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Topographic highs (“bumps”) across glaciated landscapes have the potential to temporarily slow ice sheet flow or, conversely, accelerate ice flow through subglacial strain heating and meltwater production. Isolated bumps of variable size across the deglaciated landscape of the Cordilleran Ice Sheet (CIS) of Washington State present an opportunity to study the influence of topographic highs on ice–bed interactions and ice flow organization. This work utilizes semi-automatic mapping techniques of subglacial bedforms to characterize the morphology of streamlined subglacial bedforms including elongation, surface relief, and orientation, all of which provide insight into subglacial processes during post-Last Glacial Maximum deglaciation. We identify a bump-size threshold of several cubic kilometers – around 4.5 km3 – in which bumps larger than this size will consistently and significantly disrupt both ice flow organization and subglacial sedimentary processes, which are fundamental to the genesis of streamlined subglacial bedforms. Additionally, sedimentary processes are persistent and well developed downstream of bumps, as reflected by enhanced bedform elongation and reduced surface relief, likely due to increased availability and production of subglacial sediment and meltwater. While isolated topography plays a role in disrupting ice flow, larger bumps have a greater disruption to ice flow organization, while bumps below the identified threshold seem to have little effect on ice and subglacial processes. The variable influence of isolated topographic bumps on ice flow of the CIS has significant implications for outlet glaciers of the Greenland Ice Sheet (GrIS) due to similarities in regional topography, where local bumps are largely unresolved.

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.

Meltwater drainage beneath ice sheets is a fundamental consideration for understanding ice–bed conditions and bed-modulated ice flow, with potential impacts on terminus behavior and ice-shelf mass balance. While contemporary observations reveal the presence of basal water movement in the subglacial environment and inferred styles of drainage, the geological record of former ice sheets, including sediments and landforms on land and the seafloor, aids in understanding the spatiotemporal evolution of efficient and inefficient drainage systems and their impact on ice-sheet behavior. We highlight the past decade of advances in geological studies that focus on providing process-based information on subglacial hydrology of ice sheets, how these studies inform theory, numerical models and contemporary observations, and address the needs for future research.

Recognition of how dynamic the Antarctic ice sheets and glaciers were during the late Holocene has grown in recent years. Proxy data suggests the presence of Neoglacial advances but few moraines or glacial features from this time have been dated compared to glaciated landscapes of the Northern Hemisphere. Debate continues on whether parts of Antarctica experienced glacial advance at the same time as the “Little Ice Age” (LIA), which is well-documented in the Northern Hemisphere. We provide new evidence for late Holocene glacial fluctuations at three locations along the Antarctic Peninsula. A moraine or till sheet from a tidewater glacier cross cuts a series of dated raised beaches at Tay Head, Joinville Island along the northwestern Weddell Sea. At Spark Point, on Greenwich Island, a glacier has overrun Holocene raised beaches and a shell-bearing marine deposit is reworked into a glacial diamicton. A third site in Calmette Bay within the larger Marguerite Bay also contains a recent moraine that cuts across a series of dated raised beach ridges. The new ages constraining these glacial advances are in broad agreement with the handful of other existing ages on moraines and proxy records suggestive of cooler conditions within the Antarctic Peninsula. Combining available timing constraints into a Bayesian model yields an age of 400 to 90 cal BP (1550–1860 CE; 95%) for the LIA across the Antarctica Peninsula. Consideration of a two-phase glacial advance within our Bayesian framework does fit more of the data from across the Antarctic Peninsula and suggests advances from 575 to 330 cal BP (1375–1620 CE) and 400 to 50 cal BP (1550–1900 CE). However, more work is needed to determine if such a two-phase advance occurred. Regardless, its similar timing within the Antarctic Peninsula to that of the Northern Hemisphere supports recent assertions of a volcanic or solar forcing for the LIA. These recent readvances also provide a possible mechanism for changes in the rates of Holocene relative sea-level change recorded across the Antarctic Peninsula suggesting that the Antarctic ice sheets may have been more responsive to past climate changes than previously thought and glacial isostatic adjustment from the LIA and possibly other Holocene glacial oscillations is superimposed upon the longer relaxation from the Last Glacial Maximum.

