CR3.3

Subglacial Environments of Ice Sheets and Glaciers

Subglacial environments are among the least accessible regions on Earth and represent one of the last physical frontiers of glaciological research, while emerging as a unique ecological habitat. The subglacial environment is a key component in the dynamic behaviour of ice sheets and glaciers, involving complex and precise mass and energy transfers between the ice and its substrate of water, air, bedrock, or sediment, and the oceans at ice sheet boundaries. In particular, determining the distribution and nature of water flows at the ice-mass bed is highlighted as a priority for understanding and predicting ice dynamics. For example, both remote sensing and ground-based observations across Antarctica and Greenland highlight the existence of subglacial water in a variety of forms, ranging from vast subglacial lakes (providing distinctive habitats for potentially unique life forms) to mm-thick water flows at the ice-substrate interface. Feedbacks between increased surface melting, glacier bed conditions and ice flow also affect alpine glaciers, potentially contributing to increased glacial retreat in low and mid-latitude mountain regions.

It is clear that subglacial processes impact ice dynamics, transcending ice-mass scales from valley glaciers to large ice sheets and, through feedback loops, contribute to changes in sea level, ocean circulation, and climate evolution. Quantitative characterisation of the basal environment therefore remains an outstanding glaciological problem, as does scaling of this knowledge for use in modelling ice sheet and glacier behaviour.

We invite scientific contributions that include, but are not limited to, measurements and/or modelling of: (i) flow of subglacial water at the bed and through subglacial sediments; (ii) linkages between subglacial hydrology and ice dynamics; (iii) theoretical-, field-, or laboratory-based parameterisation of subglacial processes in numerical ice-flow models; (v) formation, geometry and potential hydrological linkages between subglacial lakes; (v) subglacial and supraglacial lake drainage and subglacial floods from ice margins; and (vi) geomorphological evidence of subglacial water flows from contemporary ice-sheet margins and across formerly glaciated continental-scale regions.

Co-organized by GM7
Convener: Bryn Hubbard | Co-conveners: Rebecca Schlegel, Robert Bingham, Adam Booth, Harold Lovell
vPICO presentations
| Wed, 28 Apr, 13:30–15:00 (CEST)

vPICO presentations: Wed, 28 Apr

Chairpersons: Bryn Hubbard, Rebecca Schlegel
13:30–13:35
13:35–13:37
|
EGU21-535
Pascal Egli, Stuart Lane, James Irving, and Bruno Belotti

If tongues of temperate Alpine glaciers are subjected to high temperatures their topography may change rapidly due to the effects of differential melt related to aspect and debris cover. Independent of local surface melt, the position of subglacial conduits may have an important influence on ice creep and so on changes in topography at the ice surface. This reflects analyses that suggest that subglacial conduits at glacier margins may not be permanently pressurised; and that creep closure rates are insufficient to close subglacial conduits completely. Rapid climate warming may exacerbate this process, due both to surface-melt driven glacier thinning and over-enlargement of conduits due to high upstream melt rates. Over-enlarged conduits that are not permanently pressurised would lead to the development of structural weaknesses and eventual collapse of the ice surface into the conduits. We hypothesise that this collapse mechanism could represent an important and alternative driver of rapid glacier retreat.

In this paper we combine: (1) an extensive survey of glacier margin collapse in the Swiss Alps with (2) intensive monitoring of the dynamics of such collapse at the Otemma Glacier in the south-western Swiss Alps. Daily UAV surveys were undertaken at a high spatial resolution and with precise and accurate ground control. These datasets were used to generate surface change information using SfM-MVS photogrammetry. Surfaces of difference showed surface loss that could not be related to ablation alone. Combining them with three-dimensional ground-penetrating radar (GPR) surveys in the same zone showed that the surface loss was coincident spatially with the positions of sub-glacial conduits, for ice thicknesses between 20 m and 50 m. We show that this form of subglacial conduit collapse is also happening for several other glaciers in the Swiss Alps, and that this mechanism of snout collapse and glacier retreat has become more common than has hitherto been the case. It also leads to temporal patterns of glacier margin retreat that differ from those that might be expected due to glacier mass balance and ice mass flux effects alone.

How to cite: Egli, P., Lane, S., Irving, J., and Belotti, B.: Temperate Alpine glacier surface dynamics linked to collapsing subglacial conduits, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-535, https://doi.org/10.5194/egusphere-egu21-535, 2021.

