Basal channels, which are troughs carved into the undersides of ice shelves by buoyant plumes of water, are modulators of ice-shelf basal melt and structural stability. In this study, we track the evolution of 12 large basal channels beneath ice shelves of the Amundsen and Bellingshausen seas region in West Antarctica using the Landsat record since its start in the 1970s through 2020. We observe examples of channel growth, interactions with ice-shelf features, and systematic changes in sinuosity that give insight into the life cycles of basal channels. We use the last two decades of the record, combined with contemporary ice-flow velocity datasets, to separate channel-path evolution into components related to advection by ice flow and those controlled by other forcings, such as ocean melt or surface accumulation. Our results show that ice-flow-independent lateral channel migration is overwhelmingly to the left when viewed down-flow, suggesting that it is dominated by Coriolis-influenced ocean melt. By applying a model of channel-path evolution dominantly controlled by ice flow and ocean melt, we show that the majority of channels surveyed exhibit non-steady behavior that serves as a novel proxy for increased ocean forcing in West Antarctica starting at least in the early 1970s.

Glacier-bed characteristics that are poorly known and modeled are important in projected sea-level rise from ice-sheet changes under strong warming, especially in the Thwaites Glacier drainage of West Antarctica. Ocean warming may induce ice-shelf thinning or loss, or thinning of ice in estuarine zones, reducing backstress on grounded ice. Models indicate that, in response, more-nearly-plastic beds favor faster ice loss by causing larger flow acceleration, but more-nearly-viscous beds favor localized near-coastal thinning that could speed grounding-zone retreat into interior basins where marine-ice-sheet instability or cliff instability could develop and cause very rapid ice loss. Interpretation of available data indicates that the bed is spatially mosaicked, with both viscous and plastic regions. Flow against bedrock topography removes plastic lubricating tills, exposing bedrock that is eroded on up-glacier sides of obstacles to form moats with exposed bedrock tails extending downglacier adjacent to lee-side soft-till bedforms. Flow against topography also generates high-ice-pressure zones that prevent inflow of lubricating water over distances that scale with the obstacle size. Extending existing observations to sufficiently large regions, and developing models assimilating such data at the appropriate scale, present large, important research challenges that must be met to reliably project future forced sea-level rise.

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.

Basal channels, which form where buoyant plumes of ocean water and meltwater carve troughs upwards into ice-shelf bases, are widespread on Antarctic ice shelves. The formation of these features modulates ice-shelf basal melt by influencing the flow of buoyant plumes, and influences structural stability through concentration of strain and interactions with fractures. Because of these effects, and because basal channels can change rapidly, on timescales similar to those of ice-shelf evolution, constraining the impacts of basal channels on ice shelves is necessary for predicting future ice-shelf destabilization and retreat. We suggest that future research priorities should include constraining patterns and rates of basal channel change, determining mechanisms and detailed patterns of basal melt, and quantifying the influence that channel-related fractures have on ice-shelf stability.

Glacial hydraulic tremor (GHT) can be monitored to observe changes in location and distribution of water flow beneath glacial ice, allowing the spatiotemporal evolution of subglacial hydrology to be studied continuously and remotely. We use frequency-dependent polarization analysis (FDPA) to classify types of GHT and assess its spatio-temporal extent beneath Rhonegletscher, Switzerland, in a continuous seismic record through 2018 and 2019 at three ice-proximal bedrock-based seismometers. We determine the frequency bands composing the GHT and calculate back azimuth angles pointing to a previously known subglacial channel. We also inspect the relationship between GHT seismic power and water discharge from the glacier to compare daily and seasonal shifts with the observed GHT. We observed the seasonal shift from a distributed system to a channelized system, and our dataset allowed comparison of channel locations within and across seasons, with implications for sediment evacuation and bed erosion. The successful use of this method to assess GHT previously on Taku glacier (the methods of which this project follows) and now Rhonegletscher shows that existing ice-proximal passive seismic installations can be used to easily and continuously monitor subglacial hydraulic activity.

Glacier motion responds dynamically to changing meltwater inputs, but the multi-decadal response of basal sliding to climate remains poorly constrained due to its sensitivity across multiple timescales. Observational records of glacier motion provide critical benchmarks to decode processes influencing glacier dynamics, but multi-decadal records that precede satellite observation and modern warming are rare. Here we present a record of motion in the ablation zone of Saskatchewan Glacier that spans seven decades. We combine in situ and remote-sensing observations to inform a first-order glacier flow model used to estimate the relative contributions of sliding and internal deformation on dynamics. We find a significant increase in basal sliding rates between melt-seasons in the 1950s and those in the 1990s and 2010s and explore three process-based explanations for this anomalous behavior: (i) the glacier surface steepened over seven decades, maintaining flow-driving stresses despite sustained thinning; (ii) the formation of a proglacial lake after 1955 may support elevated basal water pressures; and (iii) subglacial topography may cause dynamic responses specific to Saskatchewan Glacier. Although further constraints are necessary to ascertain which processes are of greatest importance for Saskatchewan Glacier's dynamic evolution, this record provides a benchmark for studies of multi-decadal glacier dynamics.

