Thwaites Glacier is one of the largest, most rapidly changing glaciers on Earth, and its landward-sloping bed reaches the interior of the marine West Antarctic Ice Sheet, which impounds enough ice to yield meters of sea-level rise. Marine ice sheets with landward-sloping beds have a potentially unstable configuration in which acceleration can initiate or modulate grounding-line retreat and ice loss. Subglacial water has been observed and theorized to accelerate the flow of overlying ice dependent on whether it is hydrologically distributed or concentrated. However, the subglacial water systems of Thwaites Glacier and their control on ice flow have not been characterized by geophysical analysis. The only practical means of observing these water systems is airborne ice-penetrating radar, but existing radar analysis approaches cannot discriminate between their dynamically critical states. We use the angular distribution of energy in radar bed echoes to characterize both the extent and hydrologic state of subglacial water systems across Thwaites Glacier. We validate this approach with radar imaging, showing that substantial water volumes are ponding in a system of distributed canals upstream of a bedrock ridge that is breached and bordered by a system of concentrated channels. The transition between these systems occurs with increasing surface slope, melt-water flux, and basal shear stress. This indicates a feedback between the subglacial water system and overlying ice dynamics, which raises the possibility that subglacial water could trigger or facilitate a grounding-line retreat in Thwaites Glacier capable of spreading into the interior of the West Antarctic Ice Sheet.subglacial hydrology | radio glaciology | ice sheet stability T hwaites Glacier is an outlet glacier in the Amundsen Sea Embayment (Fig. 1A), the most rapidly changing sector of the West Antarctic Ice Sheet (WAIS) (1-3) and a leading component of deglaciation scenarios (4-6). As such, the future stability of Thwaites Glacier is a significant factor in sea-level projections. Although subglacial water has been observed to cause significant acceleration in large Antarctic outlet glaciers (7), the potential for a similar (possibly destabilizing) subglacialwater-driven acceleration in Thwaites Glacier has not been geophysically assessed. Subglacial water systems have a diverse range of configurations (8) including lakes (9), sheets (10), canals (11), channels (11), and saturated tills (12); however, their control on ice flow is principally determined by whether the water is hydrologically distributed or concentrated (10). Distributed water systems are inefficient at drainage and increase basal lubrication with increased water flux, whereas concentrated water systems are efficient at drainage and do not increase basal lubrication (10). Therefore, the sensitivity of ice-flow acceleration and grounding-line stability for Thwaites Glacier will depend on the existence, locations, and interconnections of these two kinds of water systems beneath it.Airborne soun...

Totten Glacier, the primary outlet of the Aurora Subglacial Basin, has the largest thinning rate in East Antarctica 1,2 . Thinning may be driven by enhanced basal melting due to ocean processes 3 , modulated by polynya activity 4,5 . Warm modified Circumpolar Deep Water, which has been linked to glacier retreat in West Antarctica 6 , has been observed in summer and winter on the nearby continental shelf beneath 400 to 500 m of cool Antarctic Surface Water 7,8 . Here we derive the bathymetry of the sea floor in the region from gravity 9 and magnetics 10 data as well as ice-thickness measurements 11 . We identify entrances to the ice-shelf cavity below depths of 400 to 500 m that could allow intrusions of warm water if the vertical structure of inflow is similar to nearby observations. Radar sounding reveals a previously unknown inland trough that connects the main ice-shelf cavity to the ocean. If thinning trends continue, a larger water body over the trough could potentially allow more warm water into the cavity, which may, eventually, lead to destabilization of the low-lying region between Totten Glacier and the similarly deep glacier flowing into the Reynolds Trough. We estimate that at least 3.5 m of eustatic sea level potential drains through Totten Glacier, so coastal processes in this area could have global consequences.The Totten Glacier drains into the Sabrina Coast in an area where we find coastal ice grounded below sea level and the potential for local marine ice sheet instability 12 upstream of the grounding line ( Fig. 1b). We infer the bathymetry seaward of the grounding line using inversions of gravity data 9 informed by magnetics data 10 and ice-thickness measurements 11 . The inversion reveals the southwest area of the Totten Glacier Ice Shelf (TGIS) cavity is the deepest, reaching 2.7 ± 0.19 km below sea level ( Fig. 2), comparable to the grounding line depths of Amery Ice Shelf 13 and the segment of the Moscow University Ice Shelf (MUIS) overlying the Reynolds Trough 11 . The shallowest area of the cavity (∼300 mbsl) is found beneath the calving front of the ice shelf where a large coastparallel ridge connects Law Dome with a peninsula of grounded ice protruding from the east side of the cavity (Fig. 2). The ridge extends 40 km seaward of the calving front and would have been a source of backstress on the Totten Glacier as recently as 1996 when ice rises were last detected 14 . The inversion reveals depressions located near the centre of the ridge (650 ± 190 mbsl) and to the east of the grounded ice peninsula (860 ± 190 mbsl) (Fig. 2, Profile A-A ). Looking along the long axis of the full Totten cavity we see it is an average of 500 m deeper along the western (Law Dome) side. We infer two basins on the long axis reaching depths of 2.7 ± 0.19 km and 2.0 ± 0.19 km (SW and NE, respectively; Fig. 2) separated by a narrow ridge causing an ice rise near the middle of the ice shelf (the left-hand panel in Fig. 2) 14 .Published grounding lines 14,15 indicate an area of grounded ice bounded by the MUIS t...

