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Thwaites Glacier (TG) plays an important role in future sea-level rise (SLR) contribution from the West Antarctic Ice Sheet. Recent observations show that TG is losing mass, and its grounding zone is retreating. Previous modeling has produced a wide range of results concerning whether, when, and how rapidly further retreat will occur under continued warming. These differences arise at least in part from ill-constrained processes, including friction from the bed, and future atmosphere and ocean forcing affecting ice-shelf and grounding-zone buttressing. Here, we apply the Ice Sheet and Sea-level System Model (ISSM) with a range of specifications of basal sliding behavior in response to varying ocean forcing. We find that basin-wide bed character strongly affects TG's response to sub-shelf melt by modulating how changes in driving stress are balanced by the bed as the glacier responds to external forcing. Resulting differences in dynamic thinning patterns alter modeled grounding-line retreat across Thwaites' catchment, affecting both modeled rates and magnitudes of SLR contribution from this critical sector of the ice sheet. Bed character introduces large uncertainties in projections of TG under equal external forcing, pointing to this as a crucial constraint needed in predictive models of West Antarctica.

Direct observations of the size of the Greenland Ice Sheet during Quaternary interglaciations are sparse yet valuable for testing numerical models of ice-sheet history and sea level contribution. Recent measurements of cosmogenic nuclides in bedrock from beneath the Greenland Ice Sheet collected during past deep-drilling campaigns reveal that the ice sheet was significantly smaller, and perhaps largely absent, sometime during the past 1.1 million years. These discoveries from decades-old basal samples motivate new, targeted sampling for cosmogenic-nuclide analysis beneath the ice sheet. Current drills available for retrieving bed material from the US Ice Drilling Program require < 700 m ice thickness and a frozen bed, while quartz-bearing bedrock lithologies are required for measuring a large suite of cosmogenic nuclides. We find that these and other requirements yield only ∼ 3.4 % of the Greenland Ice Sheet bed as a suitable drilling target using presently available technology. Additional factors related to scientific questions of interest are the following: which areas of the present ice sheet are the most sensitive to warming, where would a retreating ice sheet expose bare ground rather than leave a remnant ice cap, and which areas are most likely to remain frozen bedded throughout glacial cycles and thus best preserve cosmogenic nuclides? Here we identify locations beneath the Greenland Ice Sheet that are best suited for potential future drilling and analysis. These include sites bordering Inglefield Land in northwestern Greenland, near Victoria Fjord and Mylius-Erichsen Land in northern Greenland, and inland from the alpine topography along the ice margin in eastern and northeastern Greenland. Results from cosmogenic-nuclide analysis in new sub-ice bedrock cores from these areas would help to constrain dimensions of the Greenland Ice Sheet in the past.

Basal icequakes are generated as a glacier slides over its underlying bedrock, and the stick-slip motion of constant loading and unloading releases shear stresses that produce these very small magnitude (ML < 0) glacial microseisms. Detecting and locating nucleation of these fine-scale icequakes can provide highly useful insights into the deformation processes occurring at the bed and consequently the mechanisms governing glacier flow. We present icequake data derived from a seismic array installed at the grounding line of the Rutford Ice Stream in West Antarctica by Penn State University and the British Antarctic Survey during the 2018/19 austral summer. The region’s natural source seismicity was first processed using the earthquake detection and location software QuakeMigrate and the events were relatively relocated using HypoDD/GrowClust. We then clustered the events into sticky spot clusters using the unsupervised clustering algorithm DBSCAN, and finally from the clusters we selected “model” waveforms to perform template matching on the original seismic traces to create methodically comprehensive high-resolution icequake catalogs at the grounding line of Rutford. We present our methodology including the complete processing pipeline (supplemented by developed supporting open-source scripts) along with key tuning parameters, and describe how our catalogs were used to resolve glacier sliding patterns and key topographical features and characteristics of the bed like sticky spots. We additionally explore the effects of tidal modulation and Rutford ice flow motion on icequake occurrences. Our seismic traces primarily contain icequake signals that derive from stick-slip sliding, but also unique waveforms that might be derived from crevassing and teleseisms that we will also explore. Our results show that stick-slip basal icequakes and these resultant icequake catalogs are valuable data-rich resources that help improve our understanding of glacier flow dynamics and will be important toward improving glacier flow models used for constraining global mean sea level rise.

Marine-terminating glaciers lose mass through melting and iceberg calving, and we find that meltwater drainage systems influence calving timing at Helheim Glacier, a tidewater glacier in East Greenland. Meltwater feeds a buoyant subglacial discharge plume at the terminus of Helheim Glacier, which rises along the glacial front and surfaces through the mélange. Here, we use high-resolution satellite and time-lapse imagery to observe the surface expression of this meltwater plume and how plume timing and location compare with that of calving and supraglacial meltwater pooling from 2011 to 2019. The plume consistently appeared at the central terminus even as the glacier advanced and retreated, fed by a well-established channelized drainage system with connections to supraglacial water. All full-thickness calving episodes, both tabular and non-tabular, were separated from the surfacing plume by either time or by space. We hypothesize that variability in subglacial hydrology and basal coupling drive this inverse relationship between subglacial discharge plumes and full-thickness calving. Surfacing plumes likely indicate a low-pressure subglacial drainage system and grounded terminus, while full-thickness calving occurrence reflects a terminus at or close to flotation. Our records of plume appearance and full-thickness calving therefore represent proxies for the grounding state of Helheim Glacier through time.