13:37–13:39
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EGU21-1080
Stephen Livingstone, Helgi Björnsson, Jade Bowling, Winnie Chu, Christine Dow, Helen Fricker, Yan Li, Malcolm McMillan, Jill Mikucki, Felix Ng, Neil Ross, Anja Rutishauser, Rebecca Sanderson, Martin Siegert, Matthew Siegfried, Andrew Sole, and Kate Winter

Subglacial lakes provide habitats for life and can modulate ice flow, basal hydrology, biogeochemical fluxes and geomorphic activity. They have been identified widely beneath the ice sheets of Antarctica and Greenland, and detected beneath the ice caps on Devon Island and Iceland, and beneath small valley glaciers. Past investigations focussed on lakes beneath individual ice masses. A scientific synthesis of different lake populations has not been made, so a unified understanding of the mechanisms controlling subglacial lake formation, dynamics, and interaction with other parts of the Earth system is lacking. Here, we integrate existing, often disparate data into a global database of subglacial lakes, enabling subglacial lake characteristics and dynamics to be classified. We use this assessment to evaluate how subglacial lakes shape microbial ecosystems and influence ice flow, subglacial drainage, sediment transport and biogeochemical fluxes. Through our global perspective, we examine how subglacial lake characteristics and function depend on the hydrologic, dynamic and mass balance regime of the ice mass beneath which they are located. By applying this synoptic understanding and perspective, we propose a conceptual model for how subglacial lakes and their impacts on the broader environment will change in a warming world. 

How to cite: Livingstone, S., Björnsson, H., Bowling, J., Chu, W., Dow, C., Fricker, H., Li, Y., McMillan, M., Mikucki, J., Ng, F., Ross, N., Rutishauser, A., Sanderson, R., Siegert, M., Siegfried, M., Sole, A., and Winter, K.: Global synthesis of subglacial lakes and their changing role in a warming climate, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1080, https://doi.org/10.5194/egusphere-egu21-1080, 2021.

13:39–13:41
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EGU21-1099
Basile de Fleurian, Petra M. Langebroeke, and Richard Davy

In recent years, temperatures over the Greenland ice sheet have been rising, leading to an increase in surface melt. This increase however can not be reduced to a simple number. Throughout the recent years we have seen some extreme melt seasons with melt extending over the whole surface of the ice sheet (2012) or melt seasons of lower amplitudes but with a longer duration (2010). The effect of those variations on the subglacial system and hence on ice dynamic are poorly understood and are still mainly deduced from studies based on mountain glaciers.

Here we apply the Ice-sheet and Sea-level System Model (ISSM) to a synthetic glacier with a geometry similar to a Greenland ice sheet land terminating glacier. The forcing is designed such that it allows to investigate different characteristics of the melt season: its length, intensity or the spatial extension of the melt. Subglacial hydrology and ice dynamics are coupled within ISSM is coupled to a subglacial hydrology model, allowing to study the response of the system in terms of subglacial water pressure and the final impact on ice dynamics. Of particular interest is the evolution of the distribution of the efficient and inefficient component of the subglacial drainage system which directly impacts the water pressure evolution at the base of the glacier.

We note that the initiation of the melt season and the intensity of the melt at this period is a crucial parameter when studying the dynamic response of the glacier to different melt season characteristics. From those results, we can infer a more precise evolution of the dynamics of land terminating glaciers that are heavily driven by their subglacial drainage system. We also highlight which changes in the melt season pattern would be the most damageable for glacier stability in the future.

How to cite: de Fleurian, B., Langebroeke, P. M., and Davy, R.: The meltwater feedbacks on ice dynamics, influence of melt amplitude, duration and extent., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1099, https://doi.org/10.5194/egusphere-egu21-1099, 2021.

13:41–13:43
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EGU21-1809
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ECS
Shuai Yan, Donald D. Blankenship, Duncan A. Young, Jamin S. Greenbaum, Lin Li, Anja Rutishauser, Jingxue Guo, Jason L. Roberts, Tas D van Ommen, and Bo Sun

The Princess Elizabeth Land (PEL) sector of the East Antarctic Ice Sheet, one of the largest grounded ice reservoirs in Antarctica, is adjacent to regions that experienced significant change during the last glacial maximum. The identification of subglacial water in PEL (to date only inferred from satellite image data) would provide important constraints on our estimation of the basal thermal condition in this region. Also, the existence of a large subglacial hydrology system in PEL comes with potential impacts on the basal melting rate and stability of downstream ice shelves, such as the West Ice Shelf. Here we present geophysical evidences confirming the existence of a large subglacial lake in PEL, hereby referred as Lake Snow Eagle (LSE), for the first time, using recently acquired aerogeophyscial data by international collaborations. We estimate LSE to be about 42 km in length and 370 km2 in area, making it one of the largest subglacial lakes in Antarctica. LSE is shown to lie in a subglacial canyon system that is linked to the coastal ice shelves, which makes LSE the first known major Antarctic interior water body that has a potential direct hydrological pathway into the ocean. We then systematically investigate its geological characteristics and bathymetry by 2-D geophysics modellings. We estimate the water volume of LSE to be about 21 km3, while the sediment volume to be about 20 km3. Our geophysical modelling results also suggest that LSE is located along a compressional geologic boundary, indicating possible tectonic controls over LSE.

