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CR5.9

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.

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Convener: Adam Booth | Co-conveners: Robert Bingham, Christine Dow, Bryn Hubbard, Harold Lovell
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| Fri, 08 May, 10:45–12:30 (CEST)

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Chat time: Friday, 8 May 2020, 10:45–12:30

D2303 |
EGU2020-3176
Paul C Augustinus, Silvia Frisia, and Andrea Borsato

Subglacial calcite precipitates from Boggs Valley (71o55’S; 161o31’E; elevation 1,160 m asl., Northern Victoria Land, Antarctica), provided the first radiometrically-dated petrographic, geochemical and genomic evidence of thermogenic subglacial drainage events linked to subglacial eruptions during the Last Glacial Maximum (LGM). The crusts consist of two fabrics: i) a dirty (particulate-rich) microsparite, which marks catastrophic subglacial discharges of meltwater and a ii) dark columnar calcite that formed in pockets of basal melt. Synchrotron Radiation-based micro X-Ray fluorescence reveal that the dirty microsparite is S-rich, and embeds particulates characterized by high Manganese (Mn), Yttrium (Y) and Iron (Fe) concentrations. From previous work, we also know that the microsparite layers contain organic compounds, including amino acids, from which we extracted DNA fragments of microorganisms that lived in diverse sub-Antarctic environments (Frisia et al., 2017). The elongated columnar calcites are characterized by the presence of Arsenic (As) associated with low concentrations of  Mn. Both elements suggest local anaerobic, chemolitothrophic metabolism. Columnar calcite becomes increasingly rich in S near the “discharge” layers.  

Our preliminary interpretation is that during the LGM subglacial volcanism was crucial to sustain life in sub-ice sheet refugia by injecting both nutrients and diverse microbes into the basal ecosystem. The otherwise nutrient-poor, anoxic subglacial environment sustained a population of chemolithotrophs, which may have also been “allochthonous”.   

D2304 |
EGU2020-9503
Nathan Maier, Neil Humphrey, Joel Harper, and Toby Meierbachtol

Basal traction is fundamental to the dynamics of glaciers and ice sheets. On the Greenland Ice Sheet meltwater delivery to the bed and evolving drainage efficiency and connectivity modulate traction producing a characteristic seasonal velocity response. While numerical modelling and basal pressure observations have linked these velocity variations to evolving subglacial drainage, a high-fidelity record of basal traction is needed to constrain the timing and magnitude of traction changes that modulate summer ice flow.  We present a continuous summertime record of basal traction, basal ice deformation, and surface velocity measured at a densely instrumented field site in western Greenland. We use a five-station GPS network and englacial measurements of shearing and ice temperature to directly estimate the basal traction using the force balance method at the site-scale (100s of meters). Localized traction variations (10s of meters) are inferred via variations in the near-basal deformation field recorded by inclinometers installed directly above the basal interface. Combined, the data give a multi-scale perspective on how the basal traction changes during summer and relates to the conceptual model of melt season flow. Our results show the basal traction migrates between extremes during the melt season, with magnitudes greater than three times the average winter traction and near zero. The basal traction extremes correspond with the spring event, the inferred transition to efficient drainage, and the late summer velocity decline. The rapid strengthening and weakening of the basal interface show the complicated interaction of local and regional forcing that modulate melt season sliding. The near-basal deformation variations allow us to constrain the stress configuration and drainage state during each extreme traction period. Overall, the results allow us to refine the conceptual model for melt season traction changes and provide measured estimates of traction variations which can be used as quantitative targets for coupled drainage – ice dynamic models.

D2305 |
EGU2020-10710
Ugo Nanni, Florent Gimbert, Philippe Roux, and Albanne Lecointre

Subglacial hydrology strongly modulates glacier basal sliding, and thus likely exerts a major control on ice loss and sea-level rise. However, the limited direct and spatialized observations of the subglacial drainage system make difficult to assess the physical processes involved in its development. Recent work shows that detectable seismic noise is generated by subglacial water flow, such that seismic noise analysis may be used to retrieve the physical properties of subglacial channelized water flow. Yet, investigating the spatial organisation of the drainage system (e.g. channels numbers and positions) together with its evolving properties (e.g. pressure conditions) through seismic observations remains to be done. The objective of this study is to bring new insights on the subglacial hydrology spatio-temporal dynamics using dense array seismic observations.

We use 1-month long ground motion records at a hundred of sensors deployed on the Argentière Glacier (French Alps) during the onset of the melt season, when the subglacial drainage system is expected to strongly evolve in response to the rapidly increasing water input. We conduct a multi-method approach based on the analysis of both amplitude and phase maps of seismic signals. We observe characteristic spatial patterns, consistent across those independent approaches, which we attribute to the underlying subglacial drainage system.

