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The crystal orientation fabric of glacier ice impacts its strength and flow. Crystal fabric is therefore an important consideration when modeling ice flow. Here, we show that shear‐wave splitting (SWS) measured with glacial microseismicity can be used to invert seismic anisotropy and ice fabric, if represented in a statistical sense. Rutford Ice Stream (RIS) is a fast‐flowing Antarctic ice stream, a setting crucial for informing large‐scale ice sheet models. We present >200,000 SWS measurements from glacial microseismicity, registered at a 38‐station seismic network located ∼40 km upstream of the grounding line. A representative subset of these data is inverted for ice fabric. Due to the character of SWS, which accumulates along the raypath, we include information on the depth structure from radar measurements. We find that the following three‐layer configuration fits the data best: a broad vertical cone fabric near the base of RIS (500 m thick), a thick vertical girdle fabric, orientated perpendicular to flow, in the middle (1,200 m thick), and a tilted cone fabric in the uppermost 400 m. Such a variation of fabric implies a depth‐dependent strength profile of the ice with the middle layer being ∼3.5 times harder to deform along flow than across flow. At the same time, the middle layer is a factor ∼16 softer to shear than to compression or extension along flow. If such a configuration is representative for fast‐flowing ice streams, it would call for a more complex integration of viscosity in ice sheet models. Plain Language Summary: We introduce a method to derive information on the ice viscosity of a >2 km thick Antarctic Ice Stream using very small earthquakes, which are created as the ice stream flows over its bed. When ice flows, the individual ice crystals rearrange in a specific configuration. This configuration can vary, depending on the direction of applied force, making the bulk ice harder or easier to deform. This behavior can be expressed through a variable, the ice viscosity. Here, we use the recordings of icequakes recorded at seismic stations on top of the ice to infer the configuration of ice crystals and through further mathematical calculations, the viscosity between the ice‐bed and the surface. Our results show that the crystal configuration and viscosity vary with depth, which severely affects the ice stream's behavior upon deformation. Such a behavior is so far not included in many numerical models which simulate the large‐scale behavior of ice streams. Key Points: Split shear‐waves from basal microseismicity are used to invert for ice fabricThe preferred model has three depth layers of distinct fabric including a tilted cone, thick girdle, and vertical coneIce viscosity varies with depth, direction, and component of deformation, with significant implications for ice deformation [ABSTRACT FROM AUTHOR]

As in any environmental system, modeling instabilities within the glacial system is a numerical challenge of potentially high real-world relevance. Differentiating between the impacts of physical system processes and numerical noise is not straightforward. Here we use an idealized North American geometry and climate representation (similar to the HEINO experiments, Calov et al., 2010) to examine the numerical sensitivity of ice stream surge cycling in glaciological models. Through sensitivity tests, we identify some numerical requirements for a robust model configuration for such contexts. To partly address model-specific dependencies, we use both the Glacial Systems Model (GSM) and Parallel Ice Sheet Model (PISM). We show that modeled surge characteristics are resolution-dependent though converging (decreasing differences between resolutions) at higher horizontal grid resolutions. Discrepancies between high and coarse horizontal grid resolutions can be reduced by incorporating a resolution-dependent basal temperature ramp for basal sliding thermal activation. Inclusion of a diffusive bed thermal model reduces the surge cycling ice volume change by ~ 33 % as the additional heat storage dampens the change in basal temperature during surge events. The inclusion of basal hydrology, as well as a non-flat topography, leads to increased ice volume change during surge events (~ 20 and 17 %, respectively). Therefore, these latter three components are essential if one is endeavoring to maximize physical fidelity in ice stream surge cycle modeling. An abrupt transition between hard bedrock and soft sediment, as in the HEINO experiments, leads to ice stream propagation along this boundary but is not the cause of the main surge events. [ABSTRACT FROM AUTHOR]

A machine learning methodology for processing and visualizing high‐resolution LiDAR digital data is used to map drumlins and mega‐scale glacial lineations (MSGLs) on the bed of the Late Wisconsin Green Bay Lobe in Wisconsin, USA, which exhibited surge‐like behaviour during deglaciation. Previous work has shown that streamlined bedforms are the product of erosional streamlining of pre‐existing sediment. Analysis of bedform height and elongation ratio using curvature‐based relief separation (CBRS) and K‐means clustering of 32,003 bedforms reveals a continuum of six morphotypes ranging from drumlins, through "channeled" more elongated multi‐crested drumlins, to MSGLs. Further statistical analysis shows that morphotypes cluster into six types of streamlined surfaces (S1–S6) recording progressive elimination of an antecedent overridden topography to produce a smoother bed. Initial, relatively high‐relief drumlinized surfaces (S1, S2) occur around the slower‐flowing lateral flanks of the lobe, where a pre‐existing hummocky morainal topography was only partially modified by subglacial erosion. More streamlined surfaces (S3, S4) dominated by multi‐crested more elongate drumlins of reduced relief amplitude are transitional to flow sets of MSGL‐dominated surfaces (S5, S6) indicative of much faster‐flowing ice streaming along the lobe's axis. Estimates of basal drag based on roughness calculations for each surface type identify a 61% reduction in frictional retardation from poorly streamlined surfaces S1 and S2 to MSGL‐dominated surfaces S5 and S6, with a step‐like reduction between drumlins and channeled drumlins (S3, S4) possibly recording the rapid onset of fast flow. Subglacial streamlining is argued to be accomplished by a thin (<1 m) "third layer" of deforming subglacial debris between ice and its bed which functioned as an erodent layer. A thin (<3 m) till cap, formed by aggradation of deforming debris, rests unconformably on heterogeneous core sediments. Streamlined subglacial surfaces are comparable to the "functional" surfaces resulting from erosion by a "third layer" of wear debris in engineering tribological systems, and also by gouge on faults. Pleistocene ice sheets expanded over pre‐existing landsystems, pointing to the broader relevance of the methodology and findings reported here. [ABSTRACT FROM AUTHOR]

Antarctic ice shelves play a key role in regulating the rate of ice flow in tributary ice streams. Temporal variations in the associated ice‐shelf buttressing stress are observed to impact ice flow in glaciers and ice streams. Ephemeral grounding induced by tides is an important mechanism for modulating the buttressing stress. Here, we develop an approach to inferring variations in 3‐D surface displacements at an ice‐shelf‐stream system that explicitly accounts for ephemeral grounding. Using a temporally dense nine‐month‐long synthetic‐aperture radar image acquisition campaign collected over Rutford Ice Stream, West Antarctica, by the 4‐satellite COSMO‐SkyMed constellation, we infer the ephemeral grounding zones and the spatiotemporal variation of the fortnightly ice‐flow variability. We find ephemeral grounding zones along the western ice‐shelf margin as well as a few prominent ephemeral grounding points in the central trunk and in the vicinity of the grounding zone. Our observations provide evidence for tide‐modulated buttressing stress and the temporally asymmetric response of ice‐shelf flow to tidal forcing. Long‐term oceanic warming and ice‐shelf thinning will cause the loss of ephemeral grounding and decrease in ice‐shelf buttressing stress. Plain Language Summary: Antarctic ice shelves, the floating extensions of the Antarctic Ice Sheet, play a key role in ice‐flow dynamics by providing buttressing forces that resist the seaward flow of ice. Temporal variations in buttressing forces have been observed at glaciers and ice streams to impact ice‐flow speed. The grounding of ice shelves on the seafloor during low ocean tides, which is referred to as ephemeral grounding, causes temporal variations in buttressing forces. Here, using satellite radar observations, we develop an approach to inferring the surface displacements of an ice‐shelf‐stream system that explicitly accounts for ephemeral grounding. Using satellite radar imagery collected over Rutford Ice Stream, West Antarctica, we infer ephemeral grounding zones and the associated variability in ice flow. Our study provides evidence for the temporal variation in buttressing forces and improves our understanding of the short‐term response of ice‐shelf flow to ocean tides. In a warming climate, ice‐shelf thinning will reduce the impact of identified zones of ephemeral grounding, thereby causing long‐term ice‐flow speed to increase. Key Points: We develop an approach to inferring tide‐induced ephemeral grounding of ice shelves from synthetic‐aperture radar observationsEphemeral grounding plays a key role in the asymmetric response of ice‐shelf flows to tidal forcingLong‐term oceanic warming and ice‐shelf thinning will cause the loss of ephemeral grounding and reduction in ice‐shelf buttressing stress [ABSTRACT FROM AUTHOR]

The dynamics of glacial sliding over water-saturated tills are poorly constrained and difficult to capture realistically in large-scale models. Experiments characterize till as a plastic material with a pressuredependent yield stress, but the subglacial water pressure may fluctuate on annual to daily timescales, leading to transient adjustment of the till.We construct a continuum two-phase model of coupled fluid and solid deformation, describing the movement of water through the pore space of a till that is itself dilating and deforming. By forcing the model with timedependent effective pressure at the ice--till interface, we infer the resulting relationships between basal traction, solid fraction and rate of deformation. We find that shear dilation introduces internal pressure variations and transient dilatant strengthening emerges, leading to hysteretic behaviour in lowpermeability materials. The result is a time-dependent effective sliding law, with permeability-dependent lag between changes in effective pressure and the sliding speed. This deviation from traditional steady-state sliding laws may play an important role in a wide range of transient ice-sheet phenomena, from glacier surges to the tidal response of ice streams. [ABSTRACT FROM AUTHOR]

