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Knowledge gaps about how the ocean melts Antarctica's ice shelves, borne from a lack of observations, lead to large uncertainties in sea level predictions. Using high-resolution maps of the underside of Dotson Ice Shelf, West Antarctica, we reveal the imprint that ice shelf basal melting leaves on the ice. Convection and intermittent warm water intrusions form widespread terraced features through slow melting in quiescent areas, while shear-driven turbulence rapidly melts smooth, eroded topographies in outflow areas, as well as enigmatic teardrop-shaped indentations that result from boundary-layer flow rotation. Full-thickness ice fractures, with bases modified by basal melting and convective processes, are observed throughout the area. This new wealth of processes, all active under a single ice shelf, must be considered to accurately predict future Antarctic ice shelf melt. [ABSTRACT FROM AUTHOR]

Today, relatively warm Circumpolar Deep Water is melting Thwaites Glacier at the base of its ice shelf and at the grounding zone, contributing to significant ice retreat. Accelerating ice loss has been observed since the 1970s; however, it is unclear when this phase of significant melting initiated. We analyzed the marine sedimentary record to reconstruct Thwaites Glacier's history from the early Holocene to present. Marine geophysical surveys were carried out along the floating ice-shelf margin to identify core locations from various geomorphic settings. We use sedimentological data and physical properties to define sedimentary facies at seven core sites. Glaciomarine sediment deposits reveal that the grounded ice in the Amundsen Sea Embayment had already retreated to within ~45 km of the modern grounding zone prior to ca. 9,400 y ago. Sediments deposited within the past 100+ y record abrupt changes in environmental conditions. On seafloor highs, these shifts document ice-shelf thinning initiating at least as early as the 1940s. Sediments recovered from deep basins reflect a transition from ice proximal to slightly more distal conditions, suggesting ongoing grounding-zone retreat since the 1950s. The timing of ice-shelf unpinning from the seafloor for Thwaites Glacier coincides with similar records from neighboring Pine Island Glacier. Our work provides robust new evidence that glacier retreat in the Amundsen Sea was initiated in the mid-twentieth century, likely associated with climate variability. [ABSTRACT FROM AUTHOR]

Tunnel valleys are widespread in formerly glaciated regions such as the North Sea and record sediment transport beneath ice sheets undergoing deglaciation. However, their complex infill architecture often makes their implications for ice-sheet processes difficult to unravel. Here, we use high resolution 3D (HR3D) seismic-reflection data, improved-resolution conventional 3D seismic-reflection data, and geotechnical information from industry-acquired boreholes to image the infill architecture of buried Quaternary tunnel valleys in the North Sea in unprecedented detail. Ten cross-cutting generations of tunnel valleys are mapped beneath the seafloor of the North Sea where only seven were visible previously. Each generation of tunnel valleys potentially reflects a different glaciation, although our evidence may imply that it is possible to rapidly erode and infill multiple generations of tunnel valleys within a single glacial cycle. The infill of the oldest tunnel valley generations reflects sedimentation during relatively gradual ice-sheet retreat, with occasional episodes of overriding by re-advancing grounded ice. Tunnel valleys formed in more recent glaciations are characterised by more variable sedimentation patterns that reflect dynamic fluctuations of the ice margin, including readvances and stagnation, during valley filling and ice retreat. Numerous subglacial landforms are also imaged within the tunnel valleys; these sometimes contain shallow gas accumulations that represent geohazards for seafloor infrastructure installations. In addition, we document examples where salt diapirism has caused fluids to migrate upwards from depth through faults and into the near-surface tunnel valleys. In instances where this occurs, the relatively porous and often highly continuous subglacial landforms present within their infill may allow these fluids to spread laterally for kilometres or even escape from the seafloor; it is therefore important to consider tunnel valleys when monitoring possible CO 2 leakage from carbon capture and storage efforts. • The infill architecture of tunnel valleys in the North Sea is examined in detail. • Ten generations of tunnel valleys are mapped beneath the central North Sea where only seven were visible previously. • Multiple generations of tunnel valleys can be eroded and infilled within a single glacial cycle. • Tunnel valley sedimentation patterns became more variable in recent glaciations compared to older ones. • Tunnel valleys contain hazardous gas-filled landforms that could reduce the efficacy of carbon capture and storage efforts. [ABSTRACT FROM AUTHOR]