The phase-driven approach shows seismic noise sources that focuses in the along-flow direction as the water input increases. We identify this evolution as the development of the main subglacial channel whose position is coherent with the one expected from hydraulic potential calculations. During periods of rapid changes in water input (5 days over 31) and concomitant glacier acceleration the amplitude-driven approach shows spatial pattern highly consistent with the seismic noise sources location. At this time, we suggest that the spatial variations in the amplitude are representative of the water pressure conditions in subglacial channels and surrounding areas. Our spatialized observations therefore reveal the spatio-temporal evolution of the subglacial drainage system together with its changing pressure conditions. We observe, for instance, that channels develop at the very onset of the melt-season and rapidly capture the water from surrounding areas. Such unique observations may allow to better constrain the physics of subglacial water flow and therefore strengthen our knowledge on the dynamics of subglacial environments.

D2306 |
EGU2020-18400
Martin Siegert and Louis-Alexandre Couston

Over 250 stable and isolated subglacial lakes exist at and close to the ice-sheet center in Antarctica. The physical conditions within subglacial lakes, and the differences between distinct lake settings, are critical to evaluating how and where life may best exist. Here, we demonstrate that upward heating by Earth’s geothermal flux provides efficient stirring of Antarctic subglacial lakes’ water, in a variety of ways related to their water depth, ice overburden and ceiling slope. We show that most lakes are in a regime of hard convective turbulence, enabling efficient mixing of nutrient- and oxygen-enriched top melt-water, which is essential for biome formation. Lakes beneath a thin (about less than 3 km) ice cover and lakes with a thick (more than 3 km) ice cover experience similarly-large velocities, but the latter have significantly larger temperature fluctuations and have a stable layer up to several tens of meters thick adjacent to the ice. We discuss the implications of hydrological conditions on the concentration of particulates in the water column.

D2307 |
EGU2020-1173
Steven Franke, Daniela Jansen, John Paden, and Olaf Eisen

The onset and high upstream ice surface velocities of the North East Greenland Ice Stream (NEGIS) are not yet well reproducible in ice sheet models. A major uncertainty remains the understanding of basal sliding and a parameterization of basal conditions. In this study, we assess the slow-flowing part of the NEGIS in a systematic analysis of the basal conditions and investigate the increased ice flow. We analyze the spectral basal roughness in correlation with basal return power from an airborne radar survey with AWIs ultra-wideband radar system in 2018 and compare our results with current ice flow geometry and ice surface flow. We observe a roughness anisotropy where the ice stream widens, indicating a change from a smooth and soft bed to a harder bedrock as well as the evolution of elongated subglacial landforms. In addition, at the upstream part of the NEGIS we find a clear zoning of the bedrock return power, indicating an increased water content at the base of the ice stream. At the downstream part, we observe an increased bedrock return power throughout the entire width of the ice stream and outside its margins, indicating enhanced melting and the distribution of basal water beyond the shear zones.

D2308 |
EGU2020-9431
Jade Bowling, Amber Leeson, Malcolm McMillan, Stephen Livingstone, and Andrew Sole

A total of 63 subglacial lakes have been documented beneath the Greenland Ice Sheet using a combination of radio-echo sounding and surface elevation change measurements. Of these, only 7 lakes have shown evidence of hydrological activity over the period 2001-2018. Draining lakes have been observed to drive transient changes in local ice flow speeds in Antarctica. The sudden discharge of water during a subglacial lake outburst event causes the subglacial lake roof to subside, which propagates to the surface, resulting in the formation of collapse basins (typically ~50-70 m in depth). These surface features can be detected using remote sensing techniques.

Whilst over 100 active subglacial lakes have been identified in Antarctica, predominantly beneath ice streams, little is known about the extent, volume of water stored and residence times of active subglacial lakes in Greenland, together with any potential influence of drainage events on local ice dynamics and sediment evacuation rates. Here, we explore the potential of the high resolution ArcticDEM stereogrammetric digital surface model (DSM) open source dataset, generated from satellite optical imagery, to identify and monitor subglacial lake-derived collapse basins. The ArcticDEM provides 2 m time-stamped surface elevation data, covering ~160 million km2, offering an exciting opportunity to map elevation changes between 2009-2017. This study presents the first effort to utilise ArcticDEM data at an ice-sheet scale to identify and monitor active subglacial lakes beneath the Greenland Ice Sheet, which we hope will ultimately improve our understanding of its complex subglacial hydrological system.