Heinrich-type ice-sheet surges are one of the prominent signals of glacial climate variability. They are characterised as abrupt, quasi-periodic episodes of ice-sheet instabilities during which large numbers of icebergs are released from the Laurentide ice sheet. The mechanisms controlling the timing and occurrence of Heinrich-type ice-sheet surges remain poorly constrained to this day. Here, we use a coupled ice sheet–solid Earth model to identify and quantify the importance of boundary forcing for the surge cycle length of Heinrich-type ice-sheet surges for two prominent ice streams of the Laurentide ice sheet – the land-terminating Mackenzie ice stream and the marine-terminating Hudson ice stream. Both ice streams show responses of similar magnitude to surface mass balance and geothermal heat flux perturbations, but Mackenzie ice stream is more sensitive to ice surface temperature perturbations, a fact likely caused by the warmer climate in this region. Ocean and sea-level forcing as well as different frequencies of the same forcing have a negligible effect on the surge cycle length. The simulations also highlight the fact that only a certain parameter space exists under which ice-sheet oscillations can be maintained. Transitioning from an oscillatory state to a persistent ice streaming state can result in an ice volume loss of up to 30 % for the respective ice stream drainage basin under otherwise constant climate conditions. We show that Mackenzie ice stream is susceptible to undergoing such a transition in response to all tested positive climate perturbations. This underlines the potential of the Mackenzie region to have contributed to prominent abrupt climate change events of the last deglaciation. [ABSTRACT FROM AUTHOR]

Sediment accumulating beneath floating ice contains a record of ice dynamics in polar regions where in situ observations are rare. In 2019 a hole was melted through a 590m-thick region of the Ross Ice Shelf ∼5 km seawards of the Kamb Ice Stream (KIS) grounding line (82.7841°S, 155.2626°W) to access the seafloor. Imagery from a remotely operated vehicle (ROV, Icefin) shows ocean current-generated ripples likely formed by tidal flow parallel to the grounding line (GL). Observed current speeds <0.15 m s−1 suggest these bedforms may be relict. Larger, dm-scale, 'furrows' parallel to the former direction of KIS flow may relate to past grounding line processes. A 0.49 m-long gravity core collected from the seafloor contains weakly stratified diamicton. The sediment matrix comprises variable mixtures of reworked Tertiary biogenic silica, predominantly diatoms, and arkose material. Sediment ε Nd values of ∼7 are consistent with derivation from the WAIS, as is the U-Pb age distribution and modelled late Holocene ice flows. Ramped pyrolysis 14C analysis shows all fractions are either >30 ka or 14C dead. By contrast, 210Pb-210 activity of >30 Bq Kg−1 indicates deposition within the last 120 years. The combination of features suggests rapid rainout deposition from melting of a sediment-laden basal debris layer as the GL retreated, followed by some reworking by ocean currents and little modern accumulation. Although Tertiary diatoms are abundant, unambiguously Late Quaternary forms are absent and we speculate on the implications for Ross Ice Shelf stability. [239 wds] • First imagery, cores and analysis of the seafloor at the Kamb Ice Stream grounding line. • Core comprises diamicton interpreted as grounding line retreat sequence since the 19th Century. • Diatom and provenance data are interpreted in terms of WAIS stability. [ABSTRACT FROM AUTHOR]

Earlier reconstructions of the glacial history of the western Canadian Arctic Archipelago, commonly portrayed Banks Island as an ice-free biological refugium during the Last Glacial Maximum, ostensibly constituting the northeast extremity of Beringia. This has now been contradicted by widespread fieldwork across the northern and western coasts documenting their inundation by the northwest Laurentide Ice Sheet that extended onto the polar continental shelf. Here we report the surficial geology and deglacial sea levels across southern Banks Island completing the island-wide evidence for a pervasive ice cover during the Late Wisconsinan. Moraines and erratics confirm the passage of an ice stream at least 1.1 km thick through Amundsen Gulf that onlapped the south coast of the island. The inland margin of the ice stream is marked by a prominent 80 km long shear moraine (Sachs moraine) that contacted thin, cold-based Laurentide ice crossing the island's interior. Subsequent thinning and retreat of the Amundsen Gulf Ice Stream from the Sachs moraine occurred in concert with its separation from topographically-confined ice lobes in the interior recorded by prominent ice-marginal meltwater channels. As the ice stream retreated offshore, a prominent kettled outwash plain was deposited along the coast marking marine limit at 20 m asl. From an unknown position offshore, the ice stream readvanced deforming the south shore of the kettled lowland marked by the 40 km long 'Sand Hills moraine'. Based on updated field mapping, the 'Sand Hills moraine' is now recognized as the westward extension of the more widespread Jesse moraine belt. Deglacial marine limit rises eastward across the island from 11 to 40 m asl, bordering Prince of Wales Strait. Marine limit and all lower shorelines across the island's south coast (for ∼ 200 km) are barren of marine shells. This is because sea level had regressed from marine limit to a lowstand offshore before Pacific molluscs recolonized the western Canadian Arctic at 13.7 cal ka BP, their entry occasioned by the resubmergence of Bering Strait. This requires that the breakup of the western Amundsen Gulf Ice Stream (including the M'Clure Strait Ice Stream along northern Banks Island) occurred before 14 cal ka BP, documenting that the NW Laurentide Ice Sheet must have contributed to Meltwater-Pulse-1A. Although the retreat from the outermost parts of the Jesse moraine belt also pre-date the arrival of shells, we show that the youngest (central) segment along eastern Amundsen Gulf dates to ∼12.9 cal ka BP, documenting that the Jesse moraine belt is highly time transgressive (spanning >13.7 to 12.9 cal ka BP). Radiocarbon dates on woody plants and bryophytes demonstrate that the island was biologically viable as early as 12.6 cal ka BP and that all coastlines were submergent soon afterwards and remain so today. [ABSTRACT FROM AUTHOR]

The correction notice in Nature Communications addresses an error in the original article titled "Holocene thinning in central Greenland controlled by the Northeast Greenland Ice Stream." The correction involves a section discussing flow factors at the NEGIS, with a reference citation correction from 16 to 45. The correction has been made in both the PDF and HTML versions of the article by authors Ilaria Tabone, Alexander Robinson, Marisa Montoya, and Jorge Alvarez-Solas. [Extracted from the article]

It is speculated that an united East Siberian-Chukchi Ice Sheet (ESCIS) existed on the East Siberian-Chukchi margin during the Pleistocene era. However, details regarding the frequency of its occurrence and the intensity of each glaciation remain unclear. Critical information lies in the northern part of the East Siberian-Chukchi margin because this region preserves relatively complete glacial landforms and deposits, especially in areas of deep-water depths. In this paper, new full-coverage multibeam data from the outer shelf–ocean basin on the western side of the Chukchi Rise of the northwestern Chukchi margin are presented, and more complete submarine glacial landforms in this area are identified. In addition, we provide a more detailed interpretation of the entire glacial strata using a compilation of new sub-bottom profiler data. Interpreted glacial landforms and buried glaciogenic debris flows (GDFs) indicate that the western side of the Chukchi Rise was affected by the Chukchi Ice Sheet. Overall, the glacial landforms can be divided into two categories, i.e. those in an ice stream setting on the Southwestern Margin and an inter-ice stream setting on the Northwestern Margin. However, in the current inter-ice stream area, large old gullies and GDFs persist, indicating dynamic changes between the ice stream and inter-ice stream zones. Since the onset of glaciation began around ∼0.8 Ma, the study area has experienced a total of nine glaciations, including three intense glaciations, five intermediate glaciations, and 1 mild glaciation. Through a comparison with the GDFs on the eastern slope of the Arliss Plateau which locates further west of the Chukchi Rise, we infer the existence of at least four unified ESCISs during the Quaternary. • New multibeam and subbottom profiler data at the northwestern Chukchi margin reveal more complete glacial landforms. • Nine glaciations of varied intensity are inferred by nine generations of glacigenic debris flows. • At least four unified East Siberian-Chukchi Ice Sheets existed during the Quaternary. [ABSTRACT FROM AUTHOR]

The analysis of seafloor glacial landforms can provide important constraints on the past behaviour of ice sheets, including their extent at the Last Glacial Maximum and the controls on their subsequent retreat. The continental shelf and slope offshore of northwest Greenland have been sparsely surveyed, however, limiting our understanding of ice sheet extent and dynamics in this sector during the pre-satellite era. Here we use newly acquired high-resolution geophysical data to map and interpret the distribution of glacial landforms across the hitherto unexplored banks of northern Melville Bay and the adjacent slope and deep-sea basin. In contrast to previous conceptual models, our seafloor observations suggest that shelf-break glaciation was attained along the entire northwest Greenland margin at the Last Glacial Maximum, including beyond the shallow banks. The landforms that we map on the continental slope provide strong support for the existence of an ice shelf spanning northern Baffin Bay. Sub-ice shelf keel scours in water depths of down to 1220 m reveal that this ice shelf was at least 1100 m thick at its grounding zone. The orientation of sub-ice shelf landforms suggests that the ice shelf was fed mainly by the supply of ice to northern Baffin Bay from the Lancaster Sound Ice Stream of the Canadian Arctic Archipelago. The Baffin Bay ice shelf buttressed several large ice streams of the Greenland, Innuitian and Laurentide ice sheets at the Last Glacial Maximum, and its break-up may have contributed to instigating the deglacial retreat of these ice streams from the shelf edge. • We use newly acquired bathymetric data to map glacial landforms in Melville Bay. • Northwest Greenland Ice Sheet reached the shelf break at the Last Glacial Maximum. • We present geomorphological evidence for a former ice shelf in northern Baffin Bay. [ABSTRACT FROM AUTHOR]

This paper examines the effect of basal topography and strength on the grounding-line position, flux and stability of rapidly-sliding ice streams. It does so by supposing that the buoyancy of the ice stream is small, and of the same order as the longitudinal stress gradient. Making this scaling assumption makes the role of the basal gradient and accumulation rate explicit in the lowest order expression for the ice flux at the grounding line and also provides the transcendental equation for the grounding-line position. It also introduces into the stability condition terms in the basal curvature and accumulation-rate gradient. These expressions revert to well-established expressions in circumstances in which the thickness gradient is large at the grounding line, a result which is shown to be the consequence of the non-linearity of the flow. The behaviour of the grounding-line flux is illustrated for a range of bed topographies and strengths. We show that, when bed topography at a horizontal scale of several tens of ice thicknesses is present, the grounding-line flux and stability have more complex dependencies on bed gradient than that associated with the ‘marine ice-sheet instability hypothesis’, and that unstable grounding-line positions can occur on prograde beds as well as stable positions on retrograde beds.