Funding: This work is from the MELT project, a component of the International Thwaites Glacier Collaboration (ITGC). Support from the National Science Foundation (NSF, grant no. 1739003) and the Natural Environment Research Council (NERC, grant no. NE/S006656/1). Logistics provided by NSF U.S. Antarctic Program and NERC British Antarctic Survey. The ship-based CTD data were supported by the ITGC TARSAN project (NERC grant nos. NE/S006419/1 and NE/S006591/1; NSF grant no. 1929991). ITGC contribution no. ITGC 047. Thwaites Glacier is one of the fastest-changing ice–ocean systems in Antarctica1,2,3. Much of the ice sheet within the catchment of Thwaites Glacier is grounded below sea level on bedrock that deepens inland4, making it susceptible to rapid and irreversible ice loss that could raise the global sea level by more than half a metre2,3,5. The rate and extent of ice loss, and whether it proceeds irreversibly, are set by the ocean conditions and basal melting within the grounding-zone region where Thwaites Glacier first goes afloat3,6, both of which are largely unknown. Here we show—using observations from a hot-water-drilled access hole—that the grounding zone of Thwaites Eastern Ice Shelf (TEIS) is characterized by a warm and highly stable water column with temperatures substantially higher than the in situ freezing point. Despite these warm conditions, low current speeds and strong density stratification in the ice–ocean boundary layer actively restrict the vertical mixing of heat towards the ice base7,8, resulting in strongly suppressed basal melting. Our results demonstrate that the canonical model of ice-shelf basal melting used to generate sea-level projections cannot reproduce observed melt rates beneath this critically important glacier, and that rapid and possibly unstable grounding-line retreat may be associated with relatively modest basal melt rates. Publisher PDF

Direct observations of the size of the Greenland Ice Sheet during Quaternary interglaciations are sparse yet valuable for testing numerical models of ice-sheet history and sea level contribution. Recent measurements of cosmogenic nuclides in bedrock from beneath the Greenland Ice Sheet collected during past deep-drilling campaigns reveal that the ice sheet was significantly smaller, and perhaps largely absent, sometime during the past 1.1 million years. These discoveries from decades-old basal samples motivate new, targeted sampling for cosmogenic-nuclide analysis beneath the ice sheet. Current drills available for retrieving bed material from the US Ice Drilling Program require < 700 m ice thickness and a frozen bed, while quartz-bearing bedrock lithologies are required for measuring a large suite of cosmogenic nuclides. We find that these and other requirements yield only ∼ 3.4 % of the Greenland Ice Sheet bed as a suitable drilling target using presently available technology. Additional factors related to scientific questions of interest are the following: which areas of the present ice sheet are the most sensitive to warming, where would a retreating ice sheet expose bare ground rather than leave a remnant ice cap, and which areas are most likely to remain frozen bedded throughout glacial cycles and thus best preserve cosmogenic nuclides? Here we identify locations beneath the Greenland Ice Sheet that are best suited for potential future drilling and analysis. These include sites bordering Inglefield Land in northwestern Greenland, near Victoria Fjord and Mylius-Erichsen Land in northern Greenland, and inland from the alpine topography along the ice margin in eastern and northeastern Greenland. Results from cosmogenic-nuclide analysis in new sub-ice bedrock cores from these areas would help to constrain dimensions of the Greenland Ice Sheet in the past.

Sea-level rise of even one meter will have drastic global impacts. Melting the Greenland Ice Sheet (GIS) would raise sea level by 7.4 meters. There is an urgent need to improve predictions of how quickly the GIS will contribute its first meter of sea-level rise, and from where on the ice sheet that water will come. Estimating the volume of Greenland ice that was lost during past warm periods offers a way to constrain the ice sheet’s likely response to future warming. Here, we assess the sea-level potential across Greenland, based on an ensemble of ice-sheet model simulations that represent a wide range of plausible deglaciation styles. The most vulnerable region of the ice sheet is in West Greenland between approximately 64ºN and 76ºN, ranging from ~10 to ~150 km behind the present-day ice margin. The ensemble spread for the most stable regions of the GIS is sensitive to lithospheric feedbacks, while the most vulnerable GIS region is predominantly sensitive to spatial climatology and precipitation lapse rate. These results can guide future subglacial sampling by identifying regions and locations where such data will have the greatest impact on our understanding of ice-sheet vulnerability/contribution to sea-level rise in a warming world.

Seismic studies of glaciers yield insights into spatio-temporal processes within and beneath glaciers on scales relevant to flow and deformation of the ice. These methods enable direct monitoring of the bed in ways that complement other geophysical techniques, such as geodetic or ground penetrating radar observations. In this work, we report on the analysis of passive seismic data collected from the interior of the North East Greenland Ice Stream, the Greenland ice sheet's largest outlet glacier. We record thousands of basal earthquakes, many of which repeat with nearly identical waveforms. We also record many long-duration glacial tremor episodes that migrate across the seismic network with slow velocities (e.g. ~4–12 m s−1). Analysis of the basal earthquakes indicates a transition between times of individual event activity and times of tremor activity. We suggest that both processes are produced by shear slip at localized asperities along the bed. The transition between discrete and quasi-continuous slipping modes may be driven by pore-water pressure transients or heterogeneous strain accumulation in the ice due to strength contrasts of the underlying till.