D2309 |
EGU2020-10204
Michael Prior-Jones, Elizabeth Bagshaw, Jonathan Lees, Lindsay Clare, Stephen Burrow, Jemma Wadham, Mauro A Werder, Nanna B Karlsson, Dorthe Dahl-Jensen, Poul Christoffersen, and Bryn Hubbard

Innovative technological solutions are required to access and observe subglacial hydrological systems beneath glaciers and ice sheets. Wireless sensing systems can be used to collect and return data without the risk of losing data from cable breakage, which is a major obstacle when studying fast flowing glaciers and other high-strain environments. However, the performance of wireless sensors in deep and fast-moving ice has yet to be evaluated formally. We report experimental results from Cryoegg: a spherical probe that can be deployed along an ice borehole and either remain fixed in place or potentially travel through the subglacial hydrological system. The probe makes measurements in-situ and sends them back to the surface via a wireless link. Cryoegg uses very high frequency (VHF) radio to transmit data through up to 1.3 km of cold ice to a surface receiving array. It measures temperature, pressure and electrical conductivity, returning all data in real time. This transmission uses Wireless M-Bus on 169 MHz; we present a simple “radio link budget” model for its performance in cold ice and confirm its validity experimentally. Power is supplied by an internal battery with sufficient capacity for two measurements per day for up to a year, although higher reporting rates are available at the expense of battery life. Field trials were conducted in 2019 at two locations in Greenland (the EastGRIP borehole and the RESPONDER project site on Sermeq Kujalleq/Store Glacier) and on the Rhone Glacier in Switzerland.  Our results from the field demonstrate Cryoegg’s utility in studying englacial channels and moulins, including estimating moulin discharge through salt dilution gauging with the instrument deployed deep within the moulin. Future iterations of the radio system will allow Cryoegg to transmit through up to 2.5 km of ice.

D2310 |
EGU2020-2885
Neil Ross and Martin Siegert

Deep-water ‘stable’ subglacial lakes likely contain microbial life adapted in isolation to extreme environmental conditions. How water is supplied into a subglacial lake, and how water outflows, is important for understanding these conditions. Isochronal radio-echo layers have been used to infer where melting occurs above Lake Vostok and Lake Concordia in East Antarctica but have not been used more widely. We examine englacial layers above and around Lake Ellsworth, West Antarctica, to establish where the ice sheet is ‘drawn down’ towards the bed and, thus, experiences melting. Layer drawdown is focused over and around the NW parts of the lake as ice, flowing obliquely to the lake axis, becomes afloat. Drawdown can be explained by a combination of basal melting and the Weertman effect, at the transition from grounded to floating ice. We evaluate the importance of these processes on englacial layering over Lake Ellsworth and discuss implications for water circulation and sediment deposition. We report evidence of a second subglacial lake near the head of the hydrological catchment and present a new high-resolution bed DEM and hydropotential model of the lake outlet zone. These observations provide insight into the connectivity between Lake Ellsworth and the wider subglacial hydrological system.

D2311 |
EGU2020-5858
Dougal Hansen, Anders Daamsgard, and Lucas Zoet

The distribution of strain in actively deforming subglacial till is an important control on the sliding velocity and sediment transport of soft-bedded glaciers. In situ field observations, laboratory experiments, and numerical simulations have demonstrated that strain accumulation within subglacial till is often greatest at the ice-bed interface and decreases monotonically with depth, forming a convex-upward profile. However, the mechanisms that set the form of the profile and depth of deformation remain unconstrained. Here we systematically test the influence of two independent variables, effective stress and sliding velocity, on the distribution of strain in a fine-grained, sandy till emplaced beneath a layer of moving ice. Laboratory sliding experiments, conducted with a brand-new ring-shear device with a transparent sample chamber, are coupled with two suites of state-of-the-art numerical experiments using 1) a discrete element model and 2) a non-local granular fluidity continuum model designed to emulate till deformation. Five effective stresses and five sliding velocities are tested with the other parameter held constant (velocity and effective stress, respectively). For the ring shear experiments, images of the till bed are acquired at regular intervals, and we quantify the displacement of sediment grains that occurs between image captures using digital image correlation. These experiments represent the first instance where the deformation of till during glacier slip can be observed in real-time and linked directly to its controlling processes. Furthermore, they provide an opportunity to juxtapose the predictions of two new granular dynamic models against empirical observations in a controlled setting, providing an invaluable ground truth for future, larger-scale implementations simulating bedform genesis and soft-bedded glacier dynamics.