In Part 1, we have considered the dynamics of topographically confined glaciers, which may undergo surge cycles when the bed becomes temperate. In this Part 2, we consider the ice discharge over a flatbed, which would self-organize into alternating stream/ridge pairs of wet/frozen beds. The meltwater drainage, no longer curbed by the bed trough, would counter the conductive cooling to render a minimum bed strength at some intermediate width, toward which the stream would evolve over centennial timescale. At this stationary state, the stream width is roughly twice the geometric mean of the stream height and length, which is commensurate with its observed width. Over a flatbed, streams invariably interact, and we deduce that the neighboring ones would exhibit compensating cycles of maximum velocity and stagnation over the centennial timescale. This deduction is consistent with observed time variation of Ross ice streams B and C (ISB/C), which is thus a manifestation of the natural cycle. Moreover, the model uncovers an overlooked mechanism of the ISC stagnation: as ISB widens following its reactivation, it narrows ISC to augment the loss of the meltwater, leading to its stagnation. This stagnation is preceded by ice thickening hence opposite to the thinning-induced surge termination.

Recovery Ice Stream has a substantial number of active subglacial lakes that are observed, with satellite altimetry, to grow and drain over multiple years. These lakes store and release water that could be important for controlling the velocity of the ice stream. We apply a subglacial hydrology model to analyze lake growth and drainage characteristics together with the simultaneous development of the ice stream hydrological network. Our outputs produce a good match between modeled lake location and those identified using satellite altimetry for many of the lakes. The modeled subglacial system demonstrates development of pressure waves that initiate at the ice stream neck and transit to within 100 km of the terminus. These waves alter the hydraulic potential of the ice stream and encourage growth and drainage of the subglacial lakes. Lake drainage can cause large R-channels to develop between basal overdeepenings that persist for multiple years. The pressure waves, along with lake growth and drainage rates, do not identically repeat over multiple years, due to basal network development. This suggests that the subglacial hydrology of Recovery Ice Stream is influenced by regional drainage development on the scale of hundreds of kilometers rather than local conditions over tens of kilometers.

This paper contucts 0.6 m icing wind tunnel tests to study the formation law of the runback ice，i. e.，start position，coverage，and type，on a horizontal tail section assembly containing an electrothermal heater under the conditions of a wind speed of 90 m/s，a temperature of -4—-9 ℃ ，liquid water content （LWC） of 0.45—1.5 g/m³，median volumetric diameter（MVD）20.1—36 μm. Test results show that the initial position of runback ice formation on airfoil surface is obviously affected by the heating power and the incoming temperature. When the heating power or the incoming temperature is lower than certain values，the type of runback ice changes from stream ice to ice ridge. With the increase of heating power or inflow temperature，the initial position of runback ice moves backward. The overflow range of runback ice is obviously affected by LWC and the heating power. The larger the LWC is，the more water is collected，and the wider the overflow range is. The influence of heating power is similar. The increase of heating power will lead to the increase of overflow water，and the overflow range will be wider. The type of runback ice formation is sensitive to the change of MVD. When the MVD changes from 20.1 μm to 3 μm，and the overflow ice changes from typical stream ice to ice ridge. [ABSTRACT FROM AUTHOR]

Firn densification profiles are an important parameter for ice‐sheet mass balance and palaeoclimate studies. One conventional method of investigating firn profiles is using seismic refraction surveys, but these are difficult to upscale to large‐area measurements. Distributed acoustic sensing (DAS) presents an opportunity for large‐scale seismic measurements of firn with dense spatial sampling and easy deployment, especially when seismic noise is used. We study the feasibility of seismic noise interferometry (SI) on DAS data for characterizing the firn layer at the Rutford Ice Stream, West Antarctica. Dominant seismic energy appears to come from anthropogenic noise and shear‐margin crevasses. The DAS cross‐correlation interferometry yields noisy Rayleigh wave signals. To overcome this, we present two strategies for cross‐correlations: (a) hybrid instruments—correlating a geophone with DAS, and (b) stacking of selected cross‐correlation panels picked in the tau‐p domain. These approaches are validated with results derived from an active survey. Using the retrieved Rayleigh wave dispersion curve, we inverted for a high‐resolution 1D S‐wave velocity profile down to a depth of 100 m. The profile shows a "kink" (velocity gradient inflection) at ∼12 m depth, resulting from a change of compaction mechanism. A triangular DAS array is used to investigate directional variation in velocity, which shows no evident variations thus suggesting a lack of azimuthal anisotropy in the firn. Our results demonstrate the potential of using DAS and SI to image the near‐surface and present a new approach to derive S‐velocity profiles from surface wave inversion in firn studies. Plain Language Summary: The density distribution (density change with depth) over tens of meters at the top of a glacier is an important feature of ice‐sheet mass balance and palaeoclimate research. It can be estimated using the empirical relationship between density and seismic P‐wave velocity. The P‐wave velocity can be measured using a seismic refraction survey with geophones and active sources. However, refraction seismic surveys are expensive for measurements over large areas. Distributed Acoustic Sensing (DAS) using fiber optic cables to detect seismic waves is an emerging dense spatial sampling seismic acquisition technology. It can be used in conjunction with seismic noise cross‐correlation to make large‐scale measurements easier and cheaper than with conventional geophones. We investigate the feasibility of this approach on Rutford Ice Stream, West Antarctica, and propose two approaches to improve DAS seismic‐noise cross‐correlation results. Surface waves are retrieved by seismic noise cross‐correlation and are used to estimate the S‐wave velocity structure. Our S‐velocity profile resembles an independently measured P‐velocity in‐shape and presents a velocity gradient inflection—related to changes in the snow compaction mechanism. We show that DAS and seismic noise interferometry can be used for future firn measurements, but also more generally in studies of the near‐surface. Key Points: Distributed acoustic sensing (DAS) is used for the first time to derive the S‐wave velocity structure and anisotropy of the firn layer in AntarcticaDAS seismic interferograms are greatly improved through selective stacking and cross‐correlation with a geophoneOur method is suitable for large‐scale measurements and is feasible in the presence of ice lenses where refraction methods are inadequate [ABSTRACT FROM AUTHOR]

Accumulating evidence suggests that glacier basal motion can produce seismogenic stick‐slip events caused by frictional resistance at the ice‐bed interface. Frequent and subsequent failure of multiple stick‐slip patches may constitute sliding tremor in seismic recordings. We show that during stick‐slip tremor at the base of an Alpine glacier, the released seismic moment increases by an order of magnitude. The tremor emerges during melt‐intensive days and is associated with water pressure peaks in a nearby subglacial channel. We propose an extended slider‐block model that explains the regular occurrence of stick‐slip tremor by coupling seismically sliding patches to surrounding aseismically sliding bed regions. The model predicts that the stick‐slip patches control the movement of their surrounding smoothly sliding bed regions via elastic coupling and stress transfer. Thus, tremor producing stick‐slip patches could affect large‐scale ice flow to a degree, which is underestimated in models, with implications on sea level rise projections. Plain Language Summary: The motion of glaciers is often assumed to be smooth and slow. But this is not always true. During hot summer days, the ice of alpine glaciers melts. The meltwater runs through the ice down to the boundary between ice and rock, and builds up pressure. At a Swiss mountain glacier, our measurements show that during times of high water pressure, the glacier changes its motion style. The slow and smooth sliding of the glacier over the underlying rock and sediment transforms into multiple sudden slip events per second, summing up to hundreds of thousands of little slips per day. Each of these little slips causes an "icequake," which can be measured by sensitive sensors on the glacier surface. We can simulate this sliding style with a computer model and learn that locations at which the glacier ice slides in this shaky style also affect the sliding motion of their surroundings. As the shaky sliding has recently also been observed beneath polar ice streams, its influence on ice sheet stability may be important. We need to understand the physical details of how polar ice streams slide to accurately predict by how much the global sea level will rise in future. Key Points: Seismogenic stick‐slip tremor occurs at the confluence of two subglacial channels during diurnal high‐melt periods on an Alpine glacierDuring tremor, the seismic moment released by stick‐slip sliding is ∼10 times higher than outside of tremor periodsAn extended spring‐loaded slider‐block model describes tremor activity with asperity loading from viscous creep and cavity formation [ABSTRACT FROM AUTHOR]

