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Improvements in the resolution of sea-floor mapping techniques have revealed extremely regular, sub-metre-scale ridge landforms produced by the tidal flexure of ice-shelf grounding lines as they retreated very rapidly (i.e. at rates of several kilometres per year). Guided by such novel sea-floor observations from Thwaites Glacier, West Antarctica, we present three mathematical models for the formation of these corrugation ridges at a tidally migrating grounding line (that is retreating at a constant rate), where each ridge is formed by either constant till flux to the grounding line, till extrusion from the grounding line, or the resuspension and transport of grains from the grounding-zone bed. We find that both till extrusion (squeezing out till like toothpaste as the ice sheet re-settles on the sea floor) and resuspension and transport of material can qualitatively reproduce regular, delicate ridges at a retreating grounding line, as described by sea-floor observations. By considering the known properties of subglacial sediments, we agree with existing schematic models that the most likely mechanism for ridge formation is till extrusion at each low-tide position, essentially preserving an imprint of the ice-sheet grounding line as it retreated. However, when realistic (shallow) bed slopes are used in the simulations, ridges start to overprint one another, suggesting that, to preserve the regular ridges that have been observed, grounding line retreat rates (driven by dynamic thinning?) may be even higher than previously thought. [ABSTRACT FROM AUTHOR]

The Ekström Ice Shelf is one of numerous small ice shelves that fringe the coastline of western Dronning Maud Land, East Antarctica. Reconstructions of past ice-sheet extent in this area are poorly constrained, due to a lack of geomorphological evidence. Here, we present a compilation of geophysical surveys in front of and beneath the Ekström Ice Shelf, to identify and interpret evidence of past ice-sheet flow, extent and retreat. The sea floor beneath the Ekström Ice Shelf is dominated by an incised trough, which extends from the modern-day grounding line onto the continental shelf. Our surveys show that mega-scale glacial lineations cover most of the mouth of this trough, terminating 11 km away from the continental shelf break, indicating the most recent minimal extent of grounded ice in this region. Beneath the front ∼30 km of the ice shelf measured from the ice shelf edge towards the inland direction, the sea floor is characterised by an acoustically transparent sedimentary unit, up to 45 m thick. This is likely composed of subglacial till, further corroborating the presence of past grounded ice cover. Further inland, the sea floor becomes rougher, interpreted as a transition from subglacial tills to a crystalline bedrock, corresponding to the outcrop of the volcanic Explora Wedge at the sea floor. Ice retreat in this region appears to have happened rapidly in the centre of the incised trough, evidenced by a lack of overprinting of the lineations at the trough mouth. At the margins of the trough uniformly spaced recessional moraines suggest ice retreated more gradually. We estimate the palaeo-ice thickness at the calving front around the Last Glacial Maximum to have been at least 305 to 320 m, based on the depth of iceberg ploughmarks within the trough and sea level reconstructions. Given the similarity of the numerous small ice shelves along the Dronning Maud Land coast, these findings are likely representative for other ice shelves in this region and provide essential boundary conditions for palaeo ice-sheet models in this severely understudied region. [ABSTRACT FROM AUTHOR]

Landfast ice is a common coastal feature in the Arctic Ocean and around the Antarctic continent. One contributing and stabilizing mechanism is the grounding of sea ice ridges in shallow water. Recently, a grounding scheme representing this effect on sea ice dynamics was developed in order to improve the simulation of landfast ice by continuum-based sea ice models. This parameterization assumes that the ridged keel thickness is proportional to the mean thickness. Results demonstrated that this simple parameterization notably improves the simulation of landfast ice in many regions such as in the East Siberian Sea, the Laptev Sea and along the Alaskan coast. Nevertheless, a weakness of this approach is that it is based solely on the mean properties of sea ice. Here, we extend the parameterization by taking into account subgrid-scale ice thickness distribution and bathymetry distribution, which are generally non-normal, and by computing the maximum seabed stress as a joint probability interaction between the sea ice and the seabed. The probabilistic approach shows a reasonably good agreement with observations and with the previously proposed grounding scheme while potentially offering more physical insights into the formation of landfast ice. [ABSTRACT FROM AUTHOR]

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 km 2 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. [ABSTRACT FROM AUTHOR]

