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Abstract Thwaites Glacier in West Antarctica has been identified as a route to destabilization of the whole West Antarctic Ice Sheet, potentially leading to several meters of sea‐level rise. However, future evolution of Thwaites Glacier remains uncertain due to a lack of detailed knowledge about its basal boundary that will affect how its retreat proceeds. Here we aim to improve understanding of the basal boundary in the lower part of Thwaites Glacier by modeling the crustal structures that are related to the bed‐type distribution and therefore influence the basal slip. We combine long‐offset seismic, and gravity‐ and magnetic‐anomaly data to model the crustal structures along two 120 km lines roughly parallel to ice flow. We find a sedimentary basin 40 km in length in the along‐flow direction, with a maximum thickness of 1.7 0.2 km, and two mafic intrusions at 5–10 km depth that vary in maximum thickness between 3.8 and 8.6 km. The sedimentary basin and major mafic intrusions we modeled are likely related to the multi‐stage tectonic evolution of the West Antarctic Rift System. Thwaites Glacier flows across a tectonic boundary within our study site, indicating it flows across tectonically formed structures. The varying geology and resulting variations in bed types demonstrate the influence of tectonics on Thwaites Glacier dynamics. 

Abstract Observation of thin sections of the WAIS (West Antarctic Ice Sheet) Divide ice core in cross-polarized light reveals a wealth of microstructures and textural characteristics indicative of strain and recovery in an anisotropic crystalline substance undergoing high-temperature plastic deformation. The appearance of abundant subgrain domains—relatively strain-free regions inside crystals (grains) surrounded by walls of dislocations across which small structural orientation changes occur—is particularly noticeable in the depth range associated with the brittle ice (∼650–1300 m). Here we describe a subgrain texture, not previously reported in ice, that resembles chessboard-pattern subgrains in β-quartz. This chessboard texture at WAIS Divide is strongly associated with the presence of bubbles. We hypothesize that chessboard-subgrain development may affect grain-size evolution, the fracture of ice cores recovered from the brittle ice zone and perhaps grain-boundary sliding as well. 

Abstract Basal channels are incised troughs formed by elevated melt beneath ice shelves. Channels often coincide with shear margins, suggesting feedbacks between channel formation and shear. However, the effect of channel position and shape on ice-shelf flow has not been systematically explored. We use a model to show that, as expected, channels concentrate deformation and increase ice-shelf flow speeds, in some cases by over 100% at the ice-shelf center and over 80% at the grounding line. The resulting increase in shear can cause stresses around the channels to exceed the threshold for failure, suggesting that rifting, calving and retreat might result. However, channels have different effects depending on their width, depth and position on an ice shelf. Channels in areas where ice shelves are spreading freely have little effect on ice flow, and even channels in confined regions of the shelf do not necessarily alter flow significantly. Nevertheless, if located in areas of vulnerability, particularly in the shear margins near the grounding line, melt channels may alter flow in a way that could lead to catastrophic ice-shelf breakup by mechanically separating shelves from their embayments. 

Abstract The rapidly changing Thwaites Ice Shelf is crucial for understanding ice‐shelf dynamical processes and their implications for sea‐level rise from Antarctica. Fractures, particularly their vertical structure, are key to ice‐shelf structural integrity but remain poorly measured. To address this, we developed a fracture‐characterization workflow using ICESat‐2 ATL03 geolocated photon heights, producing the first time‐series vertical measurements of fractures across Thwaites from 2018 to 2024. We introduced the fracture depth/freeboard ratio as a normalized metric to quantify vertical fracture extent, serving as an indicator of structural damage. This metric enabled us to track fracture evolution in both the eastern ice shelf and western glacier tongue. In the eastern section, fracturing intensified along the northwestern shear zone and near the grounding line, in a positive feedback loop between enhanced fracturing and accelerated flow. The western section maintained an active rift formation zone about 15 km downstream of the historical grounding line. Flow velocity changes in this section were primarily confined to the unconstrained downstream portion, exhibiting an overall deceleration trend, while the upstream area remained stable. This contrast highlights the role of lateral margin conditions in governing ice‐shelf fracture and flow behavior. Changes in the eastern section showed some correspondence with warm winter air temperatures, reduced sea ice, and persistent warm ocean anomalies at shallower depths, suggesting that atmosphere‐sea ice‐ocean interactions influence ice‐shelf structural integrity through basal processes. Future research should integrate satellite‐derived fracture observations with numerical models of ice fracture and flow to better capture the dynamics of ice‐shelf weakening and retreat. 