D2312 |
EGU2020-5861
Emma Lewington, Stephen Livingstone, Chris Clark, Andrew Sole, and Robert Storrar

Despite being widely studied, subglacial meltwater landforms are typically mapped and investigated individually, thus the drainage system as a whole remains poorly understood. Here, we identify and map all visible traces of subglacial meltwater flow across the Keewatin sector of the former Laurentide Ice Sheet from the ArcticDEM, generating significant new insights into the connectedness of the drainage system.

Due to similarities in spacing, morphometry and spatial location, we suggest that the 100s-1000s m wide features often flanking and connecting sections of eskers (i.e. tunnel valleys, meltwater tracks and esker splays) are varying expressions of the same phenomena and collectively term these features ‘meltwater corridors’. Based on observations from contemporary ice masses, we propose a new formation model based on the pressure fluctuations surrounding a central conduit, in which the esker records the imprint of the central conduit and the wider meltwater corridors the interactions with the surrounding distributed drainage system, or variable pressure axis (VPA).

We suggest that the widespread aerial coverage of meltwater corridors across the Keewatin sector provides constraints on the extent of basal uncoupling induced by basal water pressure fluctuation and variations in spatial distribution and evolution of the subglacial drainage system, which have important implications for ice sheet dynamics. 

D2313 |
EGU2020-7185
Emma C. Smith, Anja Diez, Olaf Eisen, Coen Hofstede, and Jack Kohler

Kongsvegen is a well-studied surge-type glacier in the Kongsfjord area of northwest Svalbard. Long-term monitoring has shown that the ice surface velocity has been increasing since around 2014; presenting a unique opportunity to study the internal ice structure, basal conditions and thermal regime, all of which play a crucial role in initiating glacier surges. In April 2019, three-component seismic vibroseis surveys were conducted at two sites on the glacier, using a small Electrodynamic Vibrator source (ElViS). The first site is in the ablation area and the second near the equilibrium line, where the greatest increase in ice-surface velocity has been observed.

Initial analysis indicates the conditions at the two sites are significantly different. At the ablation area site, the ice is around 220 m thick, and the bed is relatively flat and unvaried, with no clear change in the bed reflection along the profile. The bed appears to comprise a uniform and undisturbed sediment package ~60 m thick, and there are no clear englacial reflections within the ice column. By contrast at the second site, the ice is around 390 m thick, and the internal ice structure is much more complex. Clear internal ice reflections are visible at depths between 150-250 m, and further reflections in the 100 m above the bed indicate there could be shearing or sediment entrainment in this area. Below the bed, cross-cutting layers are clearly visible and the bed reflection itself shows changing reflection polarity – suggesting water or very wet sediment is present in some areas.  The contrast between these two sites at the onset of a surge phase allows us to investigate the physical conditions that are conducive to surge initiation, both at the ice-bed interface and within the ice column.

D2314 |
EGU2020-10035
Robert Arthern, Rosie Williams, Kelly Hogan, Alex Brisbourne, Andrew Smith, and Tom Jordan

We consider a variety of ways that the basal drag that acts to resist the sliding of an ice sheet can be inferred from satellite observations, or from in situ observations. Three approaches are considered here. (1) use of inverse methods combined with large scale models of ice flow. (2) spectral analysis of basal topography combined with a theory of ice flow near small scale undulations, and (3) seismic methods that probe the physical characteristics of the subglacial sediment. Consideration is given to which sliding relationships are consistent with the available observations, and to identifying measurements that could help reduce ambiguity in sliding laws.

D2315 |
EGU2020-13260
Basile de Fleurian, Petra Langebroek, and Paul Halas

In recent years, temperatures over the Greenland ice sheet have been rising leading to an increase in surface melt.  Projections show that this augmentation of surface melt will continue in the future and spread to higher elevations. As it increases, melt leads to two different feedbacks on the dynamic of the Greenland ice sheet. This augmentation of melt lowers the ice surface and changes its overall geometry hence impacting the ice dynamics through ice deformation. The other feedback comes into play at the base of glaciers. Here, the increase of water availability will impact the distribution of water pressure at the base of glaciers and hence their sliding velocity. The first feedback is relatively well known and relies on our knowledge of the rheology and deformation of ice. The lubrication feedback acting at the bed of glaciers is however highly uncertain on time scales longer than a season. Here we apply the  Ice  Sheet  System  Model  (ISSM)  to  a  synthetic  glacier  which  geometry  is  similar to the one of a Greenland ice sheet land terminating glacier. The dynamic contributions from ice deformation and sliding are separated to study their relative evolution. This is permitted by the use of a dynamical subglacial hydrology model that allows to link the basal sliding to the meltwater production through an appropriate friction law. The  model  is  forced  through  a  simple  temperature  distribution  and  a  Positive  Degree  Day  model which allows to apply a large range of different forcing scenarios. Of particular interest is the evolution of the distribution of the efficient and inefficient component of the subglacial drainage system and their different response to the distribution of melt during the year which directly impact the sliding regime at the base of the glacier.