Ice sheets reshape Earth’s surface. Maps of the landscape formed by past ice sheets are our best tool for reconstructing historic ice sheet behavior. But models of glacier erosion and deposition that explain mapped features are relatively untested, and without observations of landforms developing in situ, postglacial landscapes can provide only qualitative insight into past ice sheet conditions. Here we present the first swath radar data collected in Antarctica, demonstrating the ability of swath radar technology to map the subglacial environment of Thwaites Glacier (West Antarctica) at comparable resolutions to digital elevation models of deglaciated terrain. Incompatibility between measured bedform orientation and predicted subglacial water pathways indicates that ice, not water, is the primary actor in initiating bedform development at Thwaites Glacier. These data show no clear relationship between morphology and glacier speed, a weak relationship between morphology and basal shear stress, and highlight a likely role for preexisting geology in glacial bedform shape.

We describe elongate, wet, subglacial bedforms in the shear margins of the NE Greenland Ice Stream and place some constraints on their formation. Lateral shear margin moraines have been observed across the previously glaciated landscape, but little is known about the ice-flow conditions necessary to form these bedforms. Here we describe in situ sediment bedforms under the NE Greenland Ice Stream shear margins that are observed in active-source seismic and ground-penetrating radar surveys. We find bedforms in the shear margins that are ~500 m wide, ~50 m tall, and elongated nearly parallel to ice-flow, including what we believe to be the first subglacial observation of a shear margin moraine. Acoustic impedance analysis of the bedforms shows that they are composed of unconsolidated, deformable, water-saturated till. We use these geophysical observations to place constraints on the possible formation mechanism of these subglacial features.

Recent seismic measurements from upper Thwaites Glacier indicate that the bed-type variability is closely related to the along-flow basal topography. In high-relief subglacial highlands, stoss sides of topographic highs have a relatively higher acoustic impedance (‘hard’ bed) with lower acoustic impedance (‘soft’ till) on lee sides. This pattern is similar to observations of many deglaciated terrains. Subglacial hydraulic-potential gradient and its divergence show a tendency for water to diverge over the stoss sides and converge into the lee sides. Convergence favors a thicker or more widespread water system, which can more efficiently decouple ice from the underlying till. Under such circumstances, till deformation does occur but, fluxes are relatively small. Till carried from the lee sides onto stoss sides of downstream bumps should couple to the ice more efficiently, increasing the ability for transport by till deformation. In turn, this suggests that steady-state till transport can be achieved if the stoss-side till layer is thin or discontinuous. In addition, the large basal shear stress generated in the highlands seems too high for a bed lubricated by a continuous although thin deforming till, suggesting till discontinuity, which would allow debris-laden ice to erode bedrock on stoss sides, supplying additional till for transport.

We present a shear wave model of the West Antarctic upper mantle to ∼200 km depth with enhanced regional resolution from the 2016-2018 UK Antarctic Seismic Network. The model is constructed from the combination of fundamental mode Rayleigh wave phase velocities extracted from ambient noise (periods 8-25 s) and earthquake data by two-plane wave analysis (periods 20-143 s). We seek to (i) image and interpret structures against the tectonic evolution of West Antarctica, and (ii) extract information from the seismic model that can serve as boundary conditions in ice sheet and glacial isostatic adjustment modelling efforts. The distribution of low velocity anomalies in the uppermost mantle suggests that recent tectonism in the West Antarctic Rift System (WARS) is mainly concentrated beneath the rift margins and largely confined to the uppermost mantle (

We present an upper mantle P-wave velocity model for the Ross Sea Embayment (RSE) region of West Antarctica, constructed by inverting relative P-wave travel-times from 1881 teleseismic earthquakes recorded by two temporary broadband seismograph deployments on the Ross Ice Shelf, as well as by regional ice- and rock-sited seismic stations surrounding the RSE. Faster upper mantle P-wave velocities ( ∼ + 1 % ) characterize the eastern part of the RSE, indicating that the lithosphere in this part of the RSE may not have been reheated by mid-to-late Cenozoic rifting that affected other parts of the Late Cretaceous West Antarctic Rift System. Slower upper mantle velocities ( ∼ − 1 % ) characterize the western part of the RSE over a ∼500 km-wide region, extending from the central RSE to the Transantarctic Mountains (TAM). Within this region, the model shows two areas of even slower velocities ( ∼ − 1.5 % ) centered beneath Mt. Erebus and Mt. Melbourne along the TAM front. We attribute the broader region of slow velocities mainly to reheating of the lithospheric mantle by Paleogene rifting, while the slower velocities beneath the areas of recent volcanism may reflect a Neogene-present phase of rifting and/or plume activity associated with the formation of the Terror Rift. Beneath the Ford Ranges and King Edward VII Peninsula in western Marie Byrd Land, the P-wave model shows lateral variability in upper mantle velocities of ±0.5% over distances of a few hundred km. The heterogeneity in upper mantle velocities imaged beneath the RSE and western Marie Byrd Land, assuming no significant variation in mantle composition, indicates variations in upper mantle temperatures of at least 100 °C. These temperature variations could lead to differences in surface heat flow of ∼±10 mW/m2 and mantle viscosity of 102 Pa s regionally across the study area, possibly influencing the stability of the West Antarctic Ice Sheet by affecting basal ice conditions and glacial isostatic adjustment.