How to cite: Yan, S., Blankenship, D. D., Young, D. A., Greenbaum, J. S., Li, L., Rutishauser, A., Guo, J., Roberts, J. L., Ommen, T. D. V., and Sun, B.: A large tectonic-controlled subglacial lake with ocean drainage in Princess Elizabeth Land, East Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1809, https://doi.org/10.5194/egusphere-egu21-1809, 2021.

13:43–13:45
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EGU21-2183
|
ECS
James Kirkham, Kelly Hogan, Robert Larter, Ed Self, Ken Games, Mads Huuse, Margaret Stewart, Dag Ottesen, Neil Arnold, Jeremy Ely, and Julian Dowdeswell

The geological record of landforms produced beneath deglaciating ice sheets offers insights into otherwise inaccessible subglacial processes. Large subglacial channels formed by meltwater erosion of sediments (tunnel valleys) are widespread in formerly glaciated regions such as the North Sea. These features have the potential to inform basal melt rate parameterisations, realistic water routing and the interplay between basal hydrology and ice dynamics in numerical ice‑sheet models; however, the mechanisms and timescales over which tunnel valleys form remain poorly understood. Here, we present a series of modelling experiments, informed by geophysical observations from novel high-resolution 3D seismic data (6.25 m bin size, ~3.5 m vertical resolution), which test different hypotheses of tunnel valley formation and calculate the rates at which these features likely form beneath deglaciating ice sheets. Reconstructions of the former British-Irish and Fennoscandian ice sheets from a 3D thermomechanical ice‑sheet model (BRITICE CHRONO version 2) are used to calculate subglacial water routing and steady-state water discharges as these ice sheets retreated across the North Sea Basin during the last glaciation. Using these simulations, we calculate potential meltwater channel erosion rates and estimate how quickly tunnel  valleys are formed beneath deglaciating ice sheets in warmer than present-day climates. We find little evidence for widespread water ponding which may have led to channel formation through outburst floods. Instead, our results demonstrate that seasonal surface melt delivered to the bed could incise large channels of comparable dimensions to tunnel valleys over timescales of several hundred years as these ice sheets deglaciated.  

How to cite: Kirkham, J., Hogan, K., Larter, R., Self, E., Games, K., Huuse, M., Stewart, M., Ottesen, D., Arnold, N., Ely, J., and Dowdeswell, J.: Exploring mechanisms and rates of tunnel valley formation beneath deglaciating mid-latitude ice sheets using high-resolution 3D seismic data and numerical modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2183, https://doi.org/10.5194/egusphere-egu21-2183, 2021.

13:45–13:47
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EGU21-2645
Louise Borthwick, Atsuhiro Muto, and Sridhar Anandakrishnan

Subglacial topography and bed character are important controls on glacier and ice-sheet flow. Previous studies using reflection-seismic data from the upper half of Thwaites Glacier, West Antarctica, have shown variations in the bed character in the along-flow direction with continuous soft bed in the flatter “lowland” areas and a mix of soft and hard bed over more elevated, rugged “highland” areas. Here we use long-offset reflection/refraction seismic and aerogravity data over a ~40-km section 230-km inland of the current grounding line to model the upper-crustal structures and relate them to the previously identified bed-character variability. We identified a sedimentary basin ~11-km long and up to ~400-m deep beneath the lowland area with continuous soft bed. The downstream end of this sedimentary basin aligns with the transition from the lowland to highland area which indicates its existence could be related to the formation of the subglacial topography. The sedimentary basin is a graben or half-graben potentially formed due to rifting associated with the development of the West Antarctic Rift System, suggesting tectonic influence on the bed character variability and, in turn, on the glacier flow. We will further analyze the seismic reflection data and also add aeromagnetic data to model the crustal structures more accurately and clarify the potential tectonic control on bed-character variability.

How to cite: Borthwick, L., Muto, A., and Anandakrishnan, S.: Tectonic Influence on Bed-Character Variability under Thwaites Glacier, West Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2645, https://doi.org/10.5194/egusphere-egu21-2645, 2021.