In recent decades, rapid ice-shelf disintegration along the Antarctic Peninsula has had a global impact through enhancing outlet glacier flow, and hence sea level rise, and the freshening of Antarctic Bottom Water. Ice shelf thinning due to basal melting results from the circulation of relatively warm water in the underlying ocean cavity. However, the effect of sub-shelf circulation on future ice-shelf stability cannot be predicted accurately with computer simulations if the geometry of the ice-shelf cavity is unknown. To address this deficit for Larsen C Ice Shelf, west Antarctica, we integrate new water-column thickness measurements with existing observations. We present these new data here along with an updated bathymetry grid of the ocean cavity. Key findings include relatively deep seabed to the south-east of the Kenyon Peninsula, along the grounding line and around the key ice shelf pinning point of Bawden Ice Rise. In addition, we can confirm that the cavity’s southern trough stretches from Mobiloil Inlet to the open ocean. These areas of deep seabed will influence ocean circulation and tidal mixing, and will therefore affect the basal-melt distribution. These results will help constrain models of ice-shelf cavity circulation with the aim of improving our understanding of sub-shelf processes and their potential influence on ice shelf stability. The data set comprises all point measurements of seabed depth and a gridded data product, derived using additional measurements of both offshore seabed depth and the thickness of grounded ice. We present all new depth measurements here as well as a compilation of previously published measurements used in the gridding process. The gridded data product is included in the supplementary material. [ABSTRACT FROM AUTHOR]

Increasing evidence for an elaborate subglacial drainage network underneath modern Antarctic ice sheets suggests that basal meltwater has an important influence on ice stream flow. Swath bathymetry surveys from previously glaciated continental margins display morphological features indicative of subglacial meltwater flow in inner shelf areas of some paleo ice stream troughs. Over the last few years several expeditions to the eastern Amundsen Sea embayment (West Antarctica) have investigated the paleo ice streams that extended from the Pine Island and Thwaites glaciers. A compilation of high-resolution swath bathymetry data from inner Pine Island Bay reveals details of a rough seabed topography including several deep channels that connect a series of basins. This complex basin and channel network is indicative of meltwater flow beneath the paleo-Pine Island and Thwaites ice streams, along with substantial subglacial water inflow from the east. This meltwater could have enhanced ice flow over the rough bedrock topography. Meltwater features diminish with the onset of linear features north of the basins. Similar features have previously been observed in several other areas, including the Dotson-Getz Trough (western Amundsen Sea embayment) and Marguerite Bay (SW Antarctic Peninsula), suggesting that these features may be widespread around the Antarctic margin and that subglacial meltwater drainage played a major role in past ice-sheet dynamics. [ABSTRACT FROM AUTHOR]

An extensive grid of seismic reflection data collected on Ekström Ice Shelf, East Antarctica, between 2010–2018, using an on-ice vibroseis source and snowstreamer, are used to make a detailed bathymetry map of the sea floor and ice-shelf cavity. The maps shows a deep sea-floor trough, likely a paleao-ice stream, under the western side of the ice shelf. The trough contains a number of points of higher topography, indicating probable former grounding line positions. At the shelf front a sill running across the width of the shelf has implications for ocean circulation and thus ice-ocean interaction and ice shelf melt. This new bathymetry is markedly different from previous models, which show a generally flat and shallow sea floor in the region. This is presumably the case for many of the smaller ice-shelves in Dronning Maud Land, which highlights the need for better bathymetry measurements in these key threshold regions. [ABSTRACT FROM AUTHOR]

Analyses of hydrographic observation data sets revealed that modified Circumpolar DeepWater (mCDW) tends to flood the western flank (73˚ E-78˚ E, 67˚ S-68˚ S) of the FourLadies Bank (FLB) in Prydz Bay. In this study, we investigated the mechanism responsiblefor mCDW upwelling over the FLB based on an eddy-resolving coupled ocean-sea ice-iceshelf model in conjunction with the latest high-accuracy bathymetry. It was found that zonalstep-like declines in the seabed over the FLB are crucial for the mCDW onshore upwelling,through topographic dynamic effects on the alongshore Antarctic Slope Current (ASC).In the presence of meridional cross-shelf σθ isopycnals, warm mCDW at ∼500m depth in the deep sea can make its approach to the shallower continental shelf(∼200-500 m depths), characterized as a warm layer (> -1.8 ˚ C) bounded by thecross-shelf σθ isopycnal surfaces of 27.5 kg m-3 and 27.7 kg m-3. The simulated outflowof the Prydz Bay gyre is concentrated to the eastern flank of the Prydz Channel.These results suggest that, in addition to the depressions and troughs around theAntarctic, smaller topographic features such as step-like declines in the seabed near theASC are also favorable for onshore mCDW intrusion. This study demonstrates thatmCDW onshore transport around the Antarctic continental shelf might be significantlyunderestimated by numerical models with coarse spatial representations of the topography. [ABSTRACT FROM AUTHOR]

