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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.

Although hundreds of cores have been collected on the Antarctic continental shelf over the past five decades, definitive interpretations of depositional environments associated with marine-based ice advance and retreat are hindered by similarities in sediment facies and a lack of geomorphic context. The recent use of an advanced multibeam bathymetry system allows for more detailed ice sheet reconstructions from glacial landforms that were previously not resolved with older generation systems. Here we present results from a recent cruise to the Ross Sea, Antarctica that focuses on integrating sediment facies analyses into a geomorphic framework to confidently determine depositional environments and sedimentological processes since the Last Glacial Maximum. Grain-size analysis, geotechnical properties, and micropaleontology from targeted sediment cores are used to develop improved criteria for making the distinction between subglacial diamictons and ice-proximal diamictons, which can be applied to geomorphically blind sediment cores. Our consideration of geomorphic context while interpreting sediment facies has also allowed us to determine that most debris melted out of the base of paleo-ice shelves in the Ross Sea only 1.2 km seaward of the grounded ice margin. Additionally, we find that agglutinated foraminifera specimens are characteristic of sluggish marine currents and a resulting accumulation of siliceous detritus rich in organic matter; however, calcareous specimens dominate in higher-energy current settings and in ice-proximal environments with lesser amounts of siliceous detritus and associated organic material. Overall, sedimentary successions record contrasting behaviors of the West Antarctic and East Antarctic ice sheets that extended onto the continental shelf during the Last Glacial Maximum. Not only is this coupled geomorphic and sedimentologic approach useful for understanding glacimarine processes and ice-sheet retreat patterns, but it is essential for selecting appropriate facies transitions for chronological constraints of past ice sheet behavior.

The East Antarctic ice sheet was larger than present during past cold periods. Seafloor geophysical data show that in the Ross Sea, the extended ice sheet was underlain by an active hydrologic system during the glacial termination.

Studying the history of ice-sheet behaviour in the Ross Sea, Antarctica's largest drainage basin can improve our understanding of patterns and controls on marine-based ice-sheet dynamics and provide constraints for numerical ice-sheet models. Newly collected high-resolution multibeam bathymetry data, combined with two decades of legacy multibeam and seismic data, are used to map glacial landforms and reconstruct palaeo ice-sheet drainage. During the Last Glacial Maximum, grounded ice reached the continental shelf edge in the eastern but not western Ross Sea. Recessional geomorphic features in the western Ross Sea indicate virtually continuous back-stepping of the ice-sheet grounding line. In the eastern Ross Sea, well-preserved linear features and a lack of small-scale recessional landforms signify rapid lift-off of grounded ice from the bed. Physiography exerted a first-order control on regional ice behaviour, while sea floor geology played an important subsidiary role. Previously published deglacial scenarios for Ross Sea are based on low-spatial-resolution marine data or terrestrial observations; however, this study uses high-resolution basin-wide geomorphology to constrain grounding-line retreat on the continental shelf. Our analysis of retreat patterns suggests that (1) retreat from the western Ross Sea was complex due to strong physiographic controls on ice-sheet drainage; (2) retreat was asynchronous across the Ross Sea and between troughs; (3) the eastern Ross Sea largely deglaciated prior to the western Ross Sea following the formation of a large grounding-line embayment over Whales Deep; and (4) our glacial geomorphic reconstruction converges with recent numerical models that call for significant and complex East Antarctic ice sheet and West Antarctic ice sheet contributions to the ice flow in the Ross Sea.

Post-glacial rebound has preserved beaches across many ice-free parts of the Antarctic coastline. These beaches typically contain gravel- to boulder-sized deposits and are built by high-magnitude waves. Five new ages from the western Antarctic Peninsula were added to a compilation of 282 published ages from Antarctic beaches. A reduced compilation based solely on those ages that date beach formation is used to assess variations in beach formation throughout the Holocene around Antarctica. We suggest the clustering of beach ages – centered at 0.8 and 2.4 ka in the Antarctic Peninsula, 1.5, 3.6 and 6.0 ka in the Ross Sea, and a broad peak from 2.7 to 6.3 ka in East Antarctica – indicates periods of heightened wave exposure leading to increased beach formation. Measurements of clast roundness from raised beaches show more rounded material is coincident with periods of heightened beach formation, providing an independent means of assessing wave climate. A comparison of periods of increased beach formation with available sea-ice proxies suggests beach formation is synchronous with reduced sea-ice conditions and thus higher wave climate. As the presence or absence of sea ice determines open-water conditions, the factors that control sea ice directly influence Antarctic coastal evolution.