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.

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.

Basal ice of glaciers and ice sheets frequently contains a well-developed stratification of distinct, semi-continuous, alternating layers of debris-poor and debris-rich ice. Here, the nature and distribution of shear within stratified basal ice are assessed through the anisotropy of magnetic susceptibility (AMS) of samples collected from Matanuska Glacier, Alaska. Generally, the AMS reveals consistent moderate-to-strong fabrics reflecting simple shear in the direction of ice flow; however, AMS is also dependent upon debris content and morphology. While sample anisotropy is statistically similar throughout the sampled section, debris-rich basal ice composed of semi-continuous mm-scale layers (the stratified facies) possesses well-defined triaxial to oblate fabrics reflecting shear in the direction of ice flow, whereas debris-poor ice containing mm-scale star-shaped silt aggregates (the suspended facies) possesses nearly isotropic fabrics. Thus, deformation within the stratified basal ice appears concentrated in debris-rich layers, likely the result of decreased crystal size and greater availability of unfrozen water associated with high debris content. These results suggest that variations in debris-content over small spatial scales influence ice rheology and deformation in the basal zone.

Measurements of a sample from ~580 m depth in the WAIS Divide (WDC06A) ice core reveal that bubbles are preferentially elongated in the basal plane of their parent grain, as expected if bubble shape preserves the record of dominant basal glide. This suggests that a method using bubbles as strain gauges could provide insights to grain-scale ice deformation. We introduce a technique using fabric and image analyses of paired thin and thick sections. Comparison of the crystallographic orientations of 148 grains and the shape orientations of 2377 intragrain bubbles reveals a strongly preferred elongation of bubbles in the grain basal planes (R2 = 0.96). Elongation magnitudes are consistent with a balance between ice flow deformation and diffusive restoration, with larger bubbles more elongated. Assuming bubbles record ice strain, grains with greater resolved stress on their basal planes from the far-field ice flow stresses show greater deformation, but with large variability suggesting that heterogeneity of the local stress field causes deformation even in unfavorably oriented grains. A correlation is also observed among bubble elongation, grain size, and bubble size, explaining a small but significant fraction of the variance ( P< 0.05), with implications for controls on ice deformation, as discussed here.

Comparison of climate records from Antarctic and Greenland ice cores shows that the two regions respond asynchronously during millennial-scale climate changes. the apparent out-of-phase relationship between the records has been described as a climate ``seesaw’’ in which cooling in the Northern Hemisphere is balanced by warming in the Southern Hemisphere. the same relationship has also been attributed to the initiation of climate-change events in the Southern Hemisphere, rather than the North Atlantic as is conventionally assumed. A simple statistical approach−band-pass filtering combined with lag–correlation tests−used to examine the phase relationships in more detail shows that neither an anti-phase nor a phase-lag relationship adequately describes the observations. Whereas Antarctic and Greenland climate records do exhibit approximate anti-phase behavior about 50% of the time, they are generally in phase during cooling. A phase lead of Southern Hemisphere climate of 1000–1600 years is statistically indistinguishable from a lag of 400–800 years, whether for Dansgaard–Oeschger, Heinrich or longer-duration events. the ``seesaw’’ or ``Southern lead’’ appearance of the data arises from the fundamentally different characteristics of the climate time series, most importantly the absence of rapid warming events in Antarctica comparable to those in Greenland. to be consistent with the observations, climate models will need to capture these characteristics, in addition to reproducing the correct phase relationships.

Reversals in the ice/air surface slope are important in geomorphic and glaciological contexts, thus motivating consideration of the conditions under which they form. Surface slope reversals are seen in numerous places, such as ice rumples on ice shelves, as surficial lakes, and at the down-glacier end of Vostok lake, Antarctica. Such slope reversals can reduce or reverse the subglacial hydrological gradient, thereby rerouting subglacial water transport and possibly leading to the creation of subglacial lakes. Supraglacial lakes produced by slope reversals in ablation zones may aid in driving water-filled cracks that allow surface water access to the bed. Surface slope reversals, in the absence of a concomitant reversal in ice flow, indicate a local violation of the so-called ‘shallow-ice’ approximation, and in this circumstance the longitudinal deviatoric stress becomes critical in the stress equilibrium. Using a simple numerical model, we have explored the conditions under which surface slope reversals form for certain simple scenarios. The results indicate that ice which initially possesses a normal slope will tend toward a reversed slope if the ice is thinned, the bed is strengthened or the downstream buttressing is increased.

Contrary to prior expectations that warming would cause mass addition averaged over the Greenland and Antarctic ice sheets and over the next century, the ice sheets appear to be losing mass, at least partly in response to recent warming. With warming projected for the future, additional mass loss appears more likely than not.