After more than 40 years of research, there is still wide disagreement in defining when theMid-Pleistocene Transition (MPT) occurred, with climate reconstructions ranging from anabrupt versus gradual transition that began as early as 1500 ka and ended as late as 600 ka.Our recent work in the Southwest Pacific (Ocean Drilling Program Site 1123) has providedsome evidence for a rapid transition, suggesting that the MPT was initiated by an abruptincrease in global ice volume 900 thousand years ago [1]. This study uses shallow-infaunalbenthic foraminifera Uvigerina spp. to disentangle the contributions of deep-watertemperature (using Mg/Ca ratios) and ice volume to the oxygen isotopic composition offoraminiferal calcite over the last 1.5 Ma. The resulting sea-level reconstructionacross the MPT shows that the critical step in ice-volume variation was associatedwith the suppression of melting in Marine Isotope Stage (MIS) 23, followed byrenewed ice growth in MIS 22 to yield a very large ice sheet with 120 m of sea levellowering. Here, we built on this work with the aim to investigate further the abrupt eventcentered on MIS 24 to 22 (the '900-ka event’) and try to shed some light on theprocesses and mechanisms that caused the MPT. Different hypotheses account for theorigin of the MPT as a response to long-term ocean cooling, perhaps because oflowering CO2. To better quantify the role of the carbon system during the MPT, wereconstruct past changes in bottom water inorganic carbon chemistry from the traceelement (B/Ca) and stable isotopic composition of calcite shells of the infaunalbenthic foraminifera Uvigerina spp. from 1100 ka to 350 ka at ODP Site 1123.This site was retrieved from Chatham Rise, east of New Zealand in the SouthwestPacific Ocean (41º47.2’S, 171º 29.9’ W, 3290 m water depth) and lies under theDeep Western Boundary Current (DWBC) that flows into the Pacific Ocean, and isresponsible for most of the deep water in that ocean; DWBC strength is directlyrelated to processes occurring around Antarctica. The ratio of boron to calcium(B/Ca) in benthic foraminifer shells has proven to be a reliable indicator of thecalcite saturation state of ocean bottom waters. The comparison between benthicforaminifera δ18O and δ13C shows a similar trend at ODP Site 1123, implying a closerelationship between these climate and carbon cycle signals, and we use our B/Ca recordreconstructed from the same samples to explore the potential processes behind this tightcoupling. These results permit preliminary discussion on the deep-water carbonate saturation stateduring glacial/interglacial cycles. Deep-water temperatures estimates using Mg/Ca andoxygen isotopic composition of seawater (δ18Osw) are available from Site 1123 for the last1.5 million years [1] and the phase relationship between the different signals is tentativelyassessed for the early/middle Pleistocene, when different patterns of climate variability havebeen inferred from marine and ice cores records. [1] Elderfield et al. (2012). Evolution of ocean temperature and ice volumethrough the Mid Pleistocene Climate Transition. Science, vol. 337, 6095, 704-709 [ABSTRACT FROM AUTHOR]

Depending on the location in the ocean, sea surface height changes are dominated by steric signals or are related to barotropic signals which are also visible in bottom pressure. From an observational point of view, how ocean bottom pressure is related to sea surface height changes, could be studied when SSH, OBP and steric changes would all be measured.The deployment of a series of 14 ocean bottom pressure recorders in a transect from the South of Africa towards Antarctica from 2011 to 2014 by the Alfred Wegener Institute, in principle allows for such a consistency check. The transect spans multiple Southern ocean fronts, and the northernmost records are situated in an eddy-active region, which makes it sensitive to local ocean bottom pressure signals.In this contribution, we compare the measurements with daily large scale ocean bottom variations from GRACE, sea surface heights from radar altimetry and Argo derived steric changes in the vicinity of the ocean bottom pressure recorders. In addition, time series from a joint inversion of GRACE, Swarm and radar altimetry data is also evaluated at the OBP sites. The observed time series are then compared to simulations from the FESOM ocean model. [ABSTRACT FROM AUTHOR]

The Southern Weddell Sea is a pivotal region for the global ocean circulation, by producing large amount of dense waters, which ultimately sink at the bottom of the oceans, the area is fuelling the overturning circulation. As they are produced on the continental shelf, some amount of dense waters interact with the largest ice-shelf of Antarctica, leading to basal melt and altering the characteristics of the outflowing dense waters. Warmer water located off the continental shelf, can also locally access the base of the ice-shelf, though their access is thought to be very limited in today's climate. A change of warm water access to the ice-shelf base could would dramatically increase basal melt, leading to global and widespread consequences on sea level rise, as well as, on water production. However, the dynamics at play in the blocking or access of warm water towards the ice-shelf base remains poorly described and understood. Here, the circulation of warm water toward ice shelves is studied with a set of experiments in order to understand the parameters governing this circulation. In addition, in situ measurements are also used to understand this circulation : a set of autonomous floats deployed in 2017 have been sampling on the continental shelf for almost a year, providing unprecedented year-round observation of top to bottom ocean characteristics directly next to ice-shelf. Theses floats allows us to describe the seasonal cycle of both the warm water flowing toward the ice shelf draft, and the dense water exiting the ice-shelf and being transformed as deep water on their way out of the Weddell Sea. [ABSTRACT FROM AUTHOR]