13:47–13:49
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EGU21-2847
|
ECS
Ole Zeising and Angelika Humbert

The origin of Greenland’s largest ice stream – the Northeast Greenland Ice Stream (NEGIS) – is located far inland of the Greenland Ice Sheet. High surface flow velocities in the center of NEGIS are attributed to the lubrication of the ice sheet base facilitated by basal melt water. In order to derive basal melt rates at the EastGRIP drill site (~2668 m thick ice), we performed in-situ measurements with an autonomous phase-sensitive radar (ApRES; Brennan et al., 2014; Nicholls et al., 2015) in two consecutive years. The precise processing method (Stewart et al., 2019 and Vankova et al., 2020) detects englacial and basal vertical displacements, but it is limited due to noisy data in the lower half of the ice column. Thus, we made assumptions for the vertical strain in the lower half and adapted simulation results (Rückamp et al., 2020). We found melt rates ranging from 0.16 to 0.22 m/a, which is extremely large for inland ice. However, our results are only slightly above melt rates from previous studies (Fahnestock et al., 2001 and MacGregor et al., 2016) which found melt rates of 0.10 m/a and more through airborne radar measurements (evaluated using radiostratigraphy methods) in the vicinity of EastGRIP. Melt rates of >0.16 m/a require a heat flux into the ice of >1.55 W/m2 which is not exclusively the geothermal heat flux, as also the subglacial hydrological system may supply a significant heat flux into the ice.

How to cite: Zeising, O. and Humbert, A.: Indication of high basal melting at EastGRIP drill site on the Northeast Greenland Ice Stream, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2847, https://doi.org/10.5194/egusphere-egu21-2847, 2021.

13:49–13:51
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EGU21-3490
Stephan Gruber, Rupesh Subedi, and Steven V. Kokelj

A 2015 drilling campaign near Lac de Gras has recovered permafrost core interpreted to contain preserved basal ice of the Laurentide Ice Sheet (Subedi et al., 2020). Previous samples of basal ice from ice sheets originate from coring, usually beneath modern ice divides, modern margins of Arctic icecaps that have preserved basal ice-sheet ice, or from studies near the margins of former ice sheets. The present study may be the first evidence of basal ice a few hundred kilometers from ice divides. In this intermediate zone, rates of erosion beneath an ice sheet increase and the thermal regime at the base varies. Our finding is of applied relevance because it highlights the mosaic character of a landscape that contains terrain types with non-negligible ground-ice content, poised for climate-driven thaw and landscape change. The occurrence and mosaic character of preserved ice may be reconciled with glaciological theory and observations from mineral prospecting using the theory on the genesis of dispersal plumes in till developed by Hooke et al. (2013). The existence of preserved basal ice opens basic-research opportunities alongside exploration, mining and infrastructure development in the area.  

Hooke, R. L. B., Cummings, D. I., Lesemann, J. E., and Sharpe, D. R.: Genesis of dispersal plumes in till, Can. Jo. Earth Sci., 50, 847–855, https://doi.org/10.1139/cjes-2013-0018, 2013.

Subedi, R., Kokelj, S. V., and Gruber, S.: Ground ice, organic carbon and soluble cations in tundra permafrost soils and sediments near a Laurentide ice divide in the Slave Geological Province, Northwest Territories, Canada, The Cryosphere, 14, 4341–4364, https://doi.org/10.5194/tc-14-4341-2020, 2020.

How to cite: Gruber, S., Subedi, R., and Kokelj, S. V.: Relict basal ice from the Laurentide Ice Sheet near Lac de Gras, Slave Geological Province, N.W.T., Canada, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3490, https://doi.org/10.5194/egusphere-egu21-3490, 2021.

13:51–13:53
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EGU21-5735
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ECS
Juan Pedro Roldan-Blasco, Luc Piard, Florent Gimbert, Christian Vincent, Adrien Gilbert, Olivier Gagliardini, Anuar Tobaigekov, and Andrea Walpersdorf

Basal sliding speed is a main component of glacier flow. However, acquiring direct observations of the velocity at the base of a glacier is a challenging task due to limited accessibility. One option consists in indirectly measuring basal speed by subtracting the internal deformation velocity from the velocity observed at the surface. Internal deformation has been mostly studied through annual surveys of borehole inclinometry that provide a snapshot of the internal velocity field of the glacier, while more recent efforts have installed continuously recording sensors at different depths. The former method provides a good resolution in depth, while the latter provides a good resolution in time, but few studies have provided both.