The geological record of landforms and sediments produced beneath deglaciating ice sheets offers insights into inaccessible glacial processes. Large subglacial valleys formed by meltwater erosion of sediments (tunnel valleys) are widespread in formerly glaciated regions such as the North Sea. Obtaining a better understanding of these features may help with the parameterisation of basal melt rates and the interplay between basal hydrology and ice dynamics in numerical models of past, present, and future ice-sheet configurations. However, the mechanisms and timescales over which tunnel valleys form remain poorly constrained. Here, we present a series of numerical modelling experiments, informed by new observations from high-resolution 3D seismic data (6.25 m bin size, ∼4 m vertical resolution), which test different hypotheses of tunnel valley formation and calculate subglacial water routing, seasonal water discharges, and the rates at which tunnel valleys are eroded beneath deglaciating ice sheets. Networks of smaller or abandoned channels, pervasive slump deposits, and subglacial landforms are imaged inside and at the base of larger tunnel valleys, indicating that these tunnel valleys were carved through the action of migrating smaller channels within tens of kilometres of the ice margin and were later widened by ice-contact erosion. Our model results imply that the drainage of extensive surface meltwater to the ice-sheet bed is the dominant mechanism responsible for tunnel valley formation; this process can drive rapid incision of networks of regularly spaced subglacial tunnel valleys beneath the fringes of retreating ice sheets within hundreds to thousands of years during deglaciation. Combined, our observations and modelling results identify how tunnel valleys form beneath deglaciating mid-latitude ice sheets and have implications for how the subglacial hydrological systems of contemporary ice sheets may respond to sustained climate warming. [Display omitted] • Numerical experiments and geophysical data are used to investigate tunnel valley formation beneath deglaciating ice sheets. • New high-resolution 3D seismic data reveal abandoned channel systems, slumps, and subglacial landforms inside tunnel valleys. • Migrating channels fed by seasonal surface meltwater erode tunnel valleys within 100s to 1000s of years during deglaciation. • Modelled tunnel valleys form time-transgressively close to the retreating ice sheet margin. • Our results explain the formation of tunnel valleys in most previously glaciated regions. [ABSTRACT FROM AUTHOR]

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 end of the Eocene greenhouse world was the most dramatic phase in the long-term Cenozoic cooling trend. Here we use 75,000 km of multi/single channel seismic reflection data from offshore Prydz Bay, Antarctica, to provide new insight on the Paleogene stratigraphic transition from greenhouse to icehouse conditions and reorganizing the ocean circulation changes that were invigorated by the cooling and glaciation. We identify a new prominent Paleogene transitional phase (Greenhouse to Icehouse) preserved in the deep-water sedimentary record by correlating from shelf to the continental slope. The occurrence of mega-Mass Transport Deposits (MTDs) on the slope during an early stage in the transition suggests significant instability and collapse of the upper part of the continental margin. A second stage of the transition is represented by the growth of a well-defined set of continental slope clinoforms. We estimate the formation age of the MTDs and clinoforms to be around Eocene-Oligocene Transition. The formation of the clinoforms in the transitional phase indicates sea level has risen, and large volumes sediment delivered to the margin by marine-terminating glaciers on the shelf. Finally, a subsequent marked migration of the margin depocenter toward the west and northwest, attests the onset of drift sedimentation and full glacial conditions, suggesting a more vigorous ocean circulation as the Earth entered the icehouse conditions after the Eocene-Oligocene boundary. [ABSTRACT FROM AUTHOR]

Interplay of deep‐water sedimentary processes is responsible for building a myriad of features and deposits across mixed turbidite–contourite systems, from <5 cm beds to >200 km long sedimentary drifts. Investigations of the spatial and temporal variability of their sedimentary facies and facies associations is crucial to reveal the dynamics between along‐slope bottom currents and down‐slope turbidity currents, as well as their impact on drift construction and channel erosion. This study focuses on extensive modern mixed (turbidite–contourite) systems, developed across the continental rise of the Pacific margin of the Antarctic Peninsula. Nine sediment cores were sampled and analysed, through grain size and geochemical methods, to study the sedimentary facies at high‐resolution (ca 1 to 20 cm). Three main facies associations have been identified across distinct morphological features (i.e. mounded drifts and trunk channels), comprising intercalations of hemipelagites, bottom current reworked sands (which include fine to coarse‐grained contourites) and gravitational facies (turbidites and mass‐transport deposits). These facies associations reflect fluctuations of the background sedimentation, oscillations of the bottom‐current velocity and of the frequency of gravity‐driven currents. The sedimentary record features cyclic alternations during the Late Quaternary (>99 kyr), suggesting that variations between along‐slope bottom currents and down‐slope turbidity currents are strongly linked to glacial–interglacial cycles during Marine Isotope Stages 1 to 6. Sedimentary records affected by bottom currents on polar margins, such as those of the Antarctic Peninsula, are essential to decipher the facies and facies sequences of bottom‐current deposits, as the low degree of bioturbation throughout most of the sediments allows us to observe the original sedimentary structures, which are poorly preserved in similar deposits from other continental margins. [ABSTRACT FROM AUTHOR]