D2316 |
EGU2020-17484
Nico Dewald, Chris D. Clark, Stephen J. Livingstone, Jeremy C. Ely, and Anna L.C. Hughes

The configuration of subglacial drainage systems has a major impact on the dynamics of ice sheets. However, the logistical challenges of measuring subglacial processes beneath contemporary ice sheets hinder our understanding about the spatio-temporal evolution of subglacial drainage systems. Furthermore, today’s observations on contemporary ice sheets are inherently limited to a short period within the process of deglaciation. Landforms generated by the flow of meltwater at the ice-bed interface offer the potential to study both large-scale (103-106 km2) and long-term (103-105 a) developments of subglacial drainage networks beneath past ice sheets. Despite collectively recording subglacial drainage, individual meltwater landform types such as eskers, meltwater channels and tunnel valleys, and hummock corridors have mostly been considered as separate entities. Using high-resolution (1-2 m) DEMs, we summarise the suite of interconnected subglacial meltwater landforms into a common drainage signature herein called a subglacial drainage route. Our integrated map of subglacial meltwater landforms presents the large-scale distribution of major subglacial drainage routes across Scandinavia and provides a basis for future research about the long-term evolution of subglacial drainage networks and its effect on ice dynamics of the Scandinavian Ice Sheet.

D2317 |
EGU2020-6042
Jacob Woodard, Lucas Zoet, Neal Iverson, and Christian Helanow

The slip of hard bedded glaciers partly depends on the morphology of their beds. Thus, constraints on subglacial bedrock morphology are imperative for accurate forecasting of glacier flow rates. Digital elevation models (DEMs) from ten valley glacier and ice-sheet forefields were used to analyze the spectral patterns of recently deglaciated bedrock. Valley glacier DEM length scales are 0.1 m - 100 m, while ice sheet DEM length scales are 10 m -1000 m. We observe a higher spectral roughness and aspect ratio (i.e. bump height/wavelength) for valley glaciers than ice-sheet forefields. However, forefield aspect ratios span a narrow range and decrease with increasing length scale at a consistent rate despite a range of bedrock lithologies analyzed. This implies that bedrock shear strength (τ) scales with length scale (L), as τ ~ L-0.37, closely matching the bulk strength scaling relation seen in fault rocks (Brodsky et al., 2016). These morphological constraints of forefields allow extrapolation of bedrock roughness beneath active glaciers that can help predict sliding rates.

D2318 |
EGU2020-6941
Christian Vincent, Andrea Walpersdorf, Adrien Gilbert, Olivier Gagliardini, Florent Gimbert, Fabien Gillet-Chaulet, Luc Piard, Bruno Jourdain, Diego Cusicanqui, Luc Moreau, Olivier Laarman, and Delphine Six

Understanding basal processes is a prerequisite for predicting the overall motion of glaciers and its response to climate change. Although a number of studies have shown that subglacial hydrology affects glacier’s basal sliding motion, the involved mechanisms remain poorly known. Several studies suggested that glacier velocity increases with englacial and subglacial water storage, but observational quantification of subglacial water storage and associated velocity changes are challenging to make due to uncertainties on velocity measurements and on vertical straining.

Here we tackle this observational challenge through analyzing numerous field measurements from the surface and from the subglacial observatory on the Argentière Glacier (French Alps). We analyze specifically the relationships between daily sliding velocities (measured continuously at the glacier base), surface horizontal and vertical velocities from DGPS observations and ice thickness changes over years 2018 and 2019. We find strong upward surface movements of about 0.5 m during the winter until the beginning of May that cannot be explained by longitudinal strain rate changes. We support that it is caused by water volume increase in subglacial cavities.

Further analyzing the relationships between cavity growth, sliding and surface velocities, we find that unlike in previous studies bed separation variations are not synchronous with sliding speed variations. Surface uplift starts in winter, which is long before the spring sliding acceleration, and surface drop occurs mid-summer, which is long before the end of summer sliding deceleration. These findings support that the link between subglacial water storage and sliding speed may not be as direct as previously thought.

D2319 |
EGU2020-6949
Pascal Egli, Bruno Belotti, Martino Sala, Stuart Lane, and James Irving

It is well understood that topography near the snout of an alpine glacier may evolve quickly due to differential melting depending on exposure to solar radiation and on debris cover thickness. However, the positioning and shape of subglacial conduits underneath shallow ice may also have an important influence on ice creep and thereby on the topography of this region. This relationship could potentially be used to determine locations of subglacial conduits via the detailed observation of glacier surface changes.