Polar research using ice-penetrating radars will benefit from the development of smaller and more flexible systems. In this paper, we present the development progression for a series of compact and frequency-reconfigurable ice-penetrating radars based on software-defined radio (SDR) technology and platforms. The Ice Radar series we developed consists of three systems, each based on a different SDR platform that afforded additional capability as we progressed. Detailed descriptions of system structure and performance are provided for each ice radar. We present laboratory measurements to support system capabilities, and provide results of field experiments at Subglacial Lake Whillans in West Antarctica for one of the systems. Finally, we discuss ongoing research of ice-penetrating radar development that utilize SDR technology.

Seismic measurements on Thwaites Glacier show a spatially variable bed character, with implications for ice-sheet stability. The West Antarctic Ice Sheet is losing mass rapidly through outlet glaciers and ice streams in the Amundsen Sea Embayment, including Thwaites Glacier, where limited observations and modeling suggest that ice-flow rates depend on bed properties. Here we characterize bed properties of Thwaites Glacier based on amplitude analysis of reflection-seismic data from a ∼40-km-long profile collected in the approximate flow direction and two ∼10-km-long profiles transverse to flow. The upstream portion of the seismic profile reveals a ∼12-km long sedimentary basin with ice-flow-aligned bedforms capped by a continuous till layer that is likely soft and deforming (porosity ∼0.4–0.45), with several locations where water has pooled at the bed. Downstream of the sedimentary basin, the bed rises by ∼400 m over ∼25 km into subglacial highlands. Our seismic survey of these subglacial highlands reveals strong spatial variations in bed character across rugged topography (∼200 m amplitude at ∼2- to 5-km wavelength) resembling crag-and-tails. Till on the stoss sides (facing upglacier) of topographic highs is more consolidated (porosity ∼0.3–0.35 or lower), whereas the lee sides (facing downglacier) and flat regions exhibit porosity similar to the till of the upstream sedimentary basin. Modeling studies could use the observed correlation between bed character and bed aspect and slope to extend our observations to other parts of Thwaites Glacier, resulting in more-realistic models of future grounding-line retreat. Our findings highlight the need for more geophysical constraints on bed properties for important outlets in Antarctica and Greenland.

The history of the Greenland Ice Sheet has been influenced by the geodynamic response to ice sheet fluctuations, and this interaction may help explain past deglaciations under modest climate forcing. We hypothesize that when the Iceland hot spot passed beneath north‐central Greenland, it thinned the lithosphere and left anomalous heat likely with partially melted rock; however, it did not break through the crust to supply voluminous flood basalts. Subsequent Plio‐Pleistocene glacial‐interglacial cycles caused large and rapidly migrating stresses, driving dike formation and other processes that shifted melted rock toward the surface. The resulting increase in surface geothermal flux favored a thinner, faster‐responding ice sheet that was more prone to deglaciation. If this hypothesis of control through changes in geothermal flux is correct, then the long‐term (10 (sup 5) to 10 (sup 6) years) trend now is toward lower geothermal flux, but with higher‐frequency (less than or equal to 10( sup 4) to 10 (sup 5) years) oscillations linked to glacial‐interglacial cycles. Whether the geothermal flux is increasing or decreasing now is not known but is of societal relevance due to its possible impact on ice flow. We infer that projections of the future of the ice sheet and its effect on sea level must integrate geologic and geophysical data as well as glaciological, atmospheric, oceanic, and paleoclimatic information.

Subglacial and ice-sheet marginal sedimentary basins have very different physical properties to crystalline bedrock and, therefore, form distinct conditions that influence the flow of ice above. Sedimentary rocks are particularly soft and erodible, and therefore capable of sustaining layers of subglacial till that may deform to facilitate fast ice flow downstream. Furthermore, sedimentary rocks are relatively permeable and thus allow for enhanced fluid flux, with associated impacts on ice-sheet dynamics, including feedbacks with subglacial hydrologic systems and transport of heat to the ice-sheet bed. Despite the importance for ice-sheet dynamics there is, at present, no comprehensive record of sedimentary basins in the Antarctic continent, limiting our capacity to investigate these influences. Here we develop the first version of an Antarctic-wide spatial database of sedimentary basins, their geometries and physical attributes. We emphasise the definition of in-situ and undeformed basins that retain their primary characteristics, including relative weakness and high permeability, and therefore are more likely to influence ice sheet dynamics. We define the likely extents and nature of sedimentary basins, considering a range of geological and geophysical data, including: outcrop observations, gravity and magnetic data, radio-echo sounding data and passive and active-source seismic data. Our interpretation also involves derivative products from these data, including analyses guided by machine learning. The database includes for each basin its defining characteristics in the source datasets, and interpreted information on likely basin age, sedimentary thickness, surface morphology and tectonic type. The database is constructed in ESRI geodatabase format and is suitable for incorporation in multifaceted data-interpretation and modelling procedures. It can be readily updated given new information. We define extensive basins in both East and West Antarctica, including major regions in the Ross and Weddell Sea embayments and the Amundsen Sea region of West Antarctica, and the Wilkes, Aurora and Recovery subglacial basins of East Antarctica. The compilation includes smaller basins within crystalline-bedrock dominated areas such as the Transantarctic Mountains, the Antarctic Peninsula and Dronning Maud Land. The distribution of sedimentary basins reveals the combined influence of the tectonic and glacial history of Antarctica on the current and future configuration of the Antarctic Ice Sheet and highlights areas in which the presence of dynamically-evolving subglacial till layers and the exchange of groundwater and heat with the ice sheet bed are more likely, contributing to dynamic behaviour of the Antarctic Ice Sheet.