Oscillations in ice sheet extent during early and middle Miocene are intermittently preserved in the sedimentary record from the Antarctic continental shelf, with widespread erosion occurring during major ice sheet advances, and open marine deposition during times of ice sheet retreat. Data from seismic reflection surveys and drill sites from Deep Sea Drilling Project Leg 28 and International Ocean Discovery Program Expedition 374, located across the present-day middle continental shelf of the central Ross Sea (Antarctica), indicate the presence of expanded early to middle Miocene sedimentary sections. These include the Miocene climate optimum (MCO ca. 17-14.6 Ma) and the middle Miocene climate transition (MMCT ca. 14.6-13.9 Ma). Here, we correlate drill core records, wireline logs and reflection seismic data to elucidate the depositional architecture of the continental shelf and reconstruct the evolution and variability of dynamic ice sheets in the Ross Sea during the Miocene. Drill-site data are used to constrain seismic isopach maps that document the evolution of different ice sheets and ice caps which influenced sedimentary processes in the Ross Sea through the early to middle Miocene. In the early Miocene, periods of localized advance of the ice margin are revealed by the formation of thick sediment wedges prograding into the basins. At this time, morainal bank complexes are distinguished along the basin margins suggesting sediment supply derived from marine-terminating glaciers. During the MCO, biosiliceous-bearing sediments are regionally mapped within the depocenters of the major sedimentary basin across the Ross Sea, indicative of widespread open marine deposition with reduced glacimarine influence. At the MMCT, a distinct erosive surface is interpreted as representing large-scale marine-based ice sheet advance over most of the Ross Sea paleo-continental shelf. The regional mapping of the seismic stratigraphic architecture and its correlation to drilling data indicate a regional transition through the Miocene from growth of ice caps and inland ice sheets with marine-terminating margins, to widespread marine-based ice sheets extending across the outer continental shelf in the Ross Sea. [ABSTRACT FROM AUTHOR]

Expanding multibeam bathymetric data coverage over the last two decades has revealed extensive networks of submarine channels incised into bedrock on the Antarctic inner continental shelf. The large dimensions and prevalence of the channels implies the presence of an active subglacial hydrological system beneath the past Antarctic Ice Sheet which we can use to learn more about inaccessible subglacial processes. Here, we map and analyse over 2700 bedrock channels situated across >100,000 km2 of continental shelf in the western Antarctic Peninsula and Amundsen Sea to produce the first inventory of submarine channels on the Antarctic inner continental shelf. Morphometric analysis reveals highly similar distributions of channel widths, depths, cross-sectional areas and geometric properties, with subtle differences between channels in the western Antarctic Peninsula compared to those in the Amundsen Sea. At 75–3400 m wide, 3–280 m deep, 160–290,000 m2 in cross-sectional area, and typically 8 times as wide as they are deep, the channels have similar morphologies to tunnel valleys and meltwater channel systems observed from other formerly glaciated landscapes despite differences in substrate geology and glaciological regime. We propose that the Antarctic bedrock channels formed over multiple glacial cycles through the episodic drainage of at least 59 former subglacial lakes identified on the inner continental shelf. • Inventory of subglacial meltwater channels on the West Antarctic continental shelf. • >2700 bedrock meltwater channels are mapped, and their morphometry is analysed. • Channels are similar to tunnel valley and meltwater channel systems elsewhere. • Repeated subglacial lake drainage formed the channels over multiple glaciations. [ABSTRACT FROM AUTHOR]