The geometry of the sea floor immediately beyond Antarctica's marine-terminating glaciers is a fundamental control on warm-water routing, but it also describes former topographic pinning points that have been important for ice-shelf buttressing. Unfortunately, this information is often lacking due to the inaccessibility of these areas for survey, leading to modelled or interpolated bathymetries being used as boundary conditions in numerical modelling simulations. At Thwaites Glacier (TG) this critical data gap was addressed in 2019 during the first cruise of the International Thwaites Glacier Collaboration (ITGC) project. We present more than 2000 km2 of new multibeam echo-sounder (MBES) data acquired in exceptional sea-ice conditions immediately offshore TG, and we update existing bathymetric compilations. The cross-sectional areas of sea-floor troughs are under-predicted by up to 40 % or are not resolved at all where MBES data are missing, suggesting that calculations of trough capacity, and thus oceanic heat flux, may be significantly underestimated. Spatial variations in the morphology of topographic highs, known to be former pinning points for the floating ice shelf of TG, indicate differences in bed composition that are supported by landform evidence. We discuss links to ice dynamics for an overriding ice mass including a potential positive feedback mechanism where erosion of soft erodible highs may lead to ice-shelf ungrounding even with little or no ice thinning. Analyses of bed roughnesses and basal drag contributions show that the sea-floor bathymetry in front of TG is an analogue for extant bed areas. Ice flow over the sea-floor troughs and ridges would have been affected by similarly high basal drag to that acting at the grounding zone today. We conclude that more can certainly be gleaned from these 3D bathymetric datasets regarding the likely spatial variability of bed roughness and bed composition types underneath TG. This work also addresses the requirements of recent numerical ice-sheet and ocean modelling studies that have recognised the need for accurate and high-resolution bathymetry to determine warm-water routing to the grounding zone and, ultimately, for predicting glacier retreat behaviour.

The geometry of the sea floor beyond Thwaites Glacier (TG) is a major control on the routing of warm ocean waters towards the ice stream’s grounding zone, which has led to increased mass loss through sub-ice-shelf melting and resulting accelerated ice flow. Nearshore topographic highs act as pinning points for the Thwaites Ice Shelf and potentially provide barriers to warm water incursions. To date, few vessels have been able to access this area due to persistent sea-ice and iceberg cover. This critical data gap was addressed in 2019 during the first cruise of the International Thwaites Glacier Collaboration (ITGC) project, with more than 2000 km2 of new multibeam echo-sounder data (MBES) were acquired offshore TG. Here, these data along with legacy MBES datasets are compiled to produce a set of standalone bathymetric grids for the inner Amundsen Sea shelf beyond both Pine Island and Thwaites glaciers. At TG, the bathymetry is dominated by a > 1200 m deep, structurally-controlled trough and discontinuous ridge, on which the Eastern Ice Shelf is pinned. The geometry and composition of the ridge varies spatially with some parts having distinctive flat-topped morphologies produced as their tops were planed-off by erosion at the base of the seaward-moving Thwaites Ice Shelf, suggesting a positive feedback mechanism for ice-shelf ungrounding. Knowing that this offshore area is a former bed for TG, we applied a novel spectral approach to investigate bed roughness and find that derived power spectra can be approximated using an inverse-square law, a result that is consistent with spectra for bed profiles from the modern TG. Using existing ice-flow theory, we also make a first assessment of the form drag (basal drag contribution) for ice flow over this topography. Ice flowing over the sea-floor troughs and ridges would have been affected by similarly high basal drag to that acting in the grounding zone today. We show that the sea-floor bathymetry is an analogue for extant bed areas of TG and that more can be gleaned from these 3D bathymetric datasets regarding the likely spatial variability of bed roughness and bed composition types underneath TG. Comparisons with existing regional bathymetric compilations for the area show that high-frequency (finer than 5 km) bathymetric variability beyond Antarctic ice shelves can only be resolved by observations such as MBES and that without these data calculations of the capacity of bathymetric troughs, and thus oceanic heat flux, may be significantly underestimated. This work meets the requirements of recent numerical ice-sheet and ocean modelling studies that have recognised the need for accurate and high-resolution bathymetry to determine warm water routing to the grounding zone and, ultimately, for predicting glacier retreat behaviour.