Ice shelves buttress ice streams and glaciers, slowing the rate at which they flow into the ocean. When this buttressing is reduced, either through increased melt or calving, the increased discharge of grounded ice upstream contributes to sea level rise. The thickness, strength, and stability of ice shelves can be influenced by channels in the ice base. Here, we focus on a subglacially sourced basal channel which is observed to have melted up to 50% of the ice shelf thickness. The channel extends 6 km upstream of the previously estimated grounding line of the stagnant Kamb Ice Stream. Using a combination of ground–based observations and remote sensing, we find that the channel is growing upstream over time. Over–snow radar surveying images the shape of the channel, constrains a steep inception, and shows that not all of the basal shape is manifest at the surface. Modern surface lowering at the upstream head of the channel is interpreted as a region of focused melt where a subglacial outlet meets the ocean cavity. We estimate this basal melt to be at least 35 m/a in a narrow (200 m × 1.5 km) zone. Downstream from the melt region, repeat phase sensitive radar observations reveal accretion contributing to the growth of a ledge on the true–right side of the channel. We conclude that the channel is likely formed by a retreating subglacial outlet which enhances basal melt episodically. Plain Language Summary: Ice flows off the Antarctic continent and floats on the surrounding ocean, forming ice shelves. These ice shelves get stuck against the Antarctic coastline and islands, blocking ice flow from the continent to the open ocean. When ice shelves break apart or thin by melting, they have less ability to block ice flow. As a result, more ice flows from the continent into the ocean resulting in sea level rise. Melting can be especially strong in channels which form underneath floating ice. This paper focuses on a channel melted into the base of the Ross Ice Shelf at the inland coastline. We used radar to map the channel and found that it has a similar shape to a river mouth, though under approximately 350 m of ice. Water from the Antarctic continent flows to the ocean through this channel. Using data from satellites we found an area on the ice surface above the channel which is lowering, likely because of melting in the channel underneath. We estimated that the ice base is melting by at least 35 m per year in a narrow zone. Downstream, we used radar to find that in the channel water is freezing to the base of the ice shelf. Key Points: Oversnow radar data reveals the shape of a subglacially sourced basal channel at the grounding line of the Kamb Ice StreamRemote sensing suggests that more than 35 m/a melt occurs at the upstream tip of the channel, and shows that the channel is growing upstreamRepeat autonomous phase‐sensitive radio‐echo sounder surveying shows accretion across the channel downstream from the channels inception [ABSTRACT FROM AUTHOR]

Geological records of ice sheet collapse can provide perspective on the ongoing retreat of grounded and floating ice. An abrupt retreat of the West Antarctic Ice Sheet (WAIS) that occurred during the early deglaciation is well recorded on the eastern Ross Sea continental shelf. There, an ice shelf breakup at 12.3 ± 0.6 cal. (calibrated) kyr BP caused accelerated ice-mass loss from the Bindschadler Ice Stream (BIS). The accelerated mass loss led to a significant negative mass balance that re-organized WAIS flow across the central and eastern Ross Sea. By ~ 11.5 ± 0.3 cal kyr BP, dynamic thinning of grounded ice triggered a retreat that opened a ~ 200-km grounding-line embayment on the Whales Deep Basin (WDB) middle continental shelf. Here, we reconstruct the pattern, duration and rate of retreat from a backstepping succession of small-scale grounding-zone ridges that formed on the embayment's eastern flank. We used two end-member paleo-sediment fluxes, i.e., accumulation rates, to convert the cumulative sediment volumes of the ridge field to elapsed time for measured distances of grounding-line retreat. The end-members fluxes correspond to deposition rates for buttressed and unbuttressed ice stream flow. Both scenarios require sustained rapid retreat that exceeded several centuries. Grounding-line retreat is estimated to have averaged between ~ 100 ± 32 and ~ 700 ± 79 ma−1. The evidence favors the latter scenario because iceberg furrows that cross cut the ridges in deep water require weakly buttressed flow as the embayment opened. In comparison with the modern grounding-zone dynamics, this paleo-perspective provides confidence in model projections that a large-scale sustained contraction of grounded ice is underway in several Pacific-Ocean sectors of the WAIS. [ABSTRACT FROM AUTHOR]

The Greenland Ice Sheet (GrIS) responds rapidly to the present climate, therefore, its response to the predicted future warming is of concern. To learn more about this, decoding its behavior during past periods of warmer than present climate is important. However, due to the scarcity of marine studies reconstructing ice sheet conditions on the Northeast Greenland shelf and adjacent fjords including the position of the ice sheet over marine regions, the timing of the deglaciation, and its connection to forcing factors including the Holocene Thermal Maximum (HTM) on NE Greenland remain poorly constrained. This paper aims to use bathymetric data and the analysis of sediment gravity cores to enhance our understanding of ice dynamics of the GrIS near the southern outlet of the Northeast Greenland Ice Stream (NGIS), as well as give insight into the timing of deglaciation and provide a palaeoenvironmental reconstruction of southwestern Dove Bugt and Bessel Fjord since the Last Glacial Maximum (LGM). The swath bathymetry data displayed in this study is the first time the bathymetry for Bessel Fjord has become available. North–south oriented glacial lineations, and the absence of pronounced moraines in southwest Dove Bugt, an inner continental shelf embayment (trough), suggests the southwards and offshore flow of the southern branch of the NGIS, Storstrømmen. Sedimentological data suggests that an ice body, theorized to be the NGIS, may have retreated from the region slightly before ~11.2 ka BP (in the Preboreal period). The seabed morphology of Bessel Fjord, a fjord terminating in southern Dove Bugt, includes numerous basins, separated by thresholds. The position of basin thresholds, which include some recessional moraines, suggest that the GrIS had undergone multiple halts or readvances during deglaciation. A minimum age of 7.2 ka BP is proposed for the retreat of ice to or west of its present-day position in the Bessel Fjord catchment area. This suggests that the GrIS retreated from the marine realm in early Holocene, around the time of the onset of the Holocene Thermal Maximum in this region, a period when the mean July temperature according to Bennike et al., (2008) was at least 2–3 °C higher than at present, and remained at or west of this onshore position for the remainder of the Holocene. The transition from predominantly mud to muddy sand layers in a mid-fjord core at ~4 ka BP may be the result of increased sediment input from nearby and growing ice caps. This shift may suggest that in late Holocene (Meghalayan), a period characterized by a temperature drop to modern values, ice caps in Bessel Fjord fluctuated with greater sensitivity to climatic conditions than the NE sector of the GrIS. [ABSTRACT FROM AUTHOR]

The BRITICE‐CHRONO consortium of researchers undertook a dating programme to constrain the timing of advance, maximum extent and retreat of the British–Irish Ice Sheet between 31 000 and 15 000 years before present. The dating campaign across Ireland and Britain and their continental shelves, and across the North Sea included 1500 days of field investigation yielding 18 000 km of marine geophysical data, 377 cores of sea floor sediments, and geomorphological and stratigraphical information at 121 sites on land; generating 690 new geochronometric ages. These findings are reported in 28 publications including synthesis into eight transect reconstructions. Here we build ice sheet‐wide reconstructions consistent with these findings and using retreat patterns and dates for the inter‐transect areas. Two reconstructions are presented, a wholly empirical version and a version that combines modelling with the new empirical evidence. Palaeoglaciological maps of ice extent, thickness, velocity, and flow geometry at thousand‐year timesteps are presented. The maximum ice volume of 1.8 m sea level equivalent occurred at 23 ka. A larger extent than previously defined is found and widespread advance of ice to the continental shelf break is confirmed during the last glacial. Asynchrony occurred in the timing of maximum extent and onset of retreat, ranging from 30 to 22 ka. The tipping point of deglaciation at 22 ka was triggered by ice stream retreat and saddle collapses. Analysis of retreat rates leads us to accept our hypothesis that the marine‐influenced sectors collapsed rapidly. First order controls on ice‐sheet demise were glacio‐isostatic loading triggering retreat of marine sectors, aided by glaciological instabilities and then climate warming finished off the smaller, terrestrial ice sheet. Overprinted on this signal were second order controls arising from variations in trough topographies and with sector‐scale ice geometric readjustments arising from dispositions in the geography of the landscape. These second order controls produced a stepped deglaciation. The retreat of the British–Irish Ice Sheet is now the world's most well‐constrained and a valuable data‐rich environment for improving ice‐sheet modelling. [ABSTRACT FROM AUTHOR]

Linear, incised and usually an order of magnitude wider than the Martian valleys, the Martian outflow channels are scoured ground commonly displaying streamlined remnants of the pre-existing terrain. A recent study used an unprecedented dataset of the Martian valley networks to propose that most of the valley networks are a result of combined surface and subglacial runoff. This also prompts to revisit One of the earliest hypotheses that the ancient ice streams might have carved the Martian outflow channels. With an exceptional focus on Mars exploration planned during the next decades, it is important to assess the regional-scale geomorphic processes to better target the future landing and sample return missions. [ABSTRACT FROM AUTHOR]

Mass loss near the ice-sheet margin is evident from remote sensing as frontal retreat and increases in ice velocities. Velocities in the ice sheet interior are orders of magnitude smaller, making it challenging to detect velocity change. Here, we analyze a 35-year record of remotely sensed velocities, and a 6-year record of repeated GPS observations, at the East Greenland Ice-core Project (EastGRIP), located in the middle of the Northeast-Greenland Ice Stream (NEGIS). We find that the shear margins of NEGIS are accelerating, indicating a widening of the ice stream. We demonstrate that the widening of the ice stream is unlikely to be a response to recent changes at the outlets of NEGIS. Modelling indicates that the observed spatial fingerprint of acceleration is more consistent with a softening of the shear margin, e.g. due to evolving fabric or temperature, than a response to external forcing at the surface or bed. A new study finds that the North East Greenland ice stream is not as stable as previously thought and that this will affect its future evolution. [ABSTRACT FROM AUTHOR]