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

Microseismicity, induced by the sliding of a glacier over its bed, can be used to characterize frictional properties of the ice-bed interface, which are a key parameter controlling ice stream flow....

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

Ice stream flow is predominantly controlled by sliding over the bed, deformation within the bed and deformation within the ice column. The significance of processes at the bed, now and in the future, remains uncertain due to a lack of knowledge of conditions at the ice stream bed. In the Austral summer of 2018/19, as part of the BEAMISH Project, three holes were drilled to the bed of Rutford Ice Stream to install instruments in the ice column and at the bed, and also sample the bed. Prior to drilling, three seismic profiles were acquired across the bed access sites. These data therefore provide a rare opportunity to compare in situ measurements of ice stream bed conditions with seismic reflection data. The seismic line acquisition was also repeated one year later to investigate any changes at the bed following the drilling and connection to the bed. We will use a combination of imaging, acoustic impedance calculation and wide-angle reflection amplitude variation to characterise the bed conditions using the seismic data.

Microseismicity, induced by the sliding of a glacier over its bed and through bed deformation, can be used to characterize frictional properties of the ice-bed interface. Together with ice column deformation, these characteristics form the key parameters controlling ice stream flow. Here, we use naturally occuring seismicity to monitor temporal and spatial changes in bed properties at Rutford Ice Stream (RIS), West Antarctica, in order to characterize ongoing basal deformation and sliding. RIS is a significant contributor to the outflow of ice from West Antarctica, with speeds of ~1.1 m/day. Past geological and geophysical surveys, including drilling into the bed itself, have revealed pronounced bed topography and a sharp change in bed character along flow direction from presumably soft deformable to stiffer sediments. These complementary data as well as Rutford’s flow characteristics allow us to interpret the seismic data in their geological context.Our data consist of three months of seismic recordings from a 35-station seismic network located ~40 km upstream the grounding line of RIS, being collected in the framework of the BEAMISH project during the 2018/19 field season. An event catalogue derived using the QuakeMigrate and Nonlinloc software packages reveals an active seismic environment (~40,000 events in three months) with locally clustered microseismicity. Microseismicity occurs near the ice-bed interface and is concentrated in the transition region between presumed-soft and presumed-hard sediments. Within the more compacted sediments further seismicity occurs, predominantly along topographic lows, which form elongated, flow parallel sub-glacial valleys. Within the regions of activity, seismicity tends to cluster in focused spots of particular high activity. Repeated basal seismicity at spatially restricted locations has been observed before and was interpreted as being caused by ‘sticky spots’ within a more ductile deforming matrix. Our results, showing a close alignment of these sticky spots along structural and topographic boundaries, may indicate that such features form major obstacles for basal glacial sliding. In addition to these spatial variations, the average event frequency varies over time. We estimate an ~15 day periodicity to the activity with as many as 1200 events/day during the active times and as few as ~100 events per day during the more-quiescent times. This roughly corresponds to the period of the spring-neap tidal cycle which has been shown to modulate the horizontal flow velocity of RIS. Time dependent variations in the frequency of microseismicity might suggest the glacial bed affected by these modulations.

Author(s): Shen, Weisen; Wiens, Douglas A; Anandakrishnan, Sridhar; Aster, Richard C; Gerstoft, Peter; Bromirski, Peter D; Hansen, Samantha E; Dalziel, Ian W. D; Heeszel, David S; Huerta, Audrey D; Nyblade, Andrew A; Stephen, Ralph; Wilson, Terry J; Winberry, J. Paul

The geoPebble system is a network of wirelessly interconnected seismic and GPS sensor nodes with geophysical sensing capabilities for the study of ice sheets in Antarctica and Greenland, as well as mountain glaciers. We describe our design methodology, which has enabled us to develop these state-of-the art units using commercial-off-the-shelf hardware combined with custom-designed hardware and software. Each geoPebble node is a self-contained, wirelessly connected sensor for collecting seismic activity and position information. Each node is built around a three-component seismic recorder, which includes an amplifier, filter, and 24-bit analog-to-digital converter that can sample incoming seismic signals up to 10 kHz. The timing for each node is available from GPS measurements and a local precision oscillator that is conditioned by the GPS timing pulses. In addition, we record the carrier-phase measurement of the L1 GPS signal in order to determine location at sub-decimeter accuracy (relative to other geoPebble nodes within a radius of a few kilometers). Each geoPebble includes 32 GB of solid-state storage, wireless communications capability to a central supervisory unit, and auxiliary measurements capability (including tilt from accelerometers, absolute orientation from magnetometers, and temperature). The geoPebble system has been successfully validated in the field in Antarctica and Greenland.