Contemporary ice stream flow is directly linked to conditions at the ice/bed interface, yet this environment is logistically difficult to access. Instead, we investigate subglacial processes important for ice stream flow by studying tills on the deglaciated Antarctic continental shelf. We test currently-accepted hypotheses surrounding subglacial processes and till properties with a Ross Sea dataset. Till shear strengths indicate a continuum of simultaneous processes acting at the bed, rather than discrete ‘deformation’ and ‘lodgement’ end-members. We identify a threshold water content representing saturated pore spaces, leading to basal sliding and meltwater channelization. Based on observations of till properties relative to glacial landforms, we challenge the assumption that low shear strength is linked to intense deformation. Spatial variability in landform morphology reflects variability in deforming processes at the sub-ice stream scale and suggests a maximum deforming bed thickness of 2 m at the grounding line. Regional till properties generally correlate with seafloor geology and deglacial history; the western Ross Sea is characterized by higher and more variable shear strengths and water contents, while lower-shear strength till was preserved in the Eastern Basin. These observations inform till interpretation and provide context for deforming beds beneath the modern ice sheet and on glaciated continental shelves.

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.

Optically stimulated luminescence (OSL) dating has recently been applied to crystalline (i.e. igneous and metamorphic) rock surfaces. With the advent of this technique, early studies can guide improvements to the methodology that will lead to more robust age constraints. Low OSL sensitivity, poor measurement reliability, and equivalent dose scatter are common problems with dating quartz from crystalline rock surfaces. We investigate OSL characteristics of quartz extracted from crystalline Antarctic beach cobbles. Comparison of OSL behavior with cathodoluminescene characteristics and mineral composition and texture does not show a distinct relationship, suggesting luminescence behavior results from the natural radiation environment and post-crystallization transport history, rather than provenance source and the nature of crystal defects. Elemental composition of processed samples used for OSL measurements indicates standard methods of chemical preparation are not always suitable to extract pure quartz samples. Additionally, heterogeneous radiation production and attenuation poses a problem for estimating dose rates, in particular regarding beta contributions that dominate effective dose rates. We determine the sensitivity of effective dose rates to water content parameters and crystal size needed to calculate dose rates for crystalline rock surfaces. An increase in the value of all modelled parameters serves to reduce effective dose rates and, thus increase OSL ages. Dose rates are most sensitive to crystal size, which can introduce large uncertainty in burial ages from crystalline rocks due to heterogeneous crystal morphology (i.e. size and shape). This study provides a step forward in understanding the challenges and improving the methodology of luminescence dating of quartz from crystalline rock surfaces.

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.

The Ross Sea Embayment is the largest drainage basin of the Antarctic Ice Sheet. Multiple ice streams sourced from both the East Antarctic and West Antarctic ice sheets merged in the Ross Sea during the Last Glacial Maximum (LGM). The continental shelf record is important for understanding processes that influence retreat, constraining deglacial patterns and assessing Antarctica's contribution to eustatic sea-level rise since the LGM. Newly acquired, high-resolution multibeam bathymetry provides a detailed record of the complex deglacial history of a sector in the western Ross Sea (Fig. 1a). Retreat of grounded ice is marked by small, ice-marginal retreat landforms that were poorly resolved prior to the use of an upgraded multibeam system. Dynamic grounding zone behaviour is influenced by physiography and the presence of subglacial meltwater. Fig. 1. ( a ) Multibeam survey south of Crary Bank between Franklin Island and the Central Basin, western Ross Sea shelf (basemap modified by L. Prothro from GeoMapApp). Acquisition system Kongsberg EM122. Frequency 12 kHz. Grid-cell size 20 m. ( b ) Ross Sea showing the location of the survey area ((a) in red box; map from IBCSO v. 1.0). LAB, Little American Basin; GCB, Glomar-Challenger Basin; RB, Ross Bank; PT, Pennell Trough; PB, Pennell Bank; JT, JOIDES Trough; MB, Mawson Bank; CB, Central Bank; CrB Crary Bank; FrIS, Franklin Island; DT, Drygalski Ice Tongue; VLB, Victoria Land Basin; McM, McMurdo Sound. ( c ) Two sets of linear features interpreted as glacial lineations separated by an abrupt boundary (grey dashed line). Inset shows seismic profile across glacial lineations. Acquisition system Knudsen CHIRP. Frequency 3.5 kHz. ( d ) Asymmetrical ridges, interpreted as GZWs, deposited along the banks of a channel and on top of a volcanic landform with lineated topsets. ( e ) Profile of 4 m GZW deposited on volcanic landform showing underlying surface reflection. ( f ) Example of channel cross-sectional profile. …

