Thwaites and Pine Island glaciers serve as main outlets for ice draining the West Antarctic Ice Sheet into the Amundsen Sea Embayment (ASE). Observational records show that these ice streams exhibit continuous and substantial thinning and grounding-line retreat since the 1940s, particularly accelerating from the 1990s onwards . Furthermore, modeling studies suggest that ASE glaciers are susceptible to runaway retreat. Thus, the rate and magnitude of potential mass loss from these ice streams presents a major source of uncertainty for future sea level rise predictions. Ocean-driven melting of the underside of ASE glacier ice-shelves, caused by the upwelling of warm Circumpolar Deep Water (CDW) at the shelf break and its advection across the continental shelf, is thought to be the main driver of mass loss. CDW upwelling onto the ASE shelf has varied due to natural decadal variability, longer centennial variability as well recent changes in anthropogenic forcing (Holland et al., 2022). However, regional observational records are limited to the last few decades, and the onset and evolution of the oceanic forcing, prior to the instrumental period, remains uncertain. Here, we present high-resolution foraminiferal geochemical data from marine sediment cores recovered from the ASE shelf, including material collected during the Thwaites Glacier Offshore Research (THOR) expeditions in 2019, 2020 and 2021. Preliminary Mg/Ca records of benthic foraminifera shells, a proxy for bottom-water temperature, accompanied by benthic foraminiferal δ13C records, used as a water mass tracer, reveal that CDW incursions onto the ASE shelf contributed to glacier retreat on centennial to millennial timescales. Future work will aim to further constrain changes in CDW advection to the inner ASE shelf, particularly in western Pine Island Bay and at the Dotson Ice Shelf front, for the 20th century and beyond.

Thwaites Glacier (TG) is thinning and accelerating while sitting on a landward-dipping bed, with an ice shelf that is rapidly disintegrating and losing its ability to buttress ice flow from upstream, and is in deep water that allows warm Circumpolar Deep Water (CDW) to reach its grounding zone. Significant retreat of TG would trigger loss of ice across the region. In recent decades, the mass balance of TG has become increasingly negative, suggesting that unstable retreat may have already begun. The Thwaites Offshore Research (THOR) group has just completed four field deployments aimed at understanding the recent history of TG and neighboring ice, including Pine Island Glacier (PIG). Three cruises on the RVIB N.B. Palmer, combined with sub-ice-shelf sediment coring, provide a suite of new data along the TG and PIG margins. Data include multibeam surveys, 3.5 kHz subbottom profiler, over 100 new sediment cores, and high-resolution seismic profiles. Break-up of floating ice cover in front of TG in 2019 allowed surveying of previously unmapped seafloor. Major calving of PIG in 2020 allowed marine surveying over the locations where sub-ice-shelf cores were collected in the past, allowing direct ties between ice-based and marine work. As of this writing in January 2022, we are entering into the Amundsen Sea for our third marine field season. Sediment cores record the history of grounding-zone retreat and ice interaction with the ocean over timescales from decades to several thousand years. Proxies used to reconstruct ice and ocean histories include sedimentary facies analysis, diatom and foraminiferal assemblage data, and geochemical analyses. Sedimentological analyses show a diverse array of lithofacies attributed to different environmental conditions. Many cores across the region contain laminated mud with sparse gravel and sand, suggesting deposition of meltwater deposits. Downcore 210Pb measurements are used to create age models of the past ~100 years. Combination of ages with facies models, including CT scans, reveals that the progressive detachment of Thwaites from pinning points began in the mid twentieth century, coincident with retreat of PIG (Smith et al., 2017) and with increasing advection of warm water onto the Amundsen shelf (Hillenbrand et al., 2017). Conversely, Cranton Bay, to the northeast of PIG and separated from Pine Island Bay by a shallow sill, appears to be characterized by cold deep water and high productivity, allowing it to serve as an endmember different from the records obtained proximal to the large glacial outlets where CDW is impinging. The satellite record of glacial retreat is inherently short. Observations are accumulating about forcing mechanisms that can impact the stability of ice, such as increased CDW on the Amundsen Sea shelf. However, without the time to observe the response of the ice, discovery of the forcing mechanisms is just half the story. The other half of the story is completed by using the paleo record to see how ice has responded to drivers in the past.