Despite the importance of grounding zone sedimentation for ice sheet stability and ice sheet history, evidence for sedimentary deposits beneath West Antarctica's modern grounding zone remains sparse. Seismic surveying shows that Kamb Ice Stream has no detectable grounding zone deposit. As grounding zone deposition relies strongly on ice flow, the absence of a deposit suggests that the transition from the ice stream to the ice shelf has moved after stagnation of Kamb Ice Stream. Further support for a recent grounding-zone occupation comes from satellite imagery of sub ice-shelf channel features that likely originated at previous grounding-zone locations. These features begin 25 km seaward of the current grounding zone and cut across ice-flow streak lines. We estimate that retreat to the modern grounding-zone position was abrupt at rates >0.2 km a−1.

Accurately monitoring and predicting the evolution of the West Antarctic Ice Sheet via secular changes in the Earth's gravity field requires knowledge of the underlying upper mantle viscosity structure. Published seismic models show the West Antarctic lithosphere to be ∼70–100 km thick and underlain by a low velocity zone extending to at least ∼200 km. Mantle viscosity is dependent on factors including temperature, grain size, the hydrogen content of olivine, the presence of partial melt and applied stress. As seismic wave propagation is particularly sensitive to thermal variations, seismic velocity provides a means of gauging mantle temperature. In 2012, a magnitude 5.6 intraplate earthquake in Marie Byrd Land was recorded on an array of POLENET-ANET seismometers deployed across West Antarctica. We modelled the waveforms recorded by six of the seismic stations in order to determine realistic estimates of temperature and lithology for the lithospheric mantle beneath Marie Byrd Land and the central West Antarctic Rift System. Published mantle xenolith and magnetotelluric data provided constraints on grain size and hydrogen content, respectively, for viscosity modelling. Considering tectonically-plausible stresses, we estimate that the viscosity of the lithospheric mantle beneath Marie Byrd Land and the central West Antarctic Rift System ranges from ∼ 10 20 – 10 22 Pa s. To extend our analysis to the sublithospheric seismic low velocity zone, we used a published shear wave model. We calculated that the velocity reduction observed between the base of the lithosphere (∼4.4–4.7 km/s) and the centre of the low velocity zone (∼4.2–4.3 km/s) beneath West Antarctica could be caused by a 0.1–0.3% melt fraction or a one order of magnitude reduction in grain size. However, the grain size reduction is inconsistent with our viscosity modelling constraints, suggesting that partial melt more feasibly explains the origin of the low velocity zone. Considering plausible asthenospheric stresses, we estimate the viscosity of the seismic low velocity zone beneath West Antarctica to be ∼ 10 18 – 10 19 Pa s. It has been shown elsewhere that the inclusion of a low viscosity layer of order 1019 Pa s in Fennoscandian models of glacial isostatic adjustment reduces disparities between predicted surface uplift rates and corresponding field observations. The incorporation of a low viscosity layer reflecting the seismic low velocity zone in Antarctic glacial isostatic adjustment models might similarly lessen the misfit with observed uplift rates.

The complex flow fields of the Antarctic and Greenland ice sheets deform layers deposited as snow at the ice-sheet surface, leaving a record of the regional flow history and/or local transitions in basal boundary conditions within the geometry of ice-sheet layers. Ice-penetrating radar reveals these layers, but interpretations are limited by the challenges of quantitatively and reproducibly comparing observations with model output. We present a conceptual framework that relates along-track reflector slope to gradients in the steady-state velocity field of ice sheets. This method makes effective use of englacial reflectors in regions where it is challenging to image continuous layers, and avoids the error propagation inherent to tracer-transport methods, developing the potential for formal radar data assimilation in future modeling studies. We apply our method to radar data collected at the grounding line of Whillans Ice Stream, where enhanced bed-friction produces characteristic reflector slopes reproducible using a higher-order ice flow model.

We describe methods for measuring crystal orientation fabric with sonic waves in an ice core borehole, with special attention paid to vertical-girdle fabrics that are prevalent at the WAIS Divide. The speed of vertically propagating compressional waves in ice is influenced by vertical clustering of the ice crystal c-axes. Shear-wave speeds – particularly the speed separation between fast and slow shear polarizations – are sensitive to azimuthal anisotropy. Sonic data from the WAIS Divide complement thin-section measurements of fabric. Thin sections show a steady transition to strong girdle fabrics in the upper 2000 m of ice, followed by a transition to vertical-pole fabrics below 2500 m depth. Compressional-wave sonic data are inconclusive in the upper ice, due to noise, as well as the method's inherent insensitivity to girdle fabrics. Compared with available thin sections, sonic data provide better resolution of the transition to pole fabrics below 2500 m, notably including an abrupt increase in vertical clustering near 3000 m. Our compressional-wave measurements resolve fabric changes occurring over depth ranges of a few meters that cannot be inferred from available thin sections, but are sensitive only to zenithal anisotropy. Future logging tools should be designed to measure shear waves in addition to compressional waves, especially for logging in regions where ice flow patterns favor the development of girdle fabrics.