The Ross Ice Shelf (RIS) is currently stable but recent observations have indicated that basal melt rates beneath the ice shelf are expected to increase. It is important to know which areas of the RIS are more sensitive to enhanced basal melting as well as other external forcings or internal material properties of the ice to understand how climate change will influence RIS mass balance. In this paper, we use automatic differentiation and the Ice Sheet and Sea-level System Model to quantify the sensitivity of the RIS to changes in basal friction, ice rigidity, surface mass balance, and basal melting. Using volume above flotation (VAF) as our quantity of interest, we find that the RIS is most sensitive to changes in basal friction and ice rigidity close to grounding lines and along shear margins of the Siple Coast Ice Streams and Transantarctic Mountains Outlet Glaciers. The RIS sensitivity to surface mass balance is uniform over grounded ice, while the sensitivity to basal melting is more spatially variable. Changes in basal melting close to the grounding lines of the Siple Coast Ice Streams and Transantarctic Mountains outlet glaciers have a larger impact on the final VAF compared to elsewhere. Additionally, the pinning points and ice shelf shear margins are highly sensitive to changes in basal melt. Our sensitivity maps allow areas of greatest future vulnerability to be identified. [ABSTRACT FROM AUTHOR]

We present a record of melt events obtained from the East Greenland Ice Core Project (EastGRIP) ice core in central northeastern Greenland, covering the largest part of the Holocene. The data were acquired visually using an optical dark-field line scanner. We detect and describe melt layers and lenses, seen as bubble-free layers and lenses, throughout the ice above the bubble–clathrate transition. This transition is located at 1150 m depth in the EastGRIP ice core, corresponding to an age of 9720 years b2k. We define the brittle zone in the EastGRIP ice core as that from 650 to 950 m depth, where we count on average more than three core breaks per meter. We analyze melt layer thicknesses, correct for ice thinning, and account for missing layers due to core breaks. Our record of melt events shows a large, distinct peak around 1014 years b2k (986 CE) and a broad peak around 7000 years b2k, corresponding to the Holocene Climatic Optimum. In total, we can identify approximately 831 mm of melt (corrected for thinning) over the past 10 000 years. We find that the melt event from 986 CE is most likely a large rain event similar to that from 2012 CE, and that these two events are unprecedented throughout the Holocene. We also compare the most recent 2500 years to a tree ring composite and find an overlap between melt events and tree ring anomalies indicating warm summers. Considering the ice dynamics of the EastGRIP site resulting from the flow of the Northeast Greenland Ice Stream (NEGIS), we find that summer temperatures must have been at least 3 ± 0.6 ∘ C warmer during the Early Holocene compared to today. [ABSTRACT FROM AUTHOR]

Results of ice-stream models that treat temperate ice deformation as a two-phase flow are sensitive to the ice permeability. We have constructed and begun using a custom, falling-head permeameter for measuring the permeability of temperate, polycrystalline ice. Chilled water is passed through an ice disk that is kept at the pressure-melting temperature while the rate of head decrease indicates the permeability. Fluorescein dye in the water allows water-vein geometry to be studied using fluorescence microscopy. Water flow over durations of seconds to hours is Darcian, and for grain diameter d increasing from 1.7 to 8.9 mm, average permeability decreases from 2 × 10−12 to 4 × 10−15 m2. In tests with dye on fine (d = 2 mm) and coarse (d = 7 mm) ice, average area-weighted vein radii are nearly equal, 41 and 34 μm, respectively. These average radii, if included in a theory slightly modified from Nye and Frank (1973), yield permeability values within a factor of 2.0 of best-fit values based on regression of the data. Permeability values depend on d−3.4, rather than d−2 as predicted by models if vein radii are considered independent of d. In future experiments, the dependence of permeability on liquid water content will be measured. [ABSTRACT FROM AUTHOR]

In the accumulation zone of glaciers and ice sheets snow is transformed into glacial ice by firn densification. Classically, this process is assumed to solely depend on temperature and overburden pressure, which is controlled by the accumulation rate. However, exceptionally thin firn layers have been observed in the high-strain shear margins of ice streams. Previously, it has been proposed that this firn thinning can be explained by an enhancement of firn densification due to the effect of strain softening inherent to power-law creep. This hypothesis has not been validated, and the greater firn densities in the presence of horizontal strain rates have not yet been reproduced by models. Here, we develop a model that corrects the firn densification rate predicted by classical, climate-forced models for the effect of strain softening. With the model it is confirmed that strain softening dominates the firn densification process when high strain rates are present. Firn densities along a cross section of the Northeast Greenland Ice Stream (NEGIS) are reproduced with good agreement, validating the accuracy of the developed model. Finally, it is shown that strain softening has significant implications for ice core dating and that it considerably affects the firn properties over wide areas of the polar ice sheet, even at low strain rates. Therefore, we suggest that, besides temperature and accumulation rate, horizontal strain rates should generally be considered as a forcing parameter in firn densification modeling. [ABSTRACT FROM AUTHOR]

Tills from moraines adjacent to major ice streams of the Weddell Sea Embayment contain distinct detrital zircon (n = 5304) and K-bearing mineral age populations (n = 323) that, when combined with pebble composition data, can be used to better understand Antarctica's subglacial geology and ice sheet history. Till representing the Institute, Foundation, Academy, Recovery and Slessor Ice Streams each have distinct detrital zircon age populations. Detrital Ar-Ar ages are mostly younger than zircon ages, and distinctive populations include 270--300 Ma (Institute), 170--190 Ma (Foundation), and 1200--1400 Ma (Recovery), which are not easily explained by known outcrops. Pebble fractions of the Foundation and Academy tills are most diverse with up to >40% exotic erratics. The southern side of the Recovery Glacier has fossiliferous limestone erratics. Mixing models created using a nonlinear squares curve-fitting approach were developed to evaluate contributors of zircons to Foundation Ice Stream till. These model results and pebble lithology data both indicate that unexposed (subglacial) bedrock is mixed with exposed rocks to produce the observed till. Notably, the model required limited local Patuxent Formation input to the Foundation till's zircon population. Our data suggest that sandstones underlie the Foundation Ice Stream and Recovery Glacier troughs, which has a bearing on basal ice flow conditions and results in geological controls on ice stream location. This geo- and thermo-chronological characterization of the ice streams will enable ice-rafted debris in Weddell Sea marine sediments to be traced back to its sources and interpreted in terms of ice stream dynamics. [ABSTRACT FROM AUTHOR]

Three holes were drilled to the bed of Rutford Ice Stream, through ice up to 2154 m thick, to investigate the basal processes and conditions associated with fast ice flow and the glacial history of the West Antarctic Ice Sheet. A narrative of the drilling, measuring and sampling activities, as well as some preliminary results and initial interpretations of subglacial conditions, is given. These were the deepest subglacial access holes ever drilled using the hot-water drilling method. Samples of bed and englacial sediments were recovered, and a number of instruments were installed in the ice column and the bed. The ice–bed interface was found to be unfrozen, with an existing, well-developed subglacial hydrological system at high pressure, within ~1% of the ice overburden. The bed itself comprises soft, water-saturated sediments, consistent with previous geophysical interpretations. Englacial sediment quantity varies significantly between two locations ~2 km apart, and possibly over even shorter (~20 m) distances. Difficulties and unusual observations encountered while connecting to the subglacial hydrological system in one hole possibly resulted from the presence of a large clast embedded in the bottom of the ice.

Ice stream discharge responds to a balance between gravity, basal friction and lateral drag. Appreciable viscous heating occurs in shear margins between ice streams and adjacent slow-moving ice ridges, altering the temperature-dependent viscosity distribution that connects lateral drag to marginal strain rates and ice stream velocity. Warmer ice deforms more easily and accommodates faster flow, whereas cold ice supplied from ice ridges drives advective cooling that counteracts viscous heating. Here, we present a two-dimensional (three velocity component), steady-state model designed to explore the thermal controls on ice stream shear margins. We validate our treatment through comparison with observed velocities for Bindschadler Ice Stream and verify that calculated temperatures are consistent with results from previous studies. Sweeping through a parameter range that encompasses conditions representative of ice streams in Antarctica, we show that modeled steady-state velocity has a modest response to different choices in forcing up until temperate zones develop in the shear margins. When temperate zones are present, velocity is much more sensitive to changes in forcing. We identify key scalings for the emergence of temperate conditions in our idealized treatment that can be used to identify where thermo-mechanical feedbacks influence the evolution of the ice sheet.

Drag at the bed and along the lateral margins are the primary forces resisting flow in outlet glaciers. Simultaneously inferring these parameters is challenging since basal drag and ice viscosity are coupled in the momentum balance, which governs ice flow. We test the ability of adjoint-based inverse methods to infer the slipperiness coefficient in a power-law sliding law and the flow-rate parameter in the constitutive relation for ice using a regularization scheme that includes coefficients weighted by surface strain rates. Using synthetic data with spatial variations in basal drag and ice rheology comparable to those in West Antarctic Ice Streams, we show that this approach allows for more accurate inferences. We apply this method to Bindschadler and MacAyeal Ice Streams in West Antarctica. Our results show relatively soft ice in the shear margins and spatially varying basal drag, with an increase in drag with distance upstream of the grounding line punctuated by localized areas of relatively high drag. We interpret soft ice to reflect a combination of heating through viscous dissipation and changes in the crystalline structure. These results suggest that adjoint-based inverse methods can provide inferences of basal drag and ice rheology when regularization is informed by strain rates.