In this study we quantify basal speed variations at both short and long timescales through the combined analysis of one year of continuous half-hour sampled borehole tilt measurements and high resolution GNSS positioning. The instrumentation campaign has been done in the framework of the SAUSSURE project, in which we drilled five boreholes in the ablation area of Argentière Glacier, a temperate mountain glacier in the French Alps. The boreholes were positioned along the center flow line, and each one was equipped with an array of ~18 sensors that recorded the tilt and azimuth at different depths as well as water pressure at the bottom and middle depth. With this dataset we are able to investigate how melt season impacts the internal dynamics of the glacier, or how the sudden accelerations of the glacier after heavy storms events are shared between changes in internal deformation and basal speed ups. We find that the yearly averaged internal deformation profile can be well described using a two dimensional Glen flow law with exponent n ~ 3.4. We observe as well that deformational velocities can represent up to 60% of the total velocity, more than previously considered for Argentière Glacier. Our findings suggest that weekly accelerations, usually observed along raises in water pressure, are due to the increase of basal speed paired with a decrease in deformation, which suggests stress reconfiguration. We don’t observe journal cycles of deformation velocity, which would indicate that journal variations of glacier velocity are due only to changes of basal speed. In contrast, glacier acceleration during melt season at monthly timescales is accommodated by  deformation velocity and not by sliding.

How to cite: Roldan-Blasco, J. P., Piard, L., Gimbert, F., Vincent, C., Gilbert, A., Gagliardini, O., Tobaigekov, A., and Walpersdorf, A.: Basal speed and deformation velocity in an alpine temperate glacier from high resolution borehole tilt measurements and GNSS surface velocity observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5735, https://doi.org/10.5194/egusphere-egu21-5735, 2021.

13:53–14:03
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EGU21-6759
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ECS
|
solicited
Winnie Chu

Airborne radar sounding observations have been instrumental in understanding subglacial environments and basal processes of ice sheets. Since the advent of analog radar-echo sounding (RES) system in the early 1970s, there have been tremendous innovations in both RES hardware and signal processing techniques. These technological advancements have provided high-resolution ice thickness measurements, improved detection and characterization of subglacial hydrology, as well as improved understanding of basal thermal conditions, bed roughness and geomorphology, and other processes that govern the basal boundary of the polar ice sheets. In this talk, I will provide an overview of the recent developments in radar processing approaches and system designs and highlight some of the new understanding of ice sheet subglacial processes that emerge from these breakthroughs. I will end by discussing areas where future radar applications and discoveries may be possible, including the utilization of machine learning algorithms, space-borne radar missions, and ground-based passive radar platforms to provide long-term monitoring of ice sheet subglacial environments.

How to cite: Chu, W.: Four decades of radar-echo sounding: The past, present, and future of radar applications for understanding subglacial environments , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6759, https://doi.org/10.5194/egusphere-egu21-6759, 2021.

14:03–14:05
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EGU21-8411
Jane Hart, Kirk Martinez, Nathaniel Baurley, and Benjamin Robson

A key element in the comprehension of the response of glaciers to climate change is an understanding of the bed conditions, and these are a vital component of ice sheet models. The West Antarctic ice streams are potentially highly unstable, with implications for rapid sea level rise. These are underlain by unconsolidated sediments (soft-bed), which have a distinct but rarely studied subglacial hydrology. We present a detailed data set from Skálafellsjökull, a soft-bedded glacier in Iceland, as an analogue for other soft-bedded glaciers. These data include wireless in situ till water pressure, meteorological, surface melt, discharge and glacier surface velocity from GPS as well as remote sensing imagery. We show how short-term warm events during winter can effect annual velocity, and how the number of warm events has increased over the last 10 years. We argue this was because water was stored in a soft-bed subglacial reservoir where it could be rapidly released during winter, with the resultant storage levels effecting the following summer dynamics.  To test whether warm winter events are unique to Iceland, we analyzed the daily air temperatures record of 18 World Glacier Monitoring Service ‘reference’ glaciers (1979-2018). We were able to show that periods of warm temperatures during winter were present in maritime locations, and the number of these events had increased in locations where winter temperatures had also increased. We propose that winter events are an important component of glacier retreat and sea level rise that have hitherto not been examined in detail.

How to cite: Hart, J., Martinez, K., Baurley, N., and Robson, B.: Increased winter warm events in Iceland drive enhanced glacier velocity and melting, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8411, https://doi.org/10.5194/egusphere-egu21-8411, 2021.

14:05–14:07
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EGU21-8685
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ECS
Helen Ockenden, Andrew Curtis, Daniel Goldberg, Antonios Giannopoulos, and Robert Bingham

Thwaites Glacier in West Antarctica is one of the regions of the fastest accelerating ice thinning and highest observed ice loss. The topography of the bed beneath the glacier is a key control of future ice loss, but is not currently well enough known to satisfy the requirements of ice sheet models predicting glacier behaviour. It has previously been suggested that in fast flowing ice streams the shapes of landforms at the bed should be reflected in the ice surface morphology, which is known to a much higher resolution. Indeed, recently published radar grids from Pine Island Glacier reveal bed landforms with a definite resemblance to the ice surface above them. Here, we present a new high resolution bed topography map of Thwaites Glacier, inverted from REMA and ITSLIVE data using linear perturbation theory, a mathematical formulation of this resemblance between bed and surface.  As it is based on linear physics, this method is faster than mass conservation and streamline diffusion interpolation, the two main techniques utilised by existing bed topography products in this region. Furthermore, as the theory is based on both mass and momentum balance, it provides a physically consistent estimate of elevation and basal slipperiness, in contrast to these more widely used methods. The resulting bed matches well with existing airborne and swath radar surveys, with significant detail between these radar lines. Variation in the results obtained using different reference models provides a measure of validity of the linear perturbation theory. Due to the importance of form drag in patterns of ice retreat, the inverted topographic features are potentially important for the future behaviour of Thwaites Glacier.