Calmette Bay within Marguerite Bay along the western side of the Antarctic Peninsula contains one of the most continuous flights of raised beaches described to date in Antarctica. Raised beaches extend to 40.8 m above sea level (masl) and are thought to reflect glacial isostatic adjustment due to the retreat of the Antarctic Peninsula Ice Sheet. Using optically stimulated luminescence (OSL), we dated quartz extracts from cobble surfaces buried in raised beaches at Calmette Bay. The beaches are separated into upper and lower beaches based on OSL ages, geomorphology, and sedimentary fabric. The two sets of beaches are separated by a prominent scarp. One of our OSL ages from the upper beaches dates to 9.3 thousand years ago (ka; as of 1950) consistent with previous extrapolation of sea-level data and the time of ice retreat from inner Marguerite Bay. However, four of the seven ages from the upper beaches date to the timing of glaciation. We interpret these ages to represent reworking of beaches deposited prior to the Last Glacial Maximum (LGM) by advancing and retreating LGM ice. Ages from the lower beaches record relative sea-level fall due to Holocene glacial-isostatic adjustment. We suggest a Holocene marine limit of 21.7 masl with an age of 5.5–7.3 ka based on OSL ages from Calmette Bay and other sea-level constraints in the area. A marine limit at 21.7 masl implies half as much relative sea-level change in Marguerite Bay during the Holocene as suggested by previous sea-level reconstructions. No evidence for a relative sea-level signature of neoglacial events, such as a decrease followed by an increase in RSL fall due to ice advance and retreat associated with the Little Ice Age, is found within Marguerite Bay indicating either: (1) no significant neoglacial advances occurred within Marguerite Bay; (2) rheological heterogeneity allows part of the Antarctic Peninsula (i.e. the South Shetland Islands) to respond to rapid ice mass changes while other regions are incapable of responding to short-lived ice advances; or (3) the magnitude of neoglacial events within Marguerite Bay is too small to resolve through relative sea-level reconstructions. Although the application of reconstructing sea-level histories using OSL-dated raised beach deposits provides a better understanding of the timing and nature of relative sea-level change in Marguerite Bay, we highlight possible problems associated with using raised beaches as sea-level indices due to post-depositional reworking by storm waves.

The stability of modern ice shelves is threatened by atmospheric and oceanic warming. The geologic record of formerly glaciated continental shelves provides a window into the past of how ice shelves responded to a warming climate. Fields of deep (-560 m), linear iceberg furrows on the outer, western Ross Sea continental shelf record an early post-Last Glacial Maximum episode of ice-shelf collapse that was followed by continuous retreat of the grounding line for ∼200 km. Runaway grounding line conditions culminated once the ice became pinned on shallow banks in the western Ross Sea. This early episode of ice-shelf collapse is not observed in the eastern Ross Sea, where more episodic grounding line retreat took place. More widespread (∼280,000 km(2)) retreat of the ancestral Ross Ice Shelf occurred during the late Holocene. This event is recorded in sediment cores by a shift from terrigenous glacimarine mud to diatomaceous open-marine sediment as well as an increase in radiogenic beryllium ((10)Be) concentrations. The timing of ice-shelf breakup is constrained by compound specific radiocarbon ages, the first application of this technique systematically applied to Antarctic marine sediments. Breakup initiated around 5 ka, with the ice shelf reaching its current configuration ∼1.5 ka. In the eastern Ross Sea, the ice shelf retreated up to 100 km in about a thousand years. Three-dimensional thermodynamic ice-shelf/ocean modeling results and comparison with ice-core records indicate that ice-shelf breakup resulted from combined atmospheric warming and warm ocean currents impinging onto the continental shelf.