• Exposure dating constrains rapid deglaciation of Antarctic ice streams ∼8 ka ago. • Paired 10Be-26Al nuclides sheds light on complex exposure history of erratics. • Ice thinning was induced by atmospheric and oceanic warming of the Early Holocene. • Eustatic and relative sea level rise paced the transition of grounded ice to ice shelf. • Buoyancy response of thinning ice stream may have accelerated grounding line retreat. The knowledge of dynamics and retreat patterns of marine-based ice streams under multiple stressors are of foremost importance for predicting Antarctic Ice Sheet response to climate changes. The Holocene palaeoglaciological record of former ice streams draining the northeast Antarctic Peninsula can elucidate the influences of changes in atmospheric and oceanic circulation and sea-level oscillations on the ice thinning and grounding line retreat. Here, terrestrial cosmogenic nuclide (TCN) dating of erratic boulders across the James Ross Island group sheds light on the pattern and timing of the ice recession along the two main arteries of the palaeo-ice drainage: Croft Trough and Prince Gustav Channel. The approach of using paired 10Be-26Al nuclides enabled an assessment of cosmogenic isotope inheritance and complex burial-exposure history, notably on the high-altitude volcanic mesas. The TCN ages suggest that the Prince Gustav Channel Ice Stream was thinning from at least ∼12 ka, with subsequent separation of the Antarctic Peninsula and James Ross Island ice masses by 10–8 ka. The transition from grounded ice to open marine conditions in the Croft Trough occurred rapidly at 8.6–7.2 ka, following the Early Holocene Warm Period, concomitant with eustatic and relative sea-level rise and incursions of warmer circumpolar waters. Grounding line retreat was possibly further accelerated by buoyancy response of thinning ice stream to low-gradient bed topography. The lessons of rapid deglaciation of James Ross Island palaeo-ice streams may provide analogues for recent or future intensification of pressures on Antarctic glaciers. [ABSTRACT FROM AUTHOR]

Jan Mayen Ridge, south of the isolated island of Jan Mayen (71°N 8°W), is located strategically near the centre of the Polar North Atlantic and surrounded by deep-water seas. Examination of about 4,100 km2 of multibeam bathymetric data allowed identification of 1700 chaotic seafloor depressions at 300–1000 m modern water depth and two sets of lineations at 400–600 m and 750–900 m deep. The chaotic depressions are interpreted as ploughmarks produced as iceberg keels impinged on a sedimentary seafloor. The deepest ploughmarks reach 1010 m depth. Multi-keel ploughmarks indicate the presence of tabular icebergs of minimum widths 0.5–2.3 km. Ploughmark orientations were W/E and SW/NE on the northern and southern ridge, respectively, suggesting that deep-keeled icebergs drifting across Jan Mayen Ridge were probably calved mainly from short floating tongues at the margins of full-glacial Greenland ice streams. The mapped seafloor lineations are morphologically similar to mega-scale glacial lineations and flutes formed by soft-sediment deformation beneath active ice. They are interpreted to indicate the possible grounding of glacier ice on Jan Mayen Ridge. Their W-E orientation precludes ice flow from a full-glacial Jan Mayen ice cap. A possible interpretation is that they may represent grounding of a large floating ice shelf probably fed by ice from the Greenland Ice Sheet during one of the very large pre-Weichselian glaciations of the Quaternary. Whereas these lineations at 70°N do not provide direct evidence for a huge ice-shelf complex flowing southwards from the Arctic Ocean, the general climatic conditions required to form and maintain large ice shelves suggest that the development of an Arctic Ocean ice shelf at some point during Quaternary full-glacials remains a possibility. • The >1,000 ploughmarks on Jan Mayen Ridge, at 600–1,000 m deep, are among the highest concentrations in the Polar seas. • Multi-keeled ploughmarks indicate drift of large tabular icebergs of minimum widths 0.5–2.3 km across Jan Mayen Ridge. • Surchanges at ploughmark terminations indicate W-E drift implying a likely source from a past Greenland Ice Sheet. • Seafloor lineations suggest the possible grounding of glacier ice on Jan Mayen Ridge in a previous glaciation. [ABSTRACT FROM AUTHOR]

The accuracy of sea-level rise predictions is highly dependent on reliably understanding the subglacial environment beneath ice streams. Bedforms result from the interaction between ice and its substrate, and therefore have the potential to shed light on poorly understood basal conditions influencing ice dynamics. However, theoretical models of bedform formation are often based on observations from deglaciated areas or from sparsely-sampled geophysical surveys over glaciated regions. Here, we use high-resolution three- and two-dimensional radar and seismic data to reveal details of the initiation and evolution of a subglacial bedform beneath Rutford Ice Stream, West Antarctica. Radar surveys at 20 m and 50 m line spacing allow detailed imaging of bed topography, including a moat up to 55 m deep, surrounding the upstream end of a 50-m high and >18-km long bedform. Many models rely on either a topographical or a locally resistant seed point to initiate bedform formation. The bedform described here is mostly composed of soft sediment (porosity >0.3) and lacks a boulder or outcrop, suggesting the bedform initiated without a topographic seed point. Sediment at the upstream end of bedforms appears stiffer over a distance of 2.3 km. We suggest sediment inhomogeneities in the initially flat bed cause the deposition of sediment, which, assuming resistant enough, acts as a seed point for bedform extension and moat erosion. The moat's geometry and its truncation of other bedforms suggest that it was eroded after the deposition of surrounding bedforms. These observations from a modern ice stream deliver information of subglacial processes involved in the initiation as well as in situ high-resolution topography and properties of bedforms and moats. Using these observations numerical models can be tested and developed accordingly. • Evidence for a bedform initiating without pre-existing topographic perturbations. • A moat around an elongated bedform revealed from high-resolution radar data. • Constraints of bedform formation mechanism from a modern ice stream. [ABSTRACT FROM AUTHOR]

The Northeast Greenland Ice Stream (NEGIS) is the main artery for ice discharge from the northeast sector of the Greenland Ice Sheet (GrIS) to the North Atlantic. Understanding the past, present and future stability of the NEGIS with respect to atmospheric and oceanic forcing is of global importance as it drains around 17% of the GrIS and has a sea-level equivalent of 1.6 m. This paper reconstructs the deglacial and Holocene history of Nioghalvfjerdsbræ (or 79N Glacier); a major outlet of the NEGIS. At high elevation (>900 m asl) autochthonous blockfield, a lack of glacially moulded bedrock and pre LGM exposure ages point to a complex exposure/burial history extending back over half a million years. However, post Marine Isotope Stage 12, enhanced glacial erosion led to fjord incision and plateaux abandonment. Between 900 and 600 m asl the terrain is largely unmodified by glacial scour but post LGM erratics indicate the advection of cold-based ice through the fjord. In contrast, below ∼600 m asl Nioghalvfjerdsfjorden exhibits a geomorphological signal indicative of a warm-based ice stream operating during the last glacial cycle. Dated ice marginal landforms and terrain along the fjord walls show initial thinning rates were slow between ∼23 and 10 ka, but post-10 ka it is evident that Nioghalvfjerdsfjorden deglaciated extremely quickly with complete fjord deglaciation below ∼500 m asl between 10.0 and 8.5 ka. Both increasing air and ocean temperatures were pivotal in driving surface lowering and submarine melt during deglaciation, but the final withdrawal of ice through Nioghalvfjerdsfjorden was facilitated by the action of marine ice sheet instability. Our estimates show that thinning and retreat rates reached a maximum of 5.29 ma−1 and 613 ma−1, respectively, as the ice margin withdrew westwards. This would place the Early Holocene disintegration of this outlet of the NEGIS at the upper bounds of contemporary thinning and retreat rates seen both in Greenland and Antarctica. Combined with recent evidence of ice stream shutdown during the Holocene, as well as predictions of changing ice flow dynamics within downstream sections of the NEGIS catchment, this suggests that significant re-organisation of the terminal zone of the ice stream is imminent over the next century. • The Northeast Greenland Ice Stream (NEGIS) thinned slowly between 23–10ka as ice retreated across the continental shelf. • At the opening of the Holocene rising air & ocean temperatures drove surface lowering & basal melt. • Between 10-8.5ka marine ice sheet instability further enhanced grounding line retreat rates into Nioghalvfjerdsfjorden. • During the Early Holocene thinning and retreat rates reached a maximum of 5.29 ma-1 and 613 ma-1 respectively. • Such rates are at the upper bounds of contemporary ice stream thinning and retreat rates in Greenland and Antarctica. [ABSTRACT FROM AUTHOR]

We examine the gravity-driven flow of thin films of viscous fluid of power-law rheology, lubricated by another power-law viscous fluid from below. Such flows are relevant to a range of geophysical and industrial settings, including the flow of ice sheets and fast-flowing ice streams lubricated from below by a layer of subglacial till. We model both layers using lubrication theory in two-dimensional and axisymmetric settings, in the limit in which vertical shear provides the dominant resistance to the flow. The flow is self-similar if the power-law exponents, describing the rheology of the two layers of fluid, are equal. We examine the similarity solutions in both geometries and describe the flow in terms of four distinct flow regimes ranging from thin films of viscous fluid coating a more viscous fluid from above, to thin layers of fluid lubricating a more viscous fluid from below. In contrast to the former scenario, a thin film of a low-viscosity fluid strikingly alters the dynamics of a more viscous fluid when it lubricates it from below: the overlying layer thins, the upper surface gradients lessen and most of the shear is confined to the lower layer. Such features amplify, and this flow regime becomes increasingly dominant when the viscous fluids are shear thinning, like the deformation of glacial ice on the large scale. This flow regime is most relevant to the flow of lubricated ice sheets, which thin and accelerate, forming fast-flowing ice streams, when they are well lubricated from below [ABSTRACT FROM AUTHOR]