We monitored the ice-marginal zone of the Otemma Glacier in the south-western Swiss Alps with daily UAV surveys at high spatial resolution and with a network of ablation stakes over a period of three weeks. After subtraction of melt measured with ablation stakes, we produced maps of changes in ice surface topography that are due to processes other than melt. In two consecutive summers we conducted three-dimensional GPR surveys in the same area of interest. By looking at these spatially dense grids of GPR measurements, we are able to identify the locations and shape of sub-glacial conduits underneath the ice marginal glacier tongue, for ice thicknesses between 20 m and 50 m. Superposition of the GPR-derived channel maps with those showing the topographic changes suggest a correlation between ice surface changes and processes operating at the glacier bed.

D2320 |
EGU2020-7271
Samuel Doyle, Bryn Hubbard, Poul Christoffersen, Marion Bougamont, Robert Law, Tom Chudley, Mike Prior-Jones, and Charlotte Schoonman

Glacier motion is resisted by basal traction that can be reduced significantly by pressurised water at the ice-bed interface. Few records of subglacial water pressure have been collected from fast-flowing, marine-terminating glaciers despite such glaciers accounting for approximately half of total ice discharge from the Greenland Ice Sheet.  The paucity of such measurements is due to the practical challenges in drilling and instrumenting boreholes to the bed, in areas that are often heavily-crevassed, through rapidly-deforming ice that ruptures sensor cables within weeks. Here, we present pressure records and drilling observations from two sites located 30 km from the calving front of Store Glacier in West Greenland, where ice flow averages ~600 m yr-1.  In 2018, boreholes were drilled 950 m to the bed near the margin of a large, rapidly-draining supraglacial lake. In 2019, multiple boreholes were drilled ~1030 m to the bed in the centre of the drained supraglacial lake, and in close proximity to a large, active moulin. All boreholes drained rapidly when they intersected or approached the ice-bed interface, which is commonly interpreted as indicating connection to an active subglacial drainage system. Neighbouring boreholes responded to the breakthrough of subsequent boreholes demonstrating hydrological or mechanical inter-connection over a distance of ~70 m. Differences in the time series of water pressure indicate that each borehole intersected a distinct component of the subglacial hydrological system. Boreholes located within 250 m of the moulin reveal clear diurnal cycles either in phase or anti-phase with moulin discharge. Pressure records from boreholes located on the lake margin, however, show smaller amplitude, and less distinct, diurnal cycles superimposed on longer-period (e.g. multiday) variability. We compare these datasets to those in the literature and investigate consistencies and inconsistencies with glacio-hydrological theory.

D2321 |
EGU2020-7644
Rebecca Schlegel, Adam Booth, Tavi Murray, Andy Smith, Alex Brisbourne, Ed King, Roger Clark, and Steph Cornford

There are numerous theoretical descriptions of the subglacial conditions (water availability, subglacial geology, flow dynamics) required for the formation of subglacial lineations, such as mega-scale glacial lineations and drumlins, that are known to be indicative of fast ice flow. Traditionally, mapping in de-glaciated areas, both onshore and offshore, has been undertaken using bathymetric maps, satellite data and field observations; here, lineations currently beneath the Rutford Ice Stream (West Antarctica) have been mapped using ground-penetrating radar (GPR) and seismic methods.

The Rutford Ice Stream is more than 2 km thick, of which 1.4 km are located below sea level. The ice surface speed at the grounding line is >1 m per day, and satellite observations indicate a stable ice flow over the past 30 years. The ice-bed interface is assumed to be at the pressure-melting point, while the bed can be divided into a region of soft, deforming sediment, and one of stiff, non-deforming, sediment. Long, elongated lineations, up to ~14 km, up to 150 m high, and 50-500 m wide, are found aligned in the ice-flow direction in the area of the soft sediment, within which the deposition of a drumlin was observed over a period of <10 years. Together with local erosion occurring in the same timescale, this demonstrates the temporal variability of ice stream beds.

To study the detailed architecture of the lineations, 3D grids of GPR data were acquired during the Antarctic Summer Season 2017/18, enabling 3D-processing and imaging of lineations. Using this unique dataset, in conjunction with previous publications plus data from the paleo record, we hope to better understand the possible mechanisms of formation of subglacial lineations as well as subglacial conditions at the Rutford Ice Stream.

