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The West Antarctic Ice Sheet (WAIS) presently holds enough ice to raise global sea level by 4.3 m if completely melted. The unknown response of the WAIS to future warming remains a significant challenge for numerical models in quantifying predictions of future sea level rise. Sea level rise is one of the clearest planet-wide signals of human-induced climate change. The Sensitivity of the West Antarctic Ice Sheet to a Warming of 2 ∘ C (SWAIS 2C) Project aims to understand past and current drivers and thresholds of WAIS dynamics to improve projections of the rate and size of ice sheet changes under a range of elevated greenhouse gas levels in the atmosphere as well as the associated average global temperature scenarios to and beyond the + 2 ∘ C target of the Paris Climate Agreement. Despite efforts through previous land and ship-based drilling on and along the Antarctic margin, unequivocal evidence of major WAIS retreat or collapse and its causes has remained elusive. To evaluate and plan for the interdisciplinary scientific opportunities and engineering challenges that an International Continental Drilling Program (ICDP) project along the Siple coast near the grounding zone of the WAIS could offer (Fig. 1), researchers, engineers, and logistics providers representing 10 countries held a virtual workshop in October 2020. This international partnership comprised of geologists, glaciologists, oceanographers, geophysicists, microbiologists, climate and ice sheet modelers, and engineers outlined specific research objectives and logistical challenges associated with the recovery of Neogene and Quaternary geological records from the West Antarctic interior adjacent to the Kamb Ice Stream and at Crary Ice Rise. New geophysical surveys at these locations have identified drilling targets in which new drilling technologies will allow for the recovery of up to 200 m of sediments beneath the ice sheet. Sub-ice-shelf records have so far proven difficult to obtain but are critical to better constrain marine ice sheet sensitivity to past and future increases in global mean surface temperature up to 2 ∘ C above pre-industrial levels. Thus, the scientific and technological advances developed through this program will enable us to test whether WAIS collapsed during past intervals of warmth and determine its sensitivity to a + 2 ∘ C global warming threshold (UNFCCC, 2015). [ABSTRACT FROM AUTHOR]