Thwaites Glacier (TG) is more vulnerable to unstable retreat than any other part of the West Antarctic Ice Sheet. This is due to its upstream-dipping bed, the absence of a large ice shelf buttressing its flow and the deep bathymetric troughs that route relatively warm Circumpolar Deep Water (CDW) to its margin. Over the past 30 years the mass balance of TG has become increasingly negative, suggesting that unstable retreat may have already begun. The International Thwaites Glacier Collaboration (ITGC) is an initiative jointly funded by the US National Science Foundation and the Natural Environment Research Council in the UK to improve knowledge of the boundary conditions and drivers of change at TG in order improve projections of its future contribution to sea level. The ITGC is funding a range of projects that are conducting on-ice and marine research, and applying numerical models to utilize results in order to predict how the glacier will change and contribute to sea level over coming decades to centuries.RV Nathaniel B Palmer cruise NBP20-02, taking place from January­ to March 2020, will be the second ITGC multi-disciplinary research cruise, building on results from NBP19-02, which took place last year. Thwaites Offshore Research Project (THOR) aims during NBP20-02 include: extending the bathymetric survey in front of TG, collecting sediment cores at sites selected from the survey data, and acquiring high-resolution seismic profiles to determine the properties of the former bed of TG that is now exposed. The detailed bathymetric data will reveal the dimensions and routing of troughs that conduct CDW to the glacier front and will image seabed landforms that provide information about past ice flow and processes at the bed when TG was more extensive. The sediment cores, together with ones collected recently beneath the ice shelf via hot-water drilled holes, will be analysed to establish a history of TG retreat, subglacial meltwater release, and CDW incursions extending back over decades, centuries and millennia before the short instrumental record. Thwaites-Amundsen Regional Survey and Network Project (TARSAN) researchers will reach islands and ice floes via zodiac boats to attach satellite data relay loggers to Elephant and Weddell seals. The loggers record ocean temperature and salinity during the seals’ dives, greatly increasing the spatial extent and time span of oceanographic observations. In addition to work that is part of the THOR and TARSAN projects, another cruise objective is to recover and redeploy long-term oceanographic moorings in the Amundsen Sea. We will present initial results from NBP20-02.

Icefin performed the first long range robotic exploration of the grounding zone of Thwaites Glacier from January 9-12 2020. Icefin was part of the MELT project of the International Thwaites Glacier Collaboration deployed to the grounding zone of Thwaites Glacier, West Antarctica over the period December 2019-February 2020. MELT is an interdisciplinary project to explore rapid change across the grounding zone, and in particular basal melting. The subglacial cavity ~2km north of the grounding zone was accessed via hot water drilling on January 7-8, 2020. Icefin, a hybrid autonomous and remotely operated underwater vehicle designed for sub-ice and borehole operations, conducted over 15km of round-trip data collection under the ice along a section of the glacier from the grounding zone extending to a point 4 km oceanward. The vehicle collected data with ten different science sensors including cameras, sonars, conductivity/temperature and dissolved oxygen. Overall, the water column ranged from ~100m downstream that narrowed quickly to an average of 50m that spanned over 2km, to a long segment of ~30m thickness before quickly narrowing over 500m towards the grounding zone. The seafloor structures run roughly parallel to ice flow direction, consisting of furrows, ridges, and grooves in some cases mirrored by the ice structure. The Icefin dives revealed a diverse set of basal ice conditions, with complex geometry, including a range of terraced features, smooth ablated surfaces, crevassing, sediment rich layers of varying kinds, as well as interspersed clear, potentially accreted freshwater ice. The ocean directly beneath the ice varies spatially, from moderately well-mixed near the grounding zone to highly stratified within and below concavities in the ice downstream. Sediments along the sea floor range from fine grained downstream to course angular gravel near the grounding zone distributed between larger boulders. We observed rocky material in the ice that ranged from fine grained layers compressed within the ice to small angular particles volumetrically distributed within ice, to gravel and cobbles, as well as trapped boulders up to meter scale. In addition to the oceanographic, glaciological and sea floor conditions, we also catalogued communities of organisms along the seafloor and ice-ocean interface. We will report the highlights and initial conclusions from Icefin’s in situ data collection, and offer perspectives on change at the grounding zone.