Following the Last Glacial Maximum (LGM; ca. 23-19 calibrated [cal.] kyr before present [BP]), atmospheric and oceanic warming, together with global sea-level rise, drove widespread deglaciation of the Antarctic Ice Sheet, increasing the flux of freshwater to the ocean and leading to substantial changes in marine biological productivity. On the Antarctic continental shelf, periods of elevated biological productivity, often preserved in the sediment record as laminated (and sometimes varved) diatomaceous oozes (LDO), have been reported from several locations and are typically associated with the formation of calving bay re-entrants during ice sheet retreat. Understanding what drives the formation and deposition of LDOs, and the impact of deglacial processes on biogenic productivity more generally, can help inform how Antarctic coastal environments will respond to current and future ice sheet melting. In this study we utilise a suite of sediment cores recovered from Anvers-Hugo Trough (AHT), western Antarctic Peninsula shelf, which documents the transition from subglacial to glacimarine conditions following retreat of an expanded ice stream after the LGM. We present quantitative absolute diatom abundance (ADA) and species assemblage data, to investigate changes in biological productivity during the Last Glacial Transition (19-11 cal kyr BP). In combination with radiocarbon dating, we show that seasonally open marine conditions were established on the mid-shelf by 13.6 cal kyr BP, but LDOs did not start to accumulate until ∼11.5 cal kyr BP. The ∼1.4 kyr delay between the onset of seasonally open marine conditions and LDO deposition indicates that physiographic changes, and specifically the establishment of a calving bay in AHT, is insufficient to explain LDO deposition alone. LDO deposition in AHT coincides with the early Holocene climatic optimum (∼11.5 – 9.0 kyr) and is therefore explained in terms of increased atmospheric/ocean temperatures, high rates of sea and glacial ice melt and the formation of a well-stratified water column in the austral spring. An implication of our study is that extensive bathymetric mapping in conjunction with detailed core analyses is required to reliably infer environmental controls on LDO deposition. • Changes in biological productivity during the Last Glacial Transition are revealed. • Intense diatom blooms are associated with high annual insolation and enhanced meltwater input. • Laminated diatomaceous ooze deposition coincides with the early Holocene climatic optimum. • Seafloor bathymetry plays an important role in controlling the thickness of laminated diatomaceous ooze deposits. [ABSTRACT FROM AUTHOR]

The present-day morpho-stratigraphy of the Ross Sea is the result of Cenozoic tectonic and cryospheric events, and constitutes a key record of Antarctica's cryospheric evolution. An enduring problem in interpreting this record in a broader regional context is that the correlation between eastern and western Ross Sea stratigraphy has remained uncertain due to the limited number of drill sites. We correlate the glacial-related features observed on a dense network of seismic reflection profiles in McMurdo Sound with those identified in the Nordenskjöld and Drygalski Basins, as well as the basins farther east in the central Ross Sea. We present an improved correlation of the regional patterns of early to middle Miocene ice-sheet variance across the Ross Sea constrained by new evaluation of seismic facies and age models from one site recovered by the Antarctic Drilling Project (ANDRILL) in the southwestern most part of McMurdo Sound. We also integrate this correlation with the recently published seismic framework in the central Ross Sea. The formation of U-shaped valleys during the early Miocene in McMurdo Sound, together with prograding sedimentary wedges in the western-most basins, and the central Ross Sea, suggest two major phases of overall advance of a marine-terminating ice sheet between ~18 Ma and ~17.4 Ma. Widespread formation of turbiditic channel-levee systems in McMurdo Sound and rapid sediment deposition in Nordernskjöld Basin point to subsequent ice-sheet retreat between ~17.4 Ma and ~15.8 Ma, coinciding with the onset of the Miocene Climate Optimum (MCO; ~17–14.5 Ma). However, the carving of troughs and formation of irregular morphologic features suggest that an extensive ice sheet still remained along the western Ross margin at ~17.4 Ma and a brief episode of ice-sheet advance occurred at ~16.8 Ma in the earliest interval of the MCO. Subsequent marine-based ice sheet advance during the Middle Miocene Climate Transition (MMCT, ~14.0–13.8 Ma) is indicated by widespread erosional features. Our results reconcile the semi-continouous seismic and drill core stratigraphy of the offshore Ross Sea continental shelf with inferences of ice sheet dynamics from continuous far-field deep sea and sea level records, as well as the highly discontinous (and heavily debated) onshore records of pre-MMCT glaciation and aridification of the Transantarctic Mountains at 14 Ma. • Core-log-seismic correlation allows regional correlation of the Ross Sea record • 2 major advances of marine-terminating ice sheet occurred between 18 and 17.4 Ma • Ice sheet retreated at the onset of the Miocene Climate Optimum • Short-term ice-sheet advance occurred at ~16.8 Ma • Widespread marine-based ice sheet correlates to the Miocene Climate Transition [ABSTRACT FROM AUTHOR]

