverse problem that can be solved iteratively. The only input data is the final layer thickness of the target, e.g., reconstructed radio echo layers from the Greenland ice sheet. To test this method and its limitations, we reconstruct the accumulation histories from the stratigraphies of simulations for which the idealized accumulation time series and spatial distributions are known. These simulations represent a two-dimensional cross section of the Greenland ice sheet. The results are robust to a wide range of realistic variations in accumulation for all but the layers closest to the bedrock where the deformation by the flow is most severe.

How to cite: Theofilopoulos, A. and Born, A.: Reconstructing surface mass balance from the Greenland ice sheet stratigraphy., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18618, https://doi.org/10.5194/egusphere-egu2020-18618, 2020.

The Cordilleran Ice Sheet (CIS) repeatedly covered western Canada during the Pleistocene and attained a volume and area similar to that of the present-day Greenland Ice Sheet. Deglaciation of the CIS following the Last Glacial Maximum (LGM) directly affected atmosphere and ocean circulation, eustatic sea level, and human migration from Asia to North America. It has recently been shown that the rapid climate oscillations at the end of the Pleistocene had a dramatic effect on the CIS. Data on glacial isostatic adjustment and cosmogenic nuclide exposure ages indicate that abrupt warming at the onset of the Bølling-Allerød caused significant thinning of the ice sheet, resulting in a fifty percent reduction in mass, while the Younger Dryas cooling caused the expansion of alpine glaciers across the mountains of western Canada. However, the mountainous subglacial terrain makes it challenging to reconstruct the regional-scale deglaciation dynamics of the ice sheet, and its configuration during this period of rapid change remains poorly constrained. 

Here we use the glacial landform record to reconstruct the ice sheet configuration for the central sector of the CIS, over the Cassiar and Omineca Mountains in northern British Columbia, during the Late Pleistocene climate reversals. We present the first regional-scale reconstruction of the CIS following the Bølling-Allerød warming, whereby the ice sheet was reduced to a labyrinth of valley glaciers fed by ice dispersal centres located over the Skeena Mountains in the south and Coast Mountains in the west. Additionally, numerous lateral and terminal late glacial moraines delineate the extent of alpine glaciers, ice caps and ice fields that regrew on mountain peaks above the CIS during the Younger Dryas. Cross-cutting relationships indicate that the valley glaciers of the CIS were slower to respond to the Younger Dryas cooling than the mountain glaciers.

How to cite: Dulfer, H. and Margold, M.: Reconstructing the Cordilleran Ice Sheet in northern British Columbia during the Late Pleistocene climate reversals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-480, https://doi.org/10.5194/egusphere-egu2020-480, 2020.

Compared with the intensely studied interval of the last glacial maximum and termination, significantly less attention has been given to the preceding glacial period and Termination 2. This is perhaps understandable as the Greenland ice cores do not stretch this far back in time and the terrestrial record of the ice sheets has in part been lost during the subsequent glacial period. However, there are many questions remaining about how these two glacial intervals differed and whether this was important in driving some of the differences between the last interglacial and our current interglacial. Here I will focus on a new speleothem record from northern Spain which records the meltwater-driven d¹⁸O anomaly in the eastern North Atlantic and provides an absolutely dated chronology (U/Th) during the penultimate glacial and Termination 2. The character of which differs dramatically from a record for Termination 1 recovered from speleothems at the same site. This record also shows structure during Termination 2 that has not been seen previously. Here I’ll discuss some possible reasons for the differences between the two glacial terminations and focus on recent ice sheet model experiments that have been run to try and test these hypotheses.

How to cite: Gasson, E., Stoll, H., and Cacho, I.: Contrasting style and retreat rates of the northern hemisphere ice sheets during the past two glacial terminations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20521, https://doi.org/10.5194/egusphere-egu2020-20521, 2020.

Glaciotectonic disturbance of sediments and tunnel valleys are often found near the margin of former ice sheets. Hence, these landforms can be used to reconstruct the dynamics of former ice sheet margins. The direction of thrusts usually points perpendicular to the ice front. Considering heterogeneity due to local ice advances, this relation can be used to infer the regional forward direction of large ice lobes. Here, we present a dense grid of high-resolution 2D multi-channel reflection seismic data from the German sector of the southeastern North Sea imaging a buried glaciotectonic complex and tunnel valleys in unprecedented detail.

We have identified individual thrust sheets in an area of approx. 650 km² (combined with recent results of Winsemann et al. (2020)). All thrust sheets are buried and partly eroded at their top. Two major phases of thrusting with two corresponding detachment surfaces have been identified in the subsurface, of which the younger phase led to the deformation of sediments several kilometers further into the foreland. The thickness of individual thrust sheets differs between 180 and 240 m. Some thrust sheets have been cut by the subsequent formation of tunnel valleys with an overall incision direction ranging from east-west to northeast-southwest. The glaciotectonic complex is limited to its southeast by an updipping reflector, which represents the margin of a source depression.

The restauration of cross-sections shows that the thrust sheets transported sediments over more than a kilometer towards the northwest to west, which relates the formation of the thrust sheets and the source depression. The landforms are very similar to a hill-hole pair that led to the foreland thrust sheets, probably as a result of combined bulldozing and gravity spreading in the foreland of the ice margin. Their occurrence and the adjacent tunnel valleys leads us to assume that we identified the marginal position of an Elsterian ice lobe in the southeastern North Sea.

Reference:
Winsemann, J., Koopmann, H., Tanner, D.C., Lutz, R., Lang, J., Brandes, C., Gaedicke, C., 2020. Seismic interpretation and structural restoration of the Heligoland glaciotectonic thrust-fault complex: Implications for multiple deformation during (pre-)Elsterian to Warthian ice advances into the southern North Sea Basin. Quat. Sci. Rev. 227, 1–15. https://doi.org/10.1016/j.quascirev.2019.106068

How to cite: Lohrberg, A., Krastel, S., Unverricht, D., and Schwarzer, K.: Glaciotectonics and tunnel valleys in the southeastern North Sea imaged by high-resolution multi-channel seismics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8691, https://doi.org/10.5194/egusphere-egu2020-8691, 2020.