We use polarimetric radar sounding to investigate ice crystal orientation fabric and its impact on ice viscosity within the near surface of Rutford Ice Stream, West Antarctica. The technique retrieves lateral and depth variations in the horizontal components of ice fabric but no direct information on the vertical fabric component. In the shallowest ice (depths 40–100 m), the fabric is consistent with flow‐induced development and correlates with the surface compression direction. Notably, toward the ice‐stream margin, the horizontal compression angle and azimuthal fabric orientation tend toward 45° relative to ice flow which is consistent with the early stages of flow‐induced fabric under simple shear. The fabric orientation in deeper ice (depths 100–300 m) is, in places, significantly misaligned with shallower ice and the surface compression direction due to sharp depth transitions in orientation. We then use a rheological model to bound effective anisotropic viscosities (directional hardness) of ice that are consistent with the radar measurements. Toward the shear margin, we show that the shallow‐ice fabric does not appreciably soften the ice to lateral shear although this may happen in deeper ice. In the center of the ice stream, we show that lateral and depth variations in the fabric alignment relative to ice flow result in corresponding changes in uniaxial ice viscosities relative to ice flow. Our results indicate that spatial variability in the fabric translates to variability in viscosity that widely used isotropic ice‐flow models are unable to consider. Plain Language Summary: The viscosity of glacier ice is dependent on the direction in which ice crystals are pointing relative to an applied load. This ice crystal orientation fabric also contains information about past ice flow. Compared with the interior of ice sheets, the relationships between fabric, viscosity, and ice flow are relatively unexplored in the ice streams and outlet glaciers which drain the Antarctic Ice Sheet. We use a ground‐based geophysical method to investigate how ice fabric varies spatially within Rutford Ice Stream (West Antarctica) and then calculate the relative viscosity of ice for deformation in different directions. Our results reveal rapidly varying fabric properties within the flow unit. In the shallowest ice, the fabric is consistent with what would be expected from the surface deformation, whereas in deeper ice, this is not always true. We then show that spatial variation in ice fabric translates to lateral and depth variations in directional ice viscosity. This poses challenges for using ice‐flow models to derive glacier bed conditions as they do not generally account for directional viscosity. Key Points: Pronounced spatial variability of ice crystal orientation fabric is inferred from radar sounding in the near surface of Rutford Ice StreamIn the shallowest ice, the fabric is generally aligned with surface compression whereas in deeper ice this is not always the caseThe fabric results in lateral and depth variations of anisotropic ice viscosity within the flow unit including sharp depth transitions [ABSTRACT FROM AUTHOR]

Antarctic basal water storage variation (BWSV) refers to mass changes of basal water beneath the Antarctic ice sheet (AIS). Identifying these variations is critical for understanding Antarctic basal hydrology variations and basal heat conduction, yet they are rarely accessible due to a lack of direct observation. This paper proposes a layered gravity density forward/inversion iteration method to investigate Antarctic BWSV based on multi-source satellite observations and relevant models. During 2003–2009, BWSV increased at an average rate of 43 ± 23 Gt/yr, which accounts for 29% of the previously documented total mass loss rate (−76 ± 20 Gt/yr) of AIS. Major uncertainty arises from satellite gravimetry, satellite altimetry, the glacial isostatic adjustment (GIA) model, and the modelled basal melting rate. We find that increases in basal water mainly occurred in regions with widespread active subglacial lakes, such as the Rockefeller Plateau, Siple Coast, Institute Ice Stream regions, and marginal regions of East Antarctic Ice Sheet (EAIS), which indicates the increased water storage in these active subglacial lakes, despite the frequent water drainage events. The Amundsen Sea coast experienced a significant loss during the same period, which is attributed to the basal meltwater discharging into the Amundsen Sea through basal channels. [ABSTRACT FROM AUTHOR]

Antarctica’s fast-flowing ice streams drain the ice sheet, with their velocity modulated by subglacial water systems. Current knowledge of these water systems is limited to the shallow portions near the ice-bed interface, but hypothesized deeper groundwater could also influence ice streaming. Here, we use magnetotelluric and passive seismic data from Whillans Ice Stream, West Antarctica, to provide the first observations of deep sub–ice stream groundwater. Our data reveal a volume of groundwater within a >1-kilometer-thick sedimentary basin that is more than an order of magnitude larger than the known subglacial system. A vertical salinity gradient indicates exchange between paleo seawater at depth and contemporary basal meltwater above. Our results provide new constraints for subglacial water systems that affect ice streaming and subglacial biogeochemical processes. [ABSTRACT FROM AUTHOR]

Full-Stokes (FS) ice sheet models provide the most sophisticated formulation of ice sheet flow. However, their applicability is often limited due to the high computational demand and numerical challenges. To balance computational demand and accuracy, the so-called Blatter–Pattyn (BP) stress regime is frequently used. Here, we explore the dynamic consequences of using simplified approaches by solving FS and the BP stress regime applied to the Northeast Greenland Ice Stream. To ensure a consistent comparison, we use one single ice sheet model to run the simulations under identical numerical conditions. A sensitivity study to the horizontal grid resolution (from 12.8 to a resolution of 0.1 km) reveals that velocity differences between the FS and BP solution emerge below ∼ 1 km horizontal resolution and continuously increase with resolution. Over the majority of the modelling domain both models reveal similar surface velocity patterns. At the grounding line of the 79 ∘ North Glacier the simulations show considerable differences whereby the BP model overestimates ice discharge of up to 50 % compared to FS. A sensitivity study to the friction type reveals that differences are stronger for a power-law friction than a linear friction law. Model differences are attributed to topographic variability and the basal drag, in which neglected stress terms in BP become important. [ABSTRACT FROM AUTHOR]

Basal ice-shelf melting is the key driver of Antarctica's increasing sea-level contribution. In diminishing the buttressing force of the ice shelves that fringe the ice sheet, the melting increases the ice discharge into the ocean. Here we contrast the influence of basal melting in two different ice-shelf regions on the time-dependent response of an isothermal, inherently buttressed ice-sheet-shelf system. In the idealized numerical simulations, the basal-melt perturbations are applied close to the grounding line in the ice-shelf's (1) ice-stream region, where the ice shelf is fed by the fastest ice masses that stream through the upstream bed trough and (2) shear margins, where the ice flow is slower. The results show that melting below one or both of the shear margins can cause a decadal to centennial increase in ice discharge that is more than twice as large compared to a similar perturbation in the ice-stream region. We attribute this to the fact that melt-induced ice-shelf thinning in the central grounding-line region is attenuated very effectively by the fast flow of the central ice stream. In contrast, the much slower ice dynamics in the lateral shear margins of the ice shelf facilitate sustained ice-shelf thinning and thereby foster buttressing reduction. Regardless of the melt location, a higher melt concentration toward the grounding line generally goes along with a stronger response. Our results highlight the vulnerability of outlet glaciers to basal melting in stagnant, buttressing-relevant ice-shelf regions, a mechanism that may gain importance under future global warming. [ABSTRACT FROM AUTHOR]

The Arctic Ocean is one of the last frontiers on Earth with many unknowns about its geological and climate history and considerable speculation on its role in the Earth's climate and ocean system. It has been proposed recently that it was occupied by a freshwater body of more than 9.5 × 106 km3 underneath a thick ice mass during part of glacial isotopic stages 6 and 4. We argue that such a dramatic scenario, implying replacement of marine waters by freshwater throughout the entire Arctic Ocean and Nordic Seas, is physically implausible. The very low 230Th excesses (230Thxs) observed in sediments from these intervals were used as evidence for the presence of a U‐depleted overlying freshwater column. We show here that they may simply result from short, sporadic sedimentary pulses, below multiyear sea ice or ice shelves, linked to deglacial ice streaming and surging events interrupting long‐duration sedimentary gaps. Due to this sporadic sedimentation regime, interpolating time from 230Thxs data or between benchmark ages in sedimentary sequences would simply be erroneous. [ABSTRACT FROM AUTHOR]

Reconstruction of deglacial ice margins provides insights into the demise of past ice sheets and ice‐marginal lakes and helps to understand how former ice sheets responded to climate change. Here, we reconstruct deglacial Laurentide Ice Sheet margins across Manitoba (Canada), a dynamic region that in MIS 2 spanned from an inner core region of the Keewatin dome to the periphery of the ice sheet (~900 km north of the Last Glacial Maximum limit). The area was also overrun by ice flowing from both the Quebec‐Labrador dome and the Hudson Bay Ice Saddle. The surficial landscape of Manitoba contains inherited relict and palimpsest glacial landscapes, which need to be separated from deglacial features. Ice‐impounded glacial Lake Hind was present in southwest Manitoba at 13.0 cal. ka BP, meaning most of Manitoba was covered by ice at the start of the Younger Dryas. Northwest drainage of glacial Lake Agassiz in front of the Highrock Lake–Cree Lake moraine could have occurred near the end of the Younger Dryas, prior to 11.5 cal. ka BP, though the volume of the lake varies greatly depending on ice‐margin reconstructions. Our interpretation is incompatible with the hypothesis that Lake Agassiz drainage to the Arctic Ocean triggered the Younger Dryas climatic cooling. Numerous ice streams developed across central and southern Manitoba during deglaciation, including the Souris, Red River, The Pas, Hayes and Quinn Lake. The dominant ice source was from the north early in deglaciation, switching to the northeast with growth of the Hudson Bay Ice Saddle and then back to the north again with demise of the saddle. The ice‐margin ages are largely unconstrained, and thus we are unable to accurately assign climatic drivers to various ice stream events. Nonetheless, we record the development and demise of terrestrial ice streams over both hard‐bed and soft‐bed substrates. [ABSTRACT FROM AUTHOR]