How to cite: Ockenden, H., Curtis, A., Goldberg, D., Giannopoulos, A., and Bingham, R.: Inverting surface-elevation data and velocity for basal topography beneath Thwaites Glacier, West Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8685, https://doi.org/10.5194/egusphere-egu21-8685, 2021.

14:07–14:09
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EGU21-9526
Jean Vérité, Édouard Ravier, Olivier Bourgeois, Stéphane Pochat, Thomas Lelandais, Christopher D. Clark, Paul Bessin, Régis Mourgues, David Peigné, and Nigel Atkinson

Over the three last decades, great efforts have been undertaken by the glaciological community to characterize the behaviour of ice streams and better constrain the dynamics of ice sheets. Studies of modern ice stream beds reveal crucial information on ice-meltwater-till-bedrock interactions, but are restricted to punctual observations limiting the understanding of ice stream dynamics as a whole. Consequently, theoretical ice stream landsystems derived from geomorphological and sedimentological observations were developed to provide wider constraints on those interactions on palaeo-ice stream beds. Within these landsystems, the spatial distribution and formation processes of subglacial periodic bedforms transverse to the ice flow direction – ribbed bedforms – remain unclear. The purpose of this study is (i) to explore the conditions under which these ribbed bedforms develop and (ii) to constrain their spatial organisation along ice stream beds.  

We performed physical experiments with silicon putty (to simulate the ice), water (to simulate the meltwater) and sand (to simulate a soft sedimentary bed) to model the dynamics of ice streams and produce analog subglacial landsystems. We compare the results of these experiments with the distribution of ribbed bedforms on selected examples of palaeo-ice stream beds of the Laurentide Ice Sheet. Based on this comparison, we can draw several conclusions regarding the significance of ribbed bedforms in ice stream contexts:

  • Ribbed bedforms tend to form where the ice flow undergoes high velocity gradients and the ice-bed interface is unlubricated. Where the ribs initiate, we hypothesize that high driving stresses generate high basal shear stresses, accommodated through bed deformation of the active uppermost part of the bed.
  • Ribbed bedforms can develop subglacially from a flat sediment surface beneath shear margins (i.e., lateral ribbed bedforms) and stagnant lobes (i.e., submarginal ribbed bedforms) of ice streams, while they do not develop beneath surging lobes.
  • The orientation of ribbed bedforms reflects the local stress state along the ice-bed interface, with transverse bedforms formed by compression beneath ice lobes and oblique bedforms formed by transgression below lateral shear margins.
  • The development of ribbed bedforms where the ice-bed interface is unlubricated reveals distinctive types of discontinuous basal drainage systems below shear and lobe margins: linked-cavities and efficient meltwater channels respectively.

Ribbed bedforms could thus constitute convenient geomorphic markers for the reconstruction of palaeo-ice stream margins, palaeo-ice flow dynamics and palaeo-meltwater drainage characteristics.

How to cite: Vérité, J., Ravier, É., Bourgeois, O., Pochat, S., Lelandais, T., Clark, C. D., Bessin, P., Mourgues, R., Peigné, D., and Atkinson, N.: Ribbed bedforms, markers of palaeo-ice stream margins, basal meltwater drainage and ice flow dynamic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9526, https://doi.org/10.5194/egusphere-egu21-9526, 2021.

14:09–14:11
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EGU21-9719
Olivier Gagliardini, Fabien Gillet-Chaulet, and Florent Gimbert

Friction at the base of ice-sheets has been shown to be one of the largest uncertainty of model projections for the contribution of ice-sheet to future sea level rise. On hard beds, most of the apparent friction is the result of ice flowing over the bumps that have a size smaller than described by the grid resolution of ice-sheet models. To account for this friction, the classical approach is to replace this under resolved roughness by an ad-hoc friction law. In an imaginary world of unlimited computing resource and highly resolved bedrock DEM, one should solve for all bed roughnesses assuming pure sliding at the bedrock-ice interface. If such solutions are not affordable at the scale of an ice-sheet or even at the scale of a glacier, the effect of small bumps can be inferred using synthetical periodic geometry. In this presentation,  beds are constructed using the superposition of up to five bed geometries made of sinusoidal bumps of decreasing wavelength and amplitudes. The contribution to the total friction of all five beds is evaluated by inverse methods using the most resolved solution as observation. It is shown that small features of few meters can contribute up to almost half of the total friction, depending on the wavelengths and amplitudes distribution. This work also confirms that the basal friction inferred using inverse method  is very sensitive to how the bed topography is described by the model grid, and therefore depends on the size of the model grid itself. 