D2322 |
EGU2020-8088
Douglas Benn, Ian Hewitt, Nanna Karlsson, and Anne Solgaard

Enthalpy balance theory predicts that dynamic oscillations (surge cycles) occur when glaciers cannot achieve stable steady states with regard to their mass and basal enthalpy (heat and water) budgets. That is, if the enthalpy produced by geothermal and frictional heat cannot be removed by conduction or water flux from the bed, sliding-heating feedbacks will lead to surging behaviour. To date, model simulations have focused on 'classic' surges, in which snow accumulation causes ice thickening in a reservoir zone during quiescence, and transition to surge occurs in response to a locally-driven sliding-heating feedbacks. However, many surges are initiated in glacier ablation zones, where surface mass balance is negative. Here, we show that such surges can be explained if the local mass and enthalpy budget is supplemented by non-local sources. Ice thickening during quiescence can occur if ice flux from upglacier exceeds losses by surface melt, and transition to surge occurs if accumulation of water from both local and non-local sources triggers the sliding-heating feedback. We illustrate these processes using data from Hagen Bræ, a major marine terminating glacier in North Greenland. The dataset, which covers the past 35 years at high temporal resolution, shows elevation changes, ice velocities and basal enthalpy budgets over recent surge cycles that are consistent with theory. The average surge cycle lasts 20-30 years while the duration of the active phase is approximately a decade based on the recent cycle. The theory has potentially wide applicability to surges in a range of climatic and topographic contexts.

D2323 |
EGU2020-9927
George Malczyk, Daniel Goldberg, Noel Gourmelen, Jan Wuite, and Thomas Nagler

Active subglacial lakes have been identified throughout Antarctica, offering a window into subglacial environments and into controls on ice dynamics. Between June 2013 and January 2014 a system of connected subglacial lakes drained in unison under the Thwaites glacier in the West Antarctic ice sheet, the first time that such a system has been observed in the Amundsen Sea Sector. Estimates based on catchment scale melt production suggested that lake drainages of this type should occur every 20 to 80 years. We collected elevations from January 2011 to December 2019 over the Thwaites lake region using the CryoSat-2 swath interferometric mode and ICEsat-2 land ice elevations, as well as ice velocity from the Sentinel-1 SAR mission since 2014. Using various elevation time series approaches, we obtain time dependent elevations over each lake. Results indicate that the upstream lakes undertake a second episode of drainage during mid-2017, only 3 years after the previous event, and that a new lake drained. Unlike the 2013-2014 episode, this new drainage episode contributed to filling one of the downstream lake with no evidence of further downstream activity. This new sub-glacial lake activity under Thwaites offer the possibility to explore lake connectivity, subglacial melt production and the interaction with ice dynamics.

D2324 |
EGU2020-12364
Lucas Zoet and Neal Iverson

Slip of marine-terminating ice streams over beds of deformable till is responsible for most of the contribution of the West Antarctic Ice Sheet to sea-level rise. Flow models of the ice sheet and till-bedded glaciers elsewhere require a law that relates slip resistance, slip velocity, and water pressure at the bed. We present results of the first experiments in which pressurized ice at its melting temperature is slid of over a water-saturated till bed. Steady-state slip resistance increases with slip velocity owing to sliding of ice across the bed, but above a threshold velocity till shears at its rate-independent, Coulomb strength. These results motivate a generalized slip law for glacier-flow models that combines processes of hard-bedded sliding and bed deformation.

D2325 |
EGU2020-17749
Eyjolfur Magnusson, Finnur Pálsson, Magnús T. Gudmundsson, Thórdís Högnadóttir, Christian Rossi, Thorsteinn Thorsteinsson, and Erik Sturkel

We present a 6 year record of repeated radio echo sounding (RES) on a profile grid (200-400 m between profiles) surveyed over the Eastern Skaftá Cauldron (ESC). ESC is an ice cauldron produced and maintained by powerful geothermal activity (~1 GW) at the glacier bed. Beneath the cauldron and 200-400 m of ice, water accumulates in a lake and is regularly released in jökulhlaups. The maximum discharge in the river Skaftá exceeded 3000 m3 s-1 in the most recent ones in 2015 and 2018. The record starts in 2014 and consists of annual measurements, obtained in June each year; the last on June 2019. Comparison of the repeated RES profiles (2D migrated) reveals the margin of the lake at different times and enables a classifying of traced reflections into lake and bedrock measurements. The bedrock measurements were obtained with the lake close to its minimum size in 2016, 2017 and 2019 (£~1 km2 compared to 4.0 km2 in 2015), hence it is possible to obtain fairly accurate digital elevation model (DEM) of the glacier/lake bed. This DEM is further constrained by two borehole measurements of the lake bed elevation at its centre. The traced lake reflections and comparison with the bedrock DEM enables creation of a lake thickness maps and an estimate of the lake volume for each survey. The lake thickness maps and volumes in June 2015 and 2018 are compared with the surface lowering pattern and water volumes drained in the jökulhlaups in October 2015 and August 2018. The drained water volume was derived by integrating the surface lowering during the jökulhlaups and adding estimated volume of crevasses formed in the events. The lowering in the 2015 jökulhlaup was obtained from TanDEM-X DEMs of September 23rd and October 10th, shortly before and after the jökulhlaup. The lowering in the 2018 jökulhlaup was derived from dense set of airborne altimetry profiles acquired on August 9th, a few days after the jökulhlaup, compared with a DEM in June 2018 (ArcticDEM in July 2017 corrected with dense GNSS profiles in June 2018). The lake volume estimate from the RES data is 240x106 m3 in June 2015 but 320±20x106 m3 drained from the cauldron in October. In June 2018 a relatively dense RES profile grid (~200 m between profiles) reveals a lake volume of 180x106 m3 while 210±30x106 m3 drained from the cauldron in August. This comparison demonstrates the applicability of our survey approach to monitor the water accumulation in the lake and thus better constrain potential hazard in jökulhlaups.