Abstract 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. 

Abstract Rapid retreat of the Larsen A and B ice shelves has provided important clues about the ice shelf destabilization processes. The Larsen C Ice Shelf, the largest remaining ice shelf on the Antarctic Peninsula, may also be vulnerable to future collapse in a warming climate. Here, we utilize multisource satellite images collected over 1963–2020 to derive multidecadal time series of ice front, flow velocities, and critical rift features over Larsen C, with the aim of understanding the controls on its retreat. We complement these observations with modeling experiments using the Ice‐sheet and Sea‐level System Model to examine how front geometry conditions and mechanical weakening due to rifts affect ice shelf dynamics. Over the past six decades, Larsen C lost over 20% of its area, dominated by rift‐induced tabular iceberg calving. The Bawden Ice Rise and Gipps Ice Rise are critical areas for rift formation, through their impact on the longitudinal deviatoric stress field. Mechanical weakening around Gipps Ice Rise is found to be an important control on localized flow acceleration and the propagation of two rifts that caused a major calving event in 2017. Capturing the time‐varying effects of rifts on ice rigidity in ice shelf models is essential for making realistic predictions of ice shelf flow dynamics and instability. In the context of the Larsen A and Larsen B collapses, we infer a chronology of destabilization processes for embayment‐confined ice shelves, which provides a useful framework for understanding the historical and future destabilization of Antarctic ice shelves. 

Abstract Bedforms of Thwaites Glacier, West Antarctica both record and affect ice flow, as shown by geophysical data and simple models. Thwaites Glacier flows across the tectonic fabric of the West Antarctic rift system with its bedrock highs and sedimentary basins. Swath radar and seismic surveys of the glacier bed have revealed soft‐sediment flutes 100 m or more high extending 15 km or more across basins downglacier from bedrock highs. Flutes end at prominent hard‐bedded moats on stoss sides of the next topographic highs. We use simple models to show that ice flow against topography increases pressure between ice and till upglacier along the bed over a distance that scales with the topography. In this basal zone of high pressure, ice‐contact water would be excluded, thus increasing basal drag by increasing ice‐till coupling and till flux, removing till to allow bedrock erosion that creates moats. Till carried across highlands would then be deposited in lee‐side positions forming bedforms that prograde downglacier over time, and that remain soft on top through feedbacks that match till‐deformational fluxes from well upglacier of the topography. The bedforms of the part of Thwaites surveyed here are prominent because ice flow has persisted over a long time on this geological setting, not because ice flow is anomalous. Bedform development likely has caused evolution of ice flow over time as till and lubricating water were redistributed, moats were eroded and bedforms grew. 

Abstract While analysis of glacial seismicity continues to be a widely used method for interpreting glacial processes, the underlying mechanics controlling glacial stick‐slip seismicity remain speculative. Here, we report on laboratory shear experiments of debris‐laden ice slid over a bedrock asperity under carefully controlled conditions. By modifying the elastic loading stiffness, we generated the first laboratory icequakes. Our work represents the first comprehensive lab observations of unstable ice‐slip events and replicates several seismological field observations of glacier slip, such as slip velocity, stress drop, and the relationship between stress drop and recurrence interval. We also observe that stick‐slips initiate above a critical driving velocity and that stress drop magnitude decreases with further increases in velocity, consistent with friction theory and rock‐on‐rock friction laboratory experiments. Our results demonstrate that glacier slip behavior can be accurately predicted by the constitutive rate‐and‐state friction laws that were developed for rock friction. 

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.