Deep Sea Drilling Project (DSDP) Site 270, located in the central high of the Ross Sea, was cored to 422.5 m below seafloor (mbsf) and recovered a thick Oligocene to lower Miocene sequence of mudstone with varying amounts of ice rafted debris (IRD), overlain by ~20 m of Pliocene to Recent diatom silty clay with IRD. This site provides important temporal constraints on regional stratigraphy and insights into late Oligocene to early Miocene ice sheet dynamics; however, previous age models were based on limited data. Here we provide a revised age model using a combination of biostratigraphy (dinoflagellate cysts, pollen, calcareous nannofossils, foraminifers, and diatoms), magnetostratigraphy, Sr-isotope stratigraphy, and K Ar dating of glauconite. We divide the sequence between 386 and 20 mbsf into four chronostratigraphic intervals (CSIs). CSI 1 (386–352 mbsf) is dated to between ~26 and 25 Ma based on glauconite K Ar dating, the highest occurrence (HO) of the dinoflagellate Lejeunecysta rotunda (24.5 Ma), and a paleomagnetic reversal tied to the C8r/C8n.2r boundary (25.987 Ma). A distinct change in the benthic foraminifer assemblage at 352 mbsf marks an unconformity and the base of CSI 2. CSI 2 (352 to 149–146 mbsf) is dated to between 25.44 Ma and 23.13 Ma based on nannofossil biostratigraphy. Within this 200 m interval we correlate seven magnetic reversals to Chrons C8n.2n to C6Cr (25.3–23.3 Ma) and our line of correlation suggests rapid sedimentation (~80 m/m.y.). Microfossil and lithologic evidence indicate a distal marine setting with a paleo-water depth of ~200 m at 345 mbsf, with deepening above. Within CSI 2 is a diamictite overlain by grounding line proximal sandstone and laminated mudstone indicating glaciomarine deposition in a grounding-line proximal setting between 245 and 230 mbsf, followed by ice sheet grounding-line retreat into a more distal setting from 230 to 146 mbsf (~24.5–23.5 Ma), likely due to basin subsidence and incursion of relatively warm deep water onto the shelf. Another unconformity between 149 and 146 mbsf marks the boundary between CSI 2 and CSI 3. The thin CSI 3 (149–146 to 121/112 mbsf) is dated to ~23 Ma and we place the Oligocene/Miocene boundary at the base of the sequence, although we cannot rule out that the boundary is represented by an unconformity. Benthic foraminifers indicate continued deepening in the earliest Miocene and finer-grained mudstone with less IRD suggests that the site remained in a grounding line distal setting, although a decrease in abundance of calcareous plankton indicates cooling climatic conditions at that time. The boundary between CSI 3 and CSI 4 is marked by an unconformity spanning at least 2.5 m.y. based on the lowest occurrence of the dinoflagellate Batiacasphaera cooperi (<20.7 Ma) at 111 mbsf. Biostratigraphic data constrain CSI 4 to the early Miocene. We tentatively identify three magnetic reversals within CSI 4 (121/112–20 mbsf) that may tie this interval to Chrons C6An.2n to C6r (~20.6–19.7 Ma). A major unconformity at 20 mbsf separates the lower Miocene sediments from the overlying Pliocene and younger sediments. Late Oligocene to early Miocene pollen assemblages from Site 270 suggest a tundra landscape with low-growing Nothofagaceae, Podocarpaceae, and Proteaceae scrub in warmer locations with a relatively stable terrestrial environment during that time. • Site 270 includes 366 m of upper Oligocene and lower Miocene glaciomarine sediment. • Revised age model for Site 270 places Oligocene/Miocene boundary at ~145 mbsf. • Deposition of diamictite at ~24.5 Ma indicates ice sheet grounding line advance. • Ice sheet grounding line retreated from the site between ~24.5 Ma and 23.5 Ma. • Ice retreat was likely driven by basin subsidence and incursion of warmer water. [ABSTRACT FROM AUTHOR]

The geometry of Antarctic ice sheets during warm periods of the geological past is difficult to determine from geological evidence, but is important to know because such reconstructions enable a more complete understanding of how the ice-sheet system responds to changes in climate. Here we investigate how Antarctica evolved under orbital and greenhouse gas conditions representative of an interglacial in the early Pliocene at 4.23 Ma, when Southern Hemisphere insolation reached a maximum. Using offlinecoupled climate and ice-sheet models, together with a new synthesis of high-latitude palaeoenvironmental proxy data to define a likely climate envelope, we simulate a range of icesheet geometries and calculate their likely contribution to sea level. In addition, we use these simulations to investigate the processes by which the West and East Antarctic ice sheets respond to environmental forcings and the timescales over which these behaviours manifest. We conclude that the Antarctic ice sheet contributed 8.6±2.8m to global sea level at this time, under an atmospheric CO2 concentration identical to present (400 ppm). Warmer-than-present ocean temperatures led to the collapse of West Antarctica over centuries, whereas higher air temperatures initiated surface melting in parts of East Antarctica that over one to two millennia led to lowering of the ice-sheet surface, flotation of grounded margins in some areas, and retreat of the ice sheet into the Wilkes Subglacial Basin. The results show that regional variations in climate, ice-sheet geometry, and topography produce long-term sea-level contributions that are non-linear with respect to the applied forcings, and which under certain conditions exhibit threshold behaviour associated with behavioural tipping points. [ABSTRACT FROM AUTHOR]