Reconstructing the advance and retreat of past ice sheets provides important long-term context for recent change(s) and enables us to better understand ice sheet responses to forcing mechanisms and external boundary conditions that regulate grounding line retreat. This study applies various radiocarbon dating techniques, guided by a detailed sedimentological analyses, to reconstruct the glacial history of Anvers-Hugo Trough (AHT), one of the largest bathymetric troughs on the western Antarctic Peninsula (WAP) shelf. Existing records from AHT indicate that the expanded Antarctic Peninsula Ice Sheet (APIS) advanced to, or close to, the continental shelf edge during the Last Glacial Maximum (LGM; 23-19 cal kyr BP [ = calibrated kiloyears before present]), with deglaciation of the outer shelf after ∼16.3 cal kyr BP. Our new chronological data show that the APIS had retreated to the middle shelf by ∼15.7 cal kyr BP. Over this 600-year interval, two large grounding-zone wedges (GZW) were deposited across the middle (GZW2) and inner shelf (GZW3), suggesting that their formation occurred on centennial rather than millennial timescales. Expanded sequences of sub-ice shelf sediments occur seaward of the inner GZW3, which suggests that the grounding line remained stationary for a prolonged period over the middle shelf. Grounding-line retreat rates indicate faster retreat across the outer to middle shelf compared to retreat across the middle to inner shelf. We suggest that variable retreat rates relate to the broad-scale morphology of the trough, which is characterised by a relatively smooth, retrograde seabed on the outer to middle shelf and rugged morphology with a locally landward shallowing bed and deep basin on the inner shelf. A slowdown in retreat rate could also have been promoted by convergent ice flow over the inner shelf and the availability of pinning points associated with bathymetric highs around Anvers Island and Hugo Island. • The deglacial history of Anvers-Hugo Trough is revealed by marine geological and geophysical data. • Application of multiple 14C dating methods allows reliable dating of ice sheet retreat. • Grounded ice had retreated to the mid-shelf by ∼15.7 cal kyr BP. • Retreat rates are controlled by the broad-scale morphology of the shelf. [ABSTRACT FROM AUTHOR]

The Southern Ocean surrounding Antarctica is a region that is key to a range of climatic and oceanographic processes with worldwide effects, and is characterised by high biological productivity and biodiversity. Since 2013, the International Bathymetric Chart of the Southern Ocean (IBCSO) has represented the most comprehensive compilation of bathymetry for the Southern Ocean south of 60°S. Recently, the IBCSO Project has combined its efforts with the Nippon Foundation - GEBCO Seabed 2030 Project supporting the goal of mapping the world's oceans by 2030. New datasets initiated a second version of IBCSO (IBCSO v2). This version extends to 50°S (covering approximately 2.4 times the area of seafloor of the previous version) including the gateways of the Antarctic Circumpolar Current and the Antarctic circumpolar frontal systems. Due to increased (multibeam) data coverage, IBCSO v2 significantly improves the overall representation of the Southern Ocean seafloor and resolves many submarine landforms in more detail. This makes IBCSO v2 the most authoritative seafloor map of the area south of 50°S., (© 2022. The Author(s).)

The mid-Cretaceous period was one of the warmest intervals of the past 140 million years 1-5 , driven by atmospheric carbon dioxide levels of around 1,000 parts per million by volume 6 . In the near absence of proximal geological records from south of the Antarctic Circle, it is disputed whether polar ice could exist under such environmental conditions. Here we use a sedimentary sequence recovered from the West Antarctic shelf-the southernmost Cretaceous record reported so far-and show that a temperate lowland rainforest environment existed at a palaeolatitude of about 82° S during the Turonian-Santonian age (92 to 83 million years ago). This record contains an intact 3-metre-long network of in situ fossil roots embedded in a mudstone matrix containing diverse pollen and spores. A climate model simulation shows that the reconstructed temperate climate at this high latitude requires a combination of both atmospheric carbon dioxide concentrations of 1,120-1,680 parts per million by volume and a vegetated land surface without major Antarctic glaciation, highlighting the important cooling effect exerted by ice albedo under high levels of atmospheric carbon dioxide.