How to cite: Gagliardini, O., Gillet-Chaulet, F., and Gimbert, F.: Relative control of bedrock roughness versus topography on global glacier bed friction, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9719, https://doi.org/10.5194/egusphere-egu21-9719, 2021.

14:11–14:13
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EGU21-10480
Adam Booth, Poul Christoffersen, Joseph Chapman, Charlotte Schoonman, Bryn Hubbard, Thomas Chudley, Samuel Doyle, Robert Law, Andy Clarke, and Athena Chalari

Distributed acoustic sensing (DAS) involves detecting seismic energy from the deformation of a length of optical fibre cable, offers considerable potential in the high-resolution monitoring of glacier systems. Subglacial conditions and sediment properties exert a strong control on the basal sliding rate of glaciers, but identifying the connectivity of drainage pathways and their hydraulic conductivity remains poorly understood. This is due in part to the limitations of instrumental methods to monitor these processes accurately, whether by locating cryoseismic emissions in passive seismic records or actively imaging the subglacial environment in seismic reflection surveys.  Here, we explore the application of a borehole survey geometry for constraining the thickness and distribution of subglacial sediment deposits around a DAS installation on Greenland’s Store Glacier.

Store Glacier is a fast-moving outlet of the Greenland Ice Sheet. The instrumented borehole is drilled near the centre of a drained supraglacial meltwater lake, 28 km upstream of the Store Glacier terminus, and within 100 m of an active moulin, representing a continuous supply of water to the glacier bed. The borehole, which terminates at the glacier bed at a depth of 1043 m depth, is instrumented throughout its length with Solifos BruSENS fibre-optic cable, and monitored with a Silixa iDASTM interrogator. A suite of ~30 vertical seismic profiles (VSPs) was recorded at various azimuths and offsets (up to 500 m) from the borehole, using a 7 kg sledgehammer source. 

Initial analyses of VSP data implied a 20 [+17, -2] m thickness of sediment immediately beneath the borehole. These analyses are refined by considering the full suite of VSP data, to map spatial variations in the thickness of subglacial sediment layers.  This is undertaken using an iterative ray-tracing scheme, which seeks to minimise the differences in the arrival-time of direct seismic energy and subglacial reflections received at various depths in the borehole. Englacial compressional (P-) wave velocities are measured from cross-correlating direct arrivals (= 3700 ± 75 m/s in the upper 800 m of the glacier, 4000 ± 75 m/s between 880-950 m, 3730 ± 75 m/s through basal ice). For the subglacial sediment, we use a P-wave velocity of 1839 m/s, consistent with a value constrained in nearby surface seismic reflection data. To improve the definition of subglacial reflections and the constraint of their arrival times, data are first enhanced using frequency-wavenumber filtering.

Our approach suggests that sediment thickness is ~30 m directly beneath the borehole, potentially thinning by 10 m approximately 75 m further south. In reality, the seismic velocity through the sediment layer is unconstrained, but travel-time variations are themselves indicative of changes in either P-wave velocity and/or sediment thickness. Our work further highlights the interpretative potential of borehole DAS approaches, in support of conventional surface-based seismic analysis.

How to cite: Booth, A., Christoffersen, P., Chapman, J., Schoonman, C., Hubbard, B., Chudley, T., Doyle, S., Law, R., Clarke, A., and Chalari, A.: Distribution of subglacial sediment layers around a DAS-instrumented borehole, Store Glacier, Greenland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10480, https://doi.org/10.5194/egusphere-egu21-10480, 2021.