A lack of reliable chronology often leaves Antarctic ice sheet reconstructions incomplete. Despite being the most heavily investigated region of the Antarctic continental shelf, the post-Last Glacial Maximum (LGM) record of marine-based ice sheet dynamics in the Ross Sea has remained largely elusive and at odds with fringing terrestrial records of ice sheet decay. Issues with radiocarbon dating stem primarily from poorly preserved carbonate, contamination by glacially-recycled carbon, and insufficient consideration for glacial geomorphic context and sedimentary facies. We assess nearly 70 newly acquired and more than 200 published radiocarbon ages in the context of a consistent sedimentary facies and geomorphic framework to interpret the timing of the local glacial maximum and subsequent retreat of the East Antarctic Ice Sheet (EAIS) in the Ross Sea and potential causes of retreat. Compound-specific radiocarbon ages help to provide good constraints on open marine onset, and we demonstrate that, with proper corrections and considerations, acid insoluble organic (AIO) radiocarbon ages can be reliable in open marine sediments and carbonate ages can be reliable in grounding-zone proximal sediments. We find that the ice flowing through Pennell and JOIDES troughs remained at maximum extent for approximately 10,000 years before retreating at 15.1 and 13 cal ka BP, respectively. An ice shelf covered the outer continental shelf until ∼9.5 cal ka BP, after which it collapsed in JOIDES Trough, creating deep iceberg furrows. Retreat through Pennell Trough was more episodic, marked by grounding-zone wedges that backstep upstream and onto lateral banks creating deglacial emergent ice rises. As ice retreated from ∼9 to ∼4 cal ka BP, there was considerable reorganization of drainage and at least one episode of readvance through southern JOIDES Trough that began drawdown of terrestrial ice near Ross Island at ∼7.8 cal ka BP. The ice shelf appears to have retreated to a near-present configuration by 2 cal ka BP. However, due to inconsistencies in calendar ages, grounding line position must be inferred from relative chronology based on glacial landforms, which suggests grounded ice retreated nearly to Drygalski Ice Tongue and lingered atop Crary Bank where it was maintained by ice from David Glacier. Our reconstruction further suggests that the EAIS in the western Ross Sea was not a discernible contributor to Meltwater Pulse 1A, but likely contributed to eustatic sea level rise well through the middle—perhaps even late—Holocene.

Many studies of Holocene relative sea-level (RSL) changes across Antarctica assume that their reconstructions record uplift from glacial isostatic adjustment caused by the demise of the Last Glacial Maximum (LGM) ice sheets. However, recent analysis of GPS observations suggests that mantle viscosity beneath the Antarctic Peninsula is weaker than previously thought, which would imply that solid Earth motion is not controlled by post-LGM ice-sheet retreat but instead by late Holocene ice-mass changes. If this hypothesis is correct, one might expect to find Holocene RSL records that do not reflect a monotonic decrease in the rate of RSL fall but show variations in the rate of RSL change through the Holocene. We present a new record of late Holocene RSL change from Torgersen Island near Palmer Station in the western Antarctic Peninsula that shows an increase in the rate of relative sea-level fall from 3.0 ± 1.2 mm/yr to 5.1 ± 1.8 mm/yr during the late Holocene. Independent studies of the glacial history of the region provide evidence of ice-sheet changes over similar time scales that may be driving this change. When our RSL records are corrected for sea-surface height changes associated with glacial isostatic adjustment (GIA), the rate of post-0.79 ka land uplift at Torgersen Island, 5.3 ± 1.8 mm/yr, is much higher than the rate of uplift recorded at a nearby GPS site at Palmer Station prior to the Larsen B breakup in 2002 AD (1998-2002 AD

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.