Using 8‐25s period Rayleigh and Love wave phase velocity dispersion data extracted from seismic ambient noise, we (i) model the 3D shear wave velocity structure of the West Antarctic crust and (ii) map variations in crustal radial anisotropy. Enhanced regional resolution is offered by the UK Antarctic Seismic Network. In the West Antarctic Rift System (WARS), a ridge of crust ~26‐30km thick extending south from Marie Byrd Land separates domains of more extended crust (~22km thick) in the Ross and Amundsen Sea Embayments, suggesting along‐strike variability in the Cenozoic evolution of the WARS. The southern margin of the WARS is defined along the southern Transantarctic Mountains (TAM) and Haag Nunataks‐Ellsworth Whitmore Mountains (HEW) block by a sharp crustal thickness gradient. Crust ~35‐40km is modelled beneath the Haag Nunataks‐Ellsworth Mountains, decreasing to $\sim$30‐32\,km km thick beneath the Whitmore Mountains, reflecting distinct structural domains within the composite HEW block. Our analysis suggests that the lower crust and potentially the mid crust is positively radially anisotropic (VSH > VSV) across West Antarctica. The strongest anisotropic signature is observed in the HEW block, emphasising its unique provenance amongst West Antarctica's crustal units, and conceivably reflects a ~13\,km thick metasedimentary succession atop Precambrian metamorphic basement. Positive radial anisotropy in the WARS crust is consistent with observations in extensional settings, and likely reflects the lattice‐preferred orientation of minerals such as mica and amphibole by extensional deformation. Our observations support a contention that anisotropy may be ubiquitous in continental crust.

We present a comprehensive ice-penetrating radar survey of a subglacial embayment and adjacent peninsula along the grounding zone of Whillans Ice Stream, West Antarctica. Through basal waveform and reflectivity analysis, we identify four distinct basal interfaces: (1) an ice–water-saturated till interface inland of grounding; (2) a complex interface in the grounding zone with variations in reflectivity and waveforms caused by reflections from fluting, sediment deposits and crevasses; (3) an interface of anomalously low reflectivity downstream of grounding in unambiguously floating areas of the embayment due to basal roughness and entrained debris; and (4) a high-reflectivity ice–seawater interface that occurs immediately seaward of grounding at the subglacial peninsula and several kilometers seaward of grounding in the embayment, occurring after basal debris and grounding-zone flutes have melted off the ice bottom. Sediment deposition via basal debris melt-out occurs in both locations. The higher basal-melt rate at the peninsula contributes to greater grounding-line stability by enabling faster construction of a stabilizing sediment wedge. In the embayment, the low slopes of the ice bottom and bed prevent development of a strong thermohaline circulation leading to a lower basal-melt rate and less-rapid sediment deposition. Thus, grounding lines in subglacial embayments are more likely to lack stabilizing sediment deposits and are more prone to external forcing, whether from the ocean, the subglacial water system, or large-scale ice dynamics. Our conclusions indicate that subglacial peninsulas and embayments should be treated differently in ice-sheet–ocean models if these models are to accurately simulate grounding-line response to external forcing.

For common-offset radar data, there is no clear way to disentangle path effects from reflector characteristics, so efforts to determine the physical properties at the bed using reflection amplitude are inherently limited by the constraints on englacial attenuation. In this study, we identify the theoretical considerations required for interpreting bistatic radar surveys, and use data collected on the Northeast Greenland Ice Stream (NEGIS) and Kamb Ice Stream to compute local attenuation profiles. We found that failing to correct for angle-dependent controls on return power (including antenna directivity, the reflection coefficient, and refractive focusing) can bias the computed attenuation rates as much as 30 dB/km for reflectors at 1 km depth. Because the radiation characteristics are the dominant source of uncertainty in our data, we recommend either a simplified survey design for the future (where the antennae are decoupled from the ice surface), or additional data collection to constrain the near-field permittivity and its effect on the radiation pattern. Depth-averaged attenuation rates computed using CMP methods for deep reflectors yield values >10 dB/km higher than attenuation rates computed using common-offset techniques with the same data. We attribute these anomolously high attenuation rates to additional wavenumber (and therefore, angle) dependent interferences between sub-wavelength reflectors.

Understanding the processes that affect streaming ice flow and the mass balance of glaciers and ice sheets requires sound knowledge of their subglacial environments. Previous studies have shown that an extensive deformable subglacial sediment layer favors fast ice-stream flow. However, areas of high basal drag, termed sticky spots, are of particular interest because they inhibit the fast flow of the overriding ice. The stick-slip behavior of Whillans Ice Stream (WIS) is perhaps the most conspicuous manifestation of a subglacial sticky spot. We present new ice-thickness and seismic-reflection measurements collected over the main sticky spot in the ice plain of WIS, allowing us to elucidate its role in the behavior of the ice stream. Ice-thickness and surface-elevation data show that the sticky spot occupies a subglacial topographic high. Water flow in response to the hydrological potential gradient will be routed around the sticky spot if effective pressures are similar on the sticky spot and elsewhere. The seismic experiment imaged a laterally continuous basal layer approximately 6 m thick, having compressional wave velocities of greater than 1800 m s −1 and density greater than 1800 kg m −3 , indicative of a till layer that is stiffer than corresponding till beneath well-lubricated parts of the ice stream. This layer likely continues to deform under the higher shear stress of the sticky spot, and some water may be pumped up onto the sticky spot during motion events.