Deformation of subglacial sediment during basal slip shapes the beds of many fast‐flowing glaciers and ice streams. The resultant sediment flux impacts glacier dynamics and rates of subglacial erosion over a range of timescales, but its fundamental dependencies are not well understood. Using a cryogenic ring shear device, we conducted experiments to investigate the effects of both effective stress and slip speed on rates of till transport. Sediment fluxes were computed using digital image correlation from a photographic time series of the till bed. We find a near‐linear relationship between sediment flux and slip speed, but a non‐monotonic, double‐valued dependency of sediment flux on effective stress. Deformation primarily occurred in a thin shear band near the ice sole, the thickness of which could vary with both parameters. Coupling between ice and till increased at higher slip speeds and effective stresses and scaled the magnitude of the flow profile. Plain Language Summary: Rapid motion of many glaciers and ice streams occurs by ice sliding over soft sediment beds. This sediment is mobilized in response to stress exerted by the overriding ice, which influences rates of erosion and generates subglacial landforms over a range of timescales. However, the physical processes that control this sediment flux are poorly understood largely due to the difficulty of isolating the influence of specific variables in the field. We address this uncertainty with a custom ring shear device capable of simulating glacier slip in the laboratory at realistic pressures, temperatures, and ice speeds. We tested the influence of two key parameters on sediment deformation—the slip speed of ice and the pressure exerted by the ice on the bed—and computed the rates of sediment transport using an image processing technique called digital image correlation. We find a near‐linear relationship between sediment flux and slip speed, but a nonlinear relationship with ice pressure. Sediment deformation was mostly concentrated in a thin zone near the ice‐sediment interface. Coupling between ice and sediment increased at faster ice velocities and higher ice pressures, which scaled sediment transport rates. Key Points: Sediment flux in deforming till varies non‐monotonically with effective stressThe relationship between sediment flux and the speed of basal ice is nearly linearCoupling between ice and till causes the grain velocity profile of the bed to scale with slip speed [ABSTRACT FROM AUTHOR]

Storstrømmen and L. Bistrup Bræ are 20‐ and 10‐km wide, surge type glaciers in North Greenland in quiescent phase that terminate in the southernmost floating ice tongue in East Greenland. Novel multi‐beam echo sounding data collected in August 2020 indicate a seabed at 350–400 m depth along a relatively uniform ice shelf front, 100 m deeper than expected, but surrounded by shallower terrain (<100 m) over a 30‐km wide region that blocks the access of warm, salty, subsurface Atlantic Intermediate Water (AIW) at +1.6°C. Conductivity temperature depth data reveal waters in front of the glaciers at −1.8°C not connected to AIW in the outer fjord, Dove Bugt. The recent grounding line retreat of the glaciers is attributed to glacier thinning at its ablation rate, with little influence of ocean waters, which illustrates the fundamental importance of knowing the bathymetry of glacial fjords. Plain Language Summary: Storstrømmen Glacier is a major glacier in Greenland that is part of the Northeast Ice Stream. The first oceanographic survey of the glacier ‐ and its neighbor L. Bistrup Bræ ‐ reveals an over‐deepening in front of the glacier, filled with cold water (−1.8°C) not connected to the deeper (400 m), warmer (+1.6°C) waters of Atlantic origin in Dove Bugt which have warmed by 0.7°C ± 0.2°C since the 1960s–1990s. Most of the grounding line retreat of Storstrømmen—and L. Bistrup Bræ—since 1996 is explained by glacier thinning at its ablation rate. Shallow bathymetry (100 m) at the glacier ice shelf fronts has protected them from the influence of warm waters of Atlantic origin. The result illustrates the fundamental importance of knowing the bathymetry of glacial fjords to understand the evolution of Greenland glaciers in warming climate. Key Points: Storstrømmen Glacier, Greenland (27‐cm sea level rise equivalent) stands in cold waters behind shallow bathymetryThe frontal regions are 100 m deeper than expected but access of warm Atlantic Intermediate Water (AIW) is limited by a wide and shallow ridgeRetreat of Storstrømmen and L. Bistrup Bræ glaciers has operated relatively independent of changes in AIW temperature in Dove Bugt [ABSTRACT FROM AUTHOR]

The deglaciation record of the Ontario Lowland and Mohawk Valley of North America is important for constraining the retreat history of the Laurentide Ice Sheet, end-Pleistocene paleoclimate, and ice-sheet processes. The Mohawk Valley was an important meltwater drainage route during the last deglaciation, with the area around modern Oneida Lake acting as a valve for meltwater discharge into the North Atlantic Ocean. The Mohawk Valley was occupied by the Oneida Lobe and Oneida Ice Stream during the last deglacial period. Multichannel seismic reflection data can be used to generate images of preglacial surfaces and internal structures of glacial bedforms and proglacial lake deposits, thus contributing to studies of deglaciation. This paper uses 217 km of offshore multichannel seismic reflection data to image the entire Quaternary section of the Oneida basin. A proglacial lake and paleo-calving margin is interpreted, which likely accelerated the Oneida Ice Stream, resulting in elongated bedforms observed west of the lake. The glacial bedforms identified in this study are buried by proglacial lake deposits, indicating the Oneida basin contains a record of glacial meltwater processes, including a 60-m-thick proglacial interval in eastern Oneida Lake. [ABSTRACT FROM AUTHOR]

The West Antarctic Ice Sheet (WAIS) presently holds enough ice to raise global sea level by 4.3 m if completely melted. The unknown response of the WAIS to future warming remains a significant challenge for numerical models in quantifying predictions of future sea level rise. Sea level rise is one of the clearest planet-wide signals of human-induced climate change. The Sensitivity of the West Antarctic Ice Sheet to a Warming of 2 ∘ C (SWAIS 2C) Project aims to understand past and current drivers and thresholds of WAIS dynamics to improve projections of the rate and size of ice sheet changes under a range of elevated greenhouse gas levels in the atmosphere as well as the associated average global temperature scenarios to and beyond the + 2 ∘ C target of the Paris Climate Agreement. Despite efforts through previous land and ship-based drilling on and along the Antarctic margin, unequivocal evidence of major WAIS retreat or collapse and its causes has remained elusive. To evaluate and plan for the interdisciplinary scientific opportunities and engineering challenges that an International Continental Drilling Program (ICDP) project along the Siple coast near the grounding zone of the WAIS could offer (Fig. 1), researchers, engineers, and logistics providers representing 10 countries held a virtual workshop in October 2020. This international partnership comprised of geologists, glaciologists, oceanographers, geophysicists, microbiologists, climate and ice sheet modelers, and engineers outlined specific research objectives and logistical challenges associated with the recovery of Neogene and Quaternary geological records from the West Antarctic interior adjacent to the Kamb Ice Stream and at Crary Ice Rise. New geophysical surveys at these locations have identified drilling targets in which new drilling technologies will allow for the recovery of up to 200 m of sediments beneath the ice sheet. Sub-ice-shelf records have so far proven difficult to obtain but are critical to better constrain marine ice sheet sensitivity to past and future increases in global mean surface temperature up to 2 ∘ C above pre-industrial levels. Thus, the scientific and technological advances developed through this program will enable us to test whether WAIS collapsed during past intervals of warmth and determine its sensitivity to a + 2 ∘ C global warming threshold (UNFCCC, 2015). [ABSTRACT FROM AUTHOR]

We present a high-resolution airborne radar data set (EGRIP-NOR-2018) for the onset region of the Northeast Greenland Ice Stream (NEGIS). The radar data were acquired in May 2018 with the Alfred Wegener Institute's multichannel ultra-wideband (UWB) radar mounted on the Polar 6 aircraft. Radar profiles cover an area of ∼24 000 km 2 and extend over the well-defined shear margins of the NEGIS. The survey area is centered at the location of the drill site of the East Greenland Ice-Core Project (EastGRIP), and several radar lines intersect at this location. The survey layout was designed to (i) map the stratigraphic signature of the shear margins with radar profiles aligned perpendicular to ice flow, (ii) trace the radar stratigraphy along several flow lines, and (iii) provide spatial coverage of ice thickness and basal properties. While we are able to resolve radar reflections in the deep stratigraphy, we cannot fully resolve the steeply inclined reflections at the tightly folded shear margins in the lower part of the ice column. The NEGIS is causing the most significant discrepancies between numerically modeled and observed ice surface velocities. Given the high likelihood of future climate and ocean warming, this extensive data set of new high-resolution radar data in combination with the EastGRIP ice core will be a key contribution to understand the past and future dynamics of the NEGIS. The EGRIP-NOR-2018 radar data products can be obtained from the PANGAEA data publisher (https://doi.pangaea.de/10.1594/PANGAEA.928569 ;). [ABSTRACT FROM AUTHOR]