Magnetic susceptibility (MS) of sediment and rock has been suggested as a tool to indicate changes in weathering patterns, source area, trace metals, and particle size, each of which may also provide a paleoclimate record. We test the utility of MS in recording previously documented climate changes within Holocene estuarine deposits from four bays along the Texas coast of the northwest Gulf of Mexico: Baffin, Copano, Corpus Christi, and Galveston Bays. MS analysis indicates three populations of anomalously high values: at or near the 8.2 ka climatic event, during the mid-Holocene at 5.5–5.0 ka, and at 3.6 ka. MS measurements from Copano and Corpus Christi Bay cores suggest that the 8.2 ka event may have been multiple events. Although the initial focus of this study was on the use of MS to record the 8.2 ka event, the younger MS anomalies that correlate within the mid-Holocene are equally important and more pronounced for the use of MS as a paleoclimate proxy. The strong correlation of anomalies between 5.5–5.0 ka within multiple cores suggests a regional response to environmental changes that affect the abundance of magnetic minerals in the bays. We use multiple analyses to attempt to determine the cause of the observed MS anomalies in the bays. Frequency-dependent MS indicates that the majority of measured samples, including the anomalies, have no biogenically produced ultrafine magnetite with the exception of anomalies at 8.5 ka, 8.3 ka, and 7.8 ka in Copano Bay. Particle size of the sediment is compared with MS measurements from Baffin and Copano Bays suggesting a weak correlation between particle size and MS. Inductively coupled plasma spectrometry and x-ray fluorescence fail to indicate anomalous values correlating with the observed MS anomalies. Both methods probably lack the resolution to detect the minor differences needed to cause the observed changes in MS. X-ray diffraction analysis indicates the possibility of a minor increase in magnetite at an MS anomaly in the Corpus Christi Bay record which may be sufficient in causing the elevated MS. We present three explanations of climate-controlled MS for the anomalous intervals. (1) Accelerated sea-level rise decreased the orbital depth of waves on the bottom of the bays. As a result, the sedimentation rate increased due to a higher accommodation for deposition. Due to rapid deposition, the flooding surface preserved authigenic ferromagnetic minerals within a redox layer which deterred the cycle of Fe from one phase to another. (2) An influx of detrital magnetite/maghemite to the bays occurred due to flash flooding during arid periods. (3) The MS anomalies observed in the cores represent an increase in background magnetic material. Models (1), (2), and (3) apply to the anomalies near 8.2 ka and only models (2) and (3) provide plausible explanations for the younger anomalies as there is no documentation of sea-level rise during those periods. Despite the uncertainties in the mechanisms behind the MS anomalies, the correlation of the climatic events within multiple bays along the Texas coast suggests that MS may provide a correlation tool within estuarine fills of the Gulf of Mexico Basin and perhaps across the globe.

The timing of the most recent Neoglacial advance in the Antarctic Peninsula is important for establishing global climate teleconnections and providing important post-glacial rebound corrections to gravity-based satellite measurements of ice loss. However, obtaining accurate ages from terrestrial geomorphic and sedimentary indicators of the most recent Neoglacial advance in Antarctica has been hampered by the lack of historical records and the difficulty of dating materials in Antarctica. Here we use a new approach to dating flights of raised beaches in the South Shetland Islands of the northern Antarctic Peninsula to bracket the age of a Neoglacial advance that occurred between 1500 and 1700 AD, broadly synchronous with compilations for the timing of the Little Ice Age in the northern hemisphere. Our approach is based on optically stimulated luminescence of the underside of buried cobbles to obtain the age of beaches previously shown to have been deposited immediately inside and outside the moraines of the most recent Neoglacial advance. In addition, these beaches mark the timing of an apparent change in the rate of isostatic rebound thought to be in response to the same glacial advance within the South Shetland Islands. We use a Maxwell viscoelastic model of glacial-isostatic adjustment (GIA) to determine whether the rates of uplift calculated from the raised beaches are realistic given the limited constraints on the ice advance during this most recent Neoglacial advance. Our rebound model suggests that the subsequent melting of an additional 16–22% increase in the volume of ice within the South Shetland Islands would result in a subsequent uplift rate of 12.5 mm/yr that lasted until 1840 AD resulting in a cumulative uplift of 2.5 m. This uplift rate and magnitude are in close agreement with observed rates and magnitudes calculated from the raised beaches since the most recent Neoglacial advance along the South Shetland Islands and falls within the range of uplift rates from similar settings such as Alaska.