The first Cenozoic ice sheets initiated in Antarctica from the Gamburtsev Subglacial Mountains and other highlands as a result of rapid global cooling ∼34 million years ago. In the subsequent 20 million years, at a time of declining atmospheric carbon dioxide concentrations and an evolving Antarctic circumpolar current, sedimentary sequence interpretation and numerical modelling suggest that cyclical periods of ice-sheet expansion to the continental margin, followed by retreat to the subglacial highlands, occurred up to thirty times. These fluctuations were paced by orbital changes and were a major influence on global sea levels. Ice-sheet models show that the nature of such oscillations is critically dependent on the pattern and extent of Antarctic topographic lowlands. Here we show that the basal topography of the Aurora Subglacial Basin of East Antarctica, at present overlain by 2-4.5 km of ice, is characterized by a series of well-defined topographic channels within a mountain block landscape. The identification of this fjord landscape, based on new data from ice-penetrating radar, provides an improved understanding of the topography of the Aurora Subglacial Basin and its surroundings, and reveals a complex surface sculpted by a succession of ice-sheet configurations substantially different from today's. At different stages during its fluctuations, the edge of the East Antarctic Ice Sheet lay pinned along the margins of the Aurora Subglacial Basin, the upland boundaries of which are currently above sea level and the deepest parts of which are more than 1 km below sea level. Although the timing of the channel incision remains uncertain, our results suggest that the fjord landscape was carved by at least two iceflow regimes of different scales and directions, each of which would have over-deepened existing topographic depressions, reversing valley floor slopes.

ABSTRACT. Several airborne radar-sounding surveys are used to trace internal reflections around the European Project for Ice Coring in Antarctica Dome C and Vostok ice core sites. Thirteen reflections, spanning the last two glacial cycles, are traced within 200 km of Dome C, a promising region for million-year-old ice, using the University of Texas Institute for Geophysics High-Capacity Radar Sounder. This provides a dated stratigraphy to 2318 m depth at Dome C. Reflection age uncertainties are calculated from the radar range precision and signal-to-noise ratio of the internal reflections. The radar stratigraphy matches well with the Multichannel Coherent Radar Depth Sounder (MCoRDS) radar stratigraphy obtained independently. We show that radar sounding enables the extension of ice core ages through the ice sheet with an additional radar-related age uncertainty of ∼1/3-1/2 that of the ice cores. Reflections are extended along the Byrd-Totten Glacier divide, using University of Texas/Technical University of Denmark and MCoRDS surveys. However, core-to-core connection is impeded by pervasive aeolian terranes, and Lake Vostok's influence on reflection geometry. Poor radar connection of the two ice cores is attributed to these effects and suboptimal survey design in affected areas. We demonstrate that, while ice sheet internal radar reflections are generally isochronal and can be mapped over large distances, careful survey planning is necessary to extend ice core chronologies to distant regions of the East Antarctic ice sheet.

Significance Thwaites Glacier is one of the West Antarctica's most prominent, rapidly evolving, and potentially unstable contributors to global sea level rise. Uncertainty in the amount and spatial pattern of geothermal flux and melting beneath this glacier is a major limitation in predicting its future behavior and sea level contribution. In this paper, a combination of radar sounding and subglacial water routing is used to show that large areas at the base of Thwaites Glacier are actively melting in response to geothermal flux consistent with rift-associated magma migration and volcanism. This supports the hypothesis that heterogeneous geothermal flux and local magmatic processes could be critical factors in determining the future behavior of the West Antarctic Ice Sheet.