Geological data indicate that there were major variations in Antarctic ice sheet volume and extent during the early to mid- Miocene. Simulating such large-scale changes is problematic because of a strong hysteresis effect, which results in stability once the ice sheets have reached continental size. A relatively narrow range of atmospheric CO2 concentrations indicated by proxy records exacerbates this problem. Here, we are able to simulate large-scale variability of the early to mid-Miocene Antarctic ice sheet because of three developments in our modeling approach. (i) We use a climate-ice sheet coupling method utilizing a high-resolution atmospheric component to account for ice sheet-climate feedbacks. (ii) The ice sheet model includes recently proposed mechanisms for retreat into deep subglacial basins caused by ice-cliff failure and ice-shelf hydrofracture. (iii)We account for changes in the oxygen isotopic composition of the ice sheet by using isotope-enabled climate and ice sheet models. We compare our modeling results with ice-proximal records emerging from a sedimentological drill core from the Ross Sea (Andrill-2A) that is presented in a companion article. The variability in Antarctic ice volume that we simulate is equivalent to a seawater oxygen isotope signal of 0.52-0.66‰, or a sea level equivalent change of 30-36 m, for a range of atmospheric CO2 between 280 and 500 ppm and a changing astronomical configuration. This result represents a substantial advance in resolving the long-standing model data conflict of Miocene Antarctic ice sheet and sea level variability. [ABSTRACT FROM AUTHOR]

Abstract: Dynamical changes in contemporary ice sheets account for significant proportions of their current rates of mass loss, but assessing whether or not these processes are a natural part of ice-sheet evolution requires inference from palaeo-glaciological records. However, a robust mechanism for translating sparse geological data into meaningful interpretations of past glacier dynamics at the continental scale is lacking, since geological archives can be ambiguous, and often their chronology is only poorly constrained. To address this, we combine the interpretation of high-resolution Antarctic ice sheet model results with continent-wide geological evidence pertinent to the dynamical configuration of the ice sheet during the last, and possibly preceding, glacial maxima. We first focus on the thermal regime of the ice sheet, its pattern and velocity of flow, variability in likely subglacial erosion and sediment transport, and how these characteristics evolve during glacial transitions. We show that rapid basal sliding was restricted to discrete outlets that eroded and advected sediment toward and across the continental shelf primarily during the early stages of advance and retreat of the ice sheet, highlighting the need to consider time-transgressive behaviour in the interpretation of geological archives. Secondly, we present new modelling that attempts to improve the fit of our numerical model to geologically-based reconstructions in the Ross Sea. By accounting for locally-enhanced ablation in McMurdo Sound, our new simulation achieves a much closer fit to empirically-derived flow patterns than previously. Growth of the modelled Last Glacial Maximum ice sheet takes place primarily by marine ice accretion in the major embayments, as a consequence of cooler ocean temperatures and reduced sub-ice-shelf melting, and at its maximal extent represents a grounded ice volume excess above present of approximately 8.3 m sea-level equivalent. This figure thus provides an upper bound on the possible Antarctic contribution to deglacial meltwater pulses. [Copyright &y& Elsevier]

Abstract: Developing a coherent Neogene history of the Antarctic ice sheet and Southern Ocean requires a high-resolution biostratigraphic age model, one that is applicable to the array of existing onshore and offshore paleoenvironmental records. This study integrates comprehensive diatom biostratigraphy magnetostratigraphy, and tephrostratigraphy from 32 Neogene sections around the Southern Ocean and Antarctic continental margin. Framing their correlation as a Constrained Optimization (CONOP) permits the use of computer implemented approaches that resemble multidimensional versions of graphic correlation. The method establishes the most parsimonious sequence of events subject to an expert''s selection of suitable input data, determination of geological feasibility (constraints) and the most appropriate measure of fit to the data (optimization). Quantitative output includes assessments of record quality and confidence intervals on all age assignments. Thorough review and testing of CONOP model settings and input datasets established the solution''s sensitivity to these factors and guided the selective removal of unreliable data to produce a robust, precise, and reproducible biochronology. Two complementary models, based on different assumptions about reworking, yield independent estimates of average local ranges and total regional ranges of fossil taxa in the southern high latitudes. The resulting composite sequences include range data of 116 diatom taxa, as well as 52 paleomagnetic reversals and two radiometrically dated ashes, which enable age calibration to within an average of ±0.08 m.y. for first and last appearances as old as 18 Ma. Deviations of new model ages from previously published estimates are generally small, and reflect the different sample sizes available for traditional versus new quantitative biostratigraphic calibrations. Our results confirm the reliability and regional isochroneity of most commonly utilized diatom zonal marker events, identify many new potentially useful events, and provide up to an order of magnitude greater temporal resolution than traditional zonations. The anticipated expansion of this new Southern Ocean chronostratigraphic framework to new localities and other microfossil groups will pave the way to further age refinement and broader application of this approach in the future. [Copyright &y& Elsevier]

