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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.