D2326 |
EGU2020-5835
Andrea Walpersdorf, Christian Vincent, Florent Gimbert, Agnès Helmstetter, Luc Moreau, Delphine Six, Stéphane Garambois, Laurent Ott, Stéphane Mercier, Olivier Laarman, Luc Piard, Ugo Nanni, Marguerite Mathey, Benoit Urruty, Christian Sue, Jean-Noël Bouvier, Martin Champon, Olivier Romeyer, Jean-Louis Mugnier, and Mathilde Radiguet and the SAUSSURE GNSS team

Five continuous GNSS stations monitor the Argentière glacier surface motion on a longitudinal profile at 2400 m altitude over a full melt season, from April to November 2019. High precision data analysis is enabled by a close-by reference station on the bedrock. This GNSS survey is part of the SAUSSURE project 2019-2022 that aims at increasing our knowledge on the physics of glacier basal sliding, by improving friction laws and validating them in a natural environment. The Argentière glacier is particularly interesting due to its long-term subglacial observatory measuring basal sliding velocity and subglacial discharge. The SAUSSURE project furthermore includes seismic, tiltmeter and piezometer measurements. The bedrock topography is obtained from a Ground Penetrating Radar.

The dense GNSS station setup permits to validate individual antenna movements. We then retrieve horizontal and vertical surface velocities on daily and sub-daily time scales. We can deduce strain rates in between the stations and their evolution in time, and relate this observable with the vertical surface motions. The confrontation of the GNSS data with independent observations allows analyzing the surface motions searching for glacier surges that combine horizontal speed-ups combined with uplift due to bed separation of the ice sheet. These events could give indications about cavity growth in spring. We will also try to investigate sub-daily motions that seem to occur in daily cycles in summer, as hinted at by the basal sliding measurements. These daily cycles are usually also seen in the seismic activity. The phase of the different features varies with respect to the daily cycles of temperature and sub-glacial water pressure. These phase offsets can give us indices on eventual mechanisms of sliding at the bedrock interface. The GNSS measurements represent a rare in situ data set that can contribute to better apprehend mechanisms of basal sliding and to provide high-resolution 3D constraints on physical models of glacier flow.

D2327 |
EGU2020-10537
Mark Johnson, Joni Mäkinen, Gustaf Peterson, Antti Ojala, Christian Öhrling, Elina Ahokangas, Izabella Remmert, Karu Kajuutti, and Jukka-Pekka Palmu

Triangular hummocks of subglacial origin have been identified in Sweden and Finland due to the increased resolution provided by LiDAR imagery. Their triangular shape is distinctive and recognizable as clearly identifiable landforms. These forms have been previously mapped in some cases as dead-ice hummocks, but geomorphic relationships with eskers, flutes ribbed moraine and De Geer moraines show these to be subglacial. We refer to these new landforms as ‘murtoos.’ Morphometric measurements show murtoos to be 50 to 200 m long and 50 to 100 m wide. The orientation of their apices strongly correlates with local ice-flow orientation. They form preferentially on beds that slope down-ice.  In many cases, they occur in patches in an ice-flow parallel path with eskers, defining corridors we believe to be of subglacial meltwater origin. Murtoos are composed primarily of heterogeneous diamicton with variable amounts of bedded sand and gravel. Murtoos are most common where glacier-melt rates were high during deglaciation (Bølling-Allerød and Holocene), and they are absent where extensive frozen-bed conditions were present. We suggest murtoos are a landform produced as a glacier-bed adjustment to increased delivery of supraglacial meltwater during deglaciation.