Grounding-zone wedges (GZW) have been mapped on the sea floor in various sectors of formerly glaciated continental shelf around Antarctica. In most cases, these wedges record periods of grounding-line stillstands during ice-sheet retreat following the Last Glacial Maximum (~26-19 ka BP). The presence of GZWs along the axis of a palaeo-ice stream trough therefore indicates a style of episodic retreat of the grounding line from its LGM to modern position. However, information about their composition and internal structure is sparse, and precise chronological constraints for both the onset and duration of stillstands they represent are still lacking. Consequently, the role of grounding-zone wedge formation in modulating post-LGM ice-sheet retreat cannot be reliably quantified. This information is vital, however, for calculating reliable retreat rates during the past, which are essential for evaluating and understanding the significance of modern retreat rates. Here we present the currently known inventory of GZWs on the continental shelf of the Amundsen Sea Embayment (ASE), West Antarctica, and introduce newly acquired data from one particular middle shelf GZW revealing the first information of its internal structure and composition. We will also discuss geological preconditions at the ice sheet bed that led to GZW formation. Furthermore, we will present our approach to i) reliably date the onset of GZW development, and ii) constrain the duration of these stillstands, including better estimations for subglacial sediment delivery rates. This knowledge will help refine available post-LGM retreat chronologies, which, in turn, serve as a basis for validating and improving ice-sheet models in an area where these are urgently needed.

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Multibeam swath bathymetry data sets collected over the past two decades have been compiled to identify palaeo-ice stream pathways in the easternmost Amundsen Sea Embayment. We mapped ~3000 glacial landforms to reconstruct ice flow in the ~250-km-long cross-shelf Abbot Trough. This bathymetric feature was occupied by a large ice stream, which was fed by two tributaries (Cosgrove and Abbot) and reached the continental shelf edge during the last maximum ice sheet advance. Geomorphological mapping has enabled a clear differentiation between subglacial landforms indicating warm- (e.g., megascale glacial lineations) and cold-based (e.g., hill–hole pairs) ice conditions on the continental shelf during the last glaciation. Grounding-zone wedges and recessional moraines, mapped within the palaeo-ice stream troughs and on adjacent sea-floor highs (referred to as inter-ice stream ridges) indicate grounding line stillstands or re-advances of the West Antarctic Ice Sheet during the last deglaciation of the shelf. We observe that the locations of grounding-zone wedges coincide with trough constrictions as well as local topographic highs of harder substrate. This combination of trough ‘bottlenecks’ and local pinning points on an otherwise retrograde slope is likely to have modified the pace of grounding-line retreat, causing the grounding zone to pause and deposit grounding-zone wedges. The episodic retreat recorded within Abbot Trough corresponds to post-glacial episodic retreat interpreted for the neighbouring Pine Island–Thwaites palaeo-ice stream trough, thus suggesting a uniform pattern of retreat across the eastern Amundsen Sea Embayment. Locally, indications are strong that a change in basal thermal regime of the ice from warm- to cold-based conditions occurred prior to final retreat, as hill–hole pairs overprint megascale glacial lineations. Further, the correlation of grounding-zone wedges with geological boundaries emphasises the influence of subglacial geology on ice stream flow. Our new geomorphological map of the easternmost Amundsen Sea Embayment resolves the pathways of palaeo-ice streams that were probably all active during the last maximum extent of the ice sheet, and the extent of adjacent inter-ice stream ridges. It reveals information about the style of, and the basal thermal regime during, the subsequent grounding line retreat. Such information provides an important empirical framework by which the accuracy of ice sheet models can be gauged.

Our research will help ice-sheet modellers to validate and improve their computer models and thus improve predictions of future sea-level rise in response to polar ice-sheet melting.