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]

• Ice sheet, climate model output is compared to geologic records of past environment. • We extrapolate continent-wide glacial conditions from local geologic datasets. • A thick but receded terrestrial ice sheet existed under high CO 2 and peak warmth. • Marine-based ice expanded when CO 2 dropped below a threshold value. • Model results support Transantarctic Mountain uplift through the mid-Miocene. Antarctic ice sheet and climate evolution during the mid-Miocene has direct relevance for understanding ice sheet (in)stability and the long-term response to elevated atmospheric CO 2 in the future. Geologic records reconstruct major fluctuations in the volume and extent of marine and terrestrial ice during the mid-Miocene, revealing a dynamic Antarctic ice-sheet response to past climatic variations. We use an ensemble of climate – ice sheet – vegetation model simulations spanning a range of CO 2 concentrations, Transantarctic Mountain uplift scenarios, and glacial/interglacial climatic conditions to identify climate and ice-sheet conditions consistent with Antarctic mid-Miocene terrestrial and marine geological records. We explore climatic variability at both continental and regional scales, focusing specifically on Victoria Land and Wilkes Land Basin regions using a high-resolution nested climate model over these domains. We find that peak warmth during the Miocene Climate Optimum is characterized by a thick terrestrial ice sheet receded from the coastline under high CO 2 concentrations. During the Middle Miocene Climate Transition, CO 2 episodically dropped below a threshold value for marine-based ice expansion. Comparison of model results with geologic data support ongoing Transantarctic Mountain uplift throughout the mid-Miocene. Modeled ice sheet dynamics over the Wilkes Land Basin were highly sensitive to CO 2 concentrations. This work provides a continental-wide context for localized geologic paleoclimate and vegetation records, integrating multiple datasets to reconstruct snapshots of ice sheet and climatic conditions during a pivotal period in Earth's history. [ABSTRACT FROM AUTHOR]

Retreat of the Antarctic ice sheets since the Last Glacial Maximum (LGM) contributed to sea-level rise, but the location, amount, and timing of ice mass loss has been controversial. This paper presents new 10 Be exposure ages from glacially transported erratics which record post LGM retreat of grounded ice in the western Ross Sea. Ice elevation in southern McMurdo Sound was ≥520 m above present day sea level on the eastern side of Mount Discovery during the LGM, and the onset of major deglaciation in the region was after 14 ka. The ice surface lowered from ∼520 to 234 m above present day sea level between 14.0 ka and 10.3 ka and from 234 m to ∼30 m between 10.3 ka and 7.4 ka. This late-glacial and Holocene deglaciation chronology from southern McMurdo Sound is consistent with other records on the margins of the Ross Embayment, and implies that the western margins of the Ross Sea Ice Sheet (RSIS) experienced most mass loss during the early to middle Holocene. These 10 Be exposure ages coupled with sediment provenance define a two-stage ice flow scenario for McMurdo Sound subdividing differing reconstructions into an early and late phase. Prior to Termination I, an expanded Koettlitz Glacier flowed north and northeast between Brown Peninsula and Mount Discovery and coalesced with northward flowing ice fed from the Skelton and Mulock Glaciers. Thinning and retreat of the Koettlitz Glacier and perhaps other outlet glaciers flowing through the Royal Society Range allowed ice grounded in the Ross Sea to flow westward and northward, north of Brown Peninsula. Grounding-line recession in the Ross Sea during the late-glacial and Holocene was likely driven by Southern Ocean warming and sea-level rise from the retreat of the Northern Hemisphere ice sheets and the outer margins of the Antarctic ice sheets. [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]