Glaciers often advance over proglacial sediments, which then may enhance basal motion. For glaciers with abundant meltwater, thermodynamic considerations indicate that the sediment–ice contact in the direction of ice flow tends toward an angle opposed to and somewhat steeper than the surface slope (by slightly more than 50%). A simple model based on this hypothesis yields the extent of over-ridden sediments as a function of sediment thickness and strength, a result that may be useful in guiding additional fieldwork for hypothesis testing. Sediment-floored as well as rock-floored overdeepenings are common features along glacier flow paths and are expected based on theories of glacier erosion, entrainment, transport and deposition.

Several different types of laterally extensive debris bands occur along the western terminus region of the Matanuska Glacier, Alaska, U.S.A. An ice-bed process, which to our knowledge has not previously been recognized and described, forms the most common and most prominent type of debris band at Matanuska Glacier’s terminus. The debris bands are composed of one or several millimeter-thick laminations of silt-rich ice having much higher sediment content than that of the surrounding ice. Samples of these bands and their surrounding englacial ice have been analyzed for anthropogenic tritium (3H), oxygen-18 (δ18O), and deuterium (δD).We interpreted the laminated, silt-rich debris bands as basal fractures, along which silt-laden, glaciohydraulically supercooled and pressurized waters flowed, healing the fractures by ice growth. This process is analogous to the inward growth of hydrothermal quartz from the sides of an open fracture.

Sufficiently deep water-filled fractures can penetrate even cold ice-sheet ice, but glaciogenic stresses are typically smaller than needed to propagate water-filled fractures that are less than a few tens of meters deep, as shown by our simplified analytical treatment based on analogous models of magmatic processes. However, water-filled fractures are inferred to reach the bed of Greenland through >1 km of ice and then collapse to form moulins, which are observed. Supraglacial lakes appear especially important among possible crack ‘nucleation’ mechanisms, because lakes can warm ice, supply water, and increase the pressure driving water flow and ice cracking.

Changes in the discharge of West Antarctic ice streams are of potential concern with respect to global sea level. The six relatively thin, fast-flowing Ross ice streams are of interest as low-slope end-members among Antarctic ice streams. Extensive research has demonstrated that these “rivers of ice” have a history of relatively high-frequency , asynchronous discharge variations with evolving lateral boundaries. Amidst this variability, a ∼1300 km grounding-line retreat has occurred since the Last Glacial Maximum. Numerical studies of Ice Stream D (Parizek and others, 2002) indicate that a proposed thermal-regulation mechanism (Clarke and Marshall, 1998; Hulbe and MacAyeal, 1999; Tulaczyk and others, 2000a, b), which could buffer the West Antarctic ice sheet against complete collapse, may be over-ridden by latent-heat transport within melt-water from beneath inland ice. Extending these studies to Ice Stream A, Whillans Ice Stream and Ice Stream C suggests that further grounding-line retreat contributing to sea-level rise is possible thermodynamically However, the efficiency of basal water distribution may be a constraint on the system. Because local thermal deficits promote basal freeze-on (especially on topographic highs), observed short-term variability is likely to persist.

Self-organizing maps (SOMs) provide a powerful, non-linear technique to optimally summarize a complex geophysical dataset using a user-selected number of ‘icons’ or SOM states, allowing rapid identification of preferred patterns, predictability of transitions, rates of transitions, and hysteresis in cycles. The use of SOMs is demonstrated here through application to a 24 year dataset (1973–96) of monthly Antarctic sea-ice edge positions. Variability in sea-ice extent, concentration and other physical characteristics is an important component of the Earth’s dynamic climate system, particularly in the Southern Hemisphere where annual changes in sea-ice extent (temporarily) double the size of the Antarctic cryosphere. SOM-based patterns concisely capture the spatial and temporal variability in these data, including the annual progression of expansion and retreat, a general eastward propagation of anomalies during the winter, and sub-annual variability in the rate of change in extent at different times of the year (e.g. retreat in January is faster than in November). There is also often a general seasonal hysteresis, i.e. monthly anomalies during cooling follow a different spatial path than during warming.

We produced four independent temperature time series derived from different sensors for the Siple Dome region of West Antarctica to investigate seasonal to interannual temperature variability over the last 20 years. We use data from automatic weather station air-temperature records (1997–99), European Centre for Medium-range Weather Forecasts surface temperature from the 15 year re-analyses (ERA-15, 1979–93), and emissivity-corrected brightness temperatures from the Special Sensor Microwave/Imager (1987–99) and the Scanning Multichannel Microwave Radiometer (1978–87). Each technique has limitations and errors, but all respond to temperature, and all agree in the large patterns of temperature variability over time. Our results show that there is high seasonal to interannual variability in both mean temperature and variance in the Siple Dome region during the study period. In particular, fluctuations in seasonal to interannual temperature variance occur on an approximately 5 year cycle and correlate with variations in the Southern Oscillation Index.