14:13–14:15
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EGU21-10726
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ECS
Sascha Barbara Bodenburg, Sönke Reiche, Christian Hübscher, and Julia Kowalski

The large variety of subglacial landforms observed on Earth are due to a complex interplay between the overlying ice sheet and the solid Earth below. While the ice cover thermally isolates the subglacial region, hence shields it from any influence by variations in the atmosphere, spatially varying geothermal heat fluxes from below may lead to the formation or reinforcement of existing subglacial landform patterns, such as tunnel valleys. An observed spatial correlation between tunnel valleys and underlying salt structures in the North German Basin is often explained mechanically. In this work, we alternatively focus on the role of heat transfer for the formation of tunnel valleys, which has not been holistically investigated until now. As salt has a higher thermal conductivity than the surrounding rocks, a local concentration of geothermal energy above salt structures may lead to increased subglacial melting rates of the overlying ice sheet. In particular, it is our goal to investigate to which extent the resulting meltwater discharge and corresponding erosion has the potential to reinforce tunnel valley formation. For our analysis, we develop a coupled computational strategy capable of determining the interplay between the temperature distribution within the heterogeneous subsurface including heat transport and ground water flow, and the overlying ice sheet. Modelling the interfacial heat flux from the subsurface into the ice sheet then allows us to infer on subglacial melt rates, which can be further assessed with respect to their role in the formation of tunnel valleys. In this contribution, we present results of a scaling analysis that takes into account the ice sheet with its internal horizontal and vertical velocity fields, the subsurface and the subglacial interfacial area. We furthermore describe a 1D computational strategy to combine the heat transport including subglacial phase change into a coupled process model allowing for investigating feedback mechanisms. Finally, we discuss strategies how this can be integrated into a full dimensional computational subsurface model, such as SHEMAT-Suite. Preliminary results for two tunnel valleys overlying salt structures in the German North Sea show that the local concentration of geothermal energy solely basing on heat conduction is only slightly augmented. The role of hydrothermal flow processes still remains to be quantified. We can therefore conclude that the geothermal distribution has a complementary effect to mechanical processes together leading to the formation of tunnel valleys.

How to cite: Bodenburg, S. B., Reiche, S., Hübscher, C., and Kowalski, J.: Feedback Mechanisms between Heterogeneous Geothermal Heat Fluxes and the Dynamic Ice Sheet Reinforce the Formation of Tunnel Valleys, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10726, https://doi.org/10.5194/egusphere-egu21-10726, 2021.

14:15–14:17
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EGU21-12022
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ECS
George Malczyk, Noel Gourmelen, Daniel Goldberg, Jan Wuite, and Thomas Nagler

Active subglacial lakes have been identified throughout Antarctica, offering a window into subglacial environments and their impact on ice sheet mass balance. We use high-resolution altimetry measurements over the Thwaites Glacier to show that a lake system underwent a second episode of drainage activity in 2017, only four years after another substantial drainage event. Our observations suggest significant modifications of the drainage system between the two events, with 2017 experiencing greater upstream discharge, faster lake-to-lake connectivity, and the transfer of water within a closed system. Measured rates of lake recharge during the inter-drainage period are significantly larger than modelled estimates, suggesting processes which drive subglacial melt production are currently underestimated. Our study highlights new methods of exploring subglacial environments through the application of altimetry, with potential applications for studying subglacial lakes across Antarctica

How to cite: Malczyk, G., Gourmelen, N., Goldberg, D., Wuite, J., and Nagler, T.: Repeat Subglacial Lake Drainage and Filling Beneath Thwaites Glacier, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12022, https://doi.org/10.5194/egusphere-egu21-12022, 2021.

14:17–14:19
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EGU21-14867
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ECS
Nathan Maier, Florent Gimbert, Fabien Gillet-Chaulet, and Adrien Gilbert

On glaciers and ice sheets, constraints on the bed physics which control the relationship between velocity and traction are critical for simulating ice flow. However, in Greenland the relationship between velocity and traction remains unquantified over much of the ice sheet. In this work, we determine the spatial relationship between velocity and traction in all eight drainage catchments of Greenland. The basal traction is estimated using three different methods over large grid cells to minimize biases associated with unconstrained rheologic parameters used in numerical inversions. We find that the velocity-traction relationships are consistent with our current understanding of basal physics in each catchment. We identify catchments that predominantly show Mohr-Coulomb-like behavior typical of deforming beds or significant cavitation, as well as catchments that predominantly show rate-strengthening behavior typical of Weertman-type hard-bed physics. Overall, the velocity-traction relationships suggest that the flow field and surface geometries over the grounded regions of the Greenland ice sheet are mainly dictated by Weertman-type physics. This data- and modeling based analysis provides a first constraint on the physics of basal motion over the grounded regions of Greenland and gives unique insight into future dynamics and vulnerabilities in a warming climate.

How to cite: Maier, N., Gimbert, F., Gillet-Chaulet, F., and Gilbert, A.: Constraints on the relationship between velocity and basal traction over the grounded regions of Greenland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14867, https://doi.org/10.5194/egusphere-egu21-14867, 2021.

14:19–14:21
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EGU21-15070
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ECS
Hydrology and biogeochemistry of subglacial environment of Greenland ice sheet
(withdrawn)
Ankit Pramanik, Nick Hayes, Frank Pattyn, and Sandra Arndt
14:21–15:00