Tomographic images have shown that there are clear high-velocity heterogeneities to the north of the Pacific Anomaly near the core–mantle boundary (CMB), but the detailed structure and origin of these heterogeneities are poorly known. In this study, we analyze PKP precursors from earthquakes in the Aleutian Islands and Kamchatka Peninsula recorded by seismic arrays in Antarctica, and find that these heterogeneities extend ∼400 km above the CMB and are distributed between 30° and 45°N in latitude. The scatterers show the largest P-wave velocity perturbation of 1.0–1.2% in the center (160–180°E) and ∼0.5% to the west and east (140–160°E, 180–200°E). ScS–S differential travel-time residuals reveal similar features. We suggest that these seismic scatterers are the remnants of ancient subducted slab material. The lateral variations may be caused either by different slabs, or by variations in slab composition resulting from their segregation process.

The seismic velocity structure of Antarctica is important, both as a constraint on the tectonic history of the continent and for understanding solid Earth interactions with the ice sheet. We use Rayleigh wave array analysis methods applied to teleseismic data from recent temporary broadband seismograph deployments to image the upper mantle structure of central and West Antarctica. Phase velocity maps are determined using a two-plane-wave tomography method, and are inverted for shear velocity using a Monte-Carlo approach to estimate three-dimensional velocity structure. Results illuminate the structural dichotomy between the East Antarctic Craton and West Antarctica, with West Antarctica showing thinner crust and slower upper mantle velocity. West Antarctica is characterized by a 70-100 km thick lithosphere, underlain by a low velocity zone to depths of at least 200 km. The slowest anomalies are beneath Ross Island and the Marie Byrd Land dome, and are interpreted as upper mantle thermal anomalies possibly due to mantle plumes. The central Transantarctic Mountains are marked by an uppermost mantle slow velocity anomaly, suggesting that the topography is thermally supported. The presence of thin, higher velocity lithosphere to depths of about 70 km beneath the West Antarctic Rift System limits estimates of the regionally averaged heat flow to less than 90 mW/m2. The Ellsworth-Whitmore block is underlain by mantle with velocities that are intermediate between those of the West Antarctic Rift System and the East Antarctic Craton. We interpret this province as Precambrian continental lithosphere that has been altered by Phanerozoic tectonic and magmatic activity.

Pine Island Glacier (PIG) in the Amundsen Sea sector of the West Antarctic Ice Sheet (WAIS) is losing mass and contributing to global sea-level rise at an accelerating rate. Although recent observations and modeling have identified the incursion of relatively warm Circumpolar Deep Water (CDW) beneath the PIG ice shelf (PIGIS) as the main driver of this ice-mass loss, the lack of precise bathymetry limits furthering our understanding of the ice-ocean interactions and improving the accuracy of modeling. Here we present updated bathymetry and sediment distribution beneath the PIGIS, modeled by the inversion of aerogravity data with constraints from active-source seismic data, observations from an autonomous underwater vehicle, and the regional gravity-anomaly field derived from satellite gravity observations. Modeled bathymetry shows a submarine ridge beneath the middle of PIGIS that rises ∼350 to 400 m above the surrounding sea floor, with a minimum water-column thickness of ∼200 m above it. This submarine ridge continues across the whole width of the 45-km wide ice shelf, with no deep troughs crossing it, confirming the general features of the previously predicted sub-ice-shelf ocean circulation. However, the relatively low resolution of the aerogravity data and limitations in our inversion method leave a possibility that there is an undetected, few-kilometers-wide or narrower trough that may alter the predicted sub-ice-shelf ocean circulation. Modeled sediment distribution indicates a sedimentary basin of up to ∼800 m thick near the current grounding zone of the main PIG trunk and extending farther inland, and a region seaward of the submarine ridge where sediments are thin or absent with exposed crystalline basement that extends seaward into Pine Island Bay. Therefore, the submarine ridge marks the transition from a thick sedimentary basin providing a smooth interface over which ice could flow easily by sliding or sediment deformation, to a region with no to little sediments and instead a rough interface over which ice flows mainly by deformation. We hypothesize that the post-Last Glacial Maximum retreat of PIG stabilized at this location because of the spatial transition in basal conditions. This in turn supports the hypothesis that the recent retreat of PIG was strongly forced, probably by changes in ocean circulation, rather than occurring because of ongoing response to the end of the ice age or other changes inland of or beneath PIG.

Preferred crystal orientation fabrics (COFs) within an ice sheet or glacier are typically found from ice cores. We conducted experiments at the North Greenland Eemian Ice Drilling (NEEM) facility ice core location, where COF data were available at Jakobshavn Isbræ west Greenland, to test if COF can be determined seismically. We used observations of anisotropic seismic wave propagation on multioffset gathers and englacial imaging from a 2D reflection profile. Anisotropy analysis of the NEEM data yielded mean c-axes distributed over a conical region of 30° to 32° from vertical. No internal ice seismic reflectors were imaged. Direct COF measurements collected in the ice core agreed with the seismic observations. At Jakobshavn Isbræ, we used a multioffset gather and a 2D reflection profile, but we lacked ice core data. Englacial reflectors allowed the determination of ice column interval properties. Anisotropy analysis found that the upper 1640 m of the ice column consisted of cold ([Formula: see text]) and mostly isotropic ice with c-axes distributed over a conical region of 80° from vertical. The lower 300 m of the ice column was characterized by warm ([Formula: see text]) ice with COF. These observations were consistent with complex ice fabric development and temperature estimations over the same region of Jakobshavn Isbræ. This study demonstrated that the ice sheet and glacier ice anisotropy information can be gained from seismic field observations.