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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]

The Weddell Sea sector is one of the main formation sites for Antarctic Bottom Water and an outlet for about one fifth of Antarctica's continental ice volume. Over the last few decades, studies on glacial–geological records in this sector have provided conflicting reconstructions of changes in ice-sheet extent and ice-sheet thickness since the Last Glacial Maximum (LGM at ca 23–19 calibrated kiloyears before present, cal ka BP). Terrestrial geomorphological records and exposure ages obtained from rocks in the hinterland of the Weddell Sea, ice-sheet thickness constraints from ice cores and some radiocarbon dates on offshore sediments were interpreted to indicate no significant ice thickening and locally restricted grounding-line advance at the LGM. Other marine geological and geophysical studies concluded that subglacial bedforms mapped on the Weddell Sea continental shelf, subglacial deposits and sediments over-compacted by overriding ice recovered in cores, and the few available radiocarbon ages from marine sediments are consistent with major ice-sheet advance at the LGM. Reflecting the geological interpretations, different ice-sheet models have reconstructed conflicting LGM ice-sheet configurations for the Weddell Sea sector. Consequently, the estimated contributions of ice-sheet build-up in the Weddell Sea sector to the LGM sea-level low-stand of ∼130 m vary considerably. In this paper, we summarise and review the geological records of past ice-sheet margins and past ice-sheet elevations in the Weddell Sea sector. We compile marine and terrestrial chronological data constraining former ice-sheet size, thereby highlighting different levels of certainty, and present two alternative scenarios of the LGM ice-sheet configuration, including time-slice reconstructions for post-LGM grounding-line retreat. Moreover, we discuss consistencies and possible reasons for inconsistencies between the various reconstructions and propose objectives for future research. The aim of our study is to provide two alternative interpretations of glacial–geological datasets on Antarctic Ice-Sheet History for the Weddell Sea sector, which can be utilised to test and improve numerical ice-sheet models. [ABSTRACT FROM AUTHOR]

Abstract: Inaccessibility due to harsh weather conditions and perennial sea ice has resulted in the Weddell Sea being one of the more under-studied regions of Antarctica. Re-examination of existing cores and unpublished multibeam bathymetric data provide new insight into the LGM configuration of the ice sheet in the region. Sediment cores from the eastern continental shelf, where the East Antarctic Ice Sheet (EAIS) drains, display a retreat stratigraphy with distal glacimarine sediments overlying proximal glacimarine sediments and till. The distinction between distal and proximal glacimarine sediments is based on degrees of marine influence based on grain-size distributions and foraminiferal abundance and diversity. Glacimarine sediments yielded AMS radiocarbon ages that span the past 30,476 cal yr BP, indicating that retreat of the EAIS from the continental shelf occurred prior to the LGM. Multibeam swath bathymetry data show mega-scale glacial lineations in two troughs on the southern continental shelf, which indicates the existence of two ice streams draining the West Antarctic Ice Sheet (WAIS). More subtle lineations within Filchner Trough imply more sluggish flow across bedrock and older strata within this trough. Although there are no radiocarbon ages to absolutely constrain the timing of this grounding event on the southern continental shelf, we interpret the lineations as LGM age based on their pristine nature. This interpretation is consistent with results from other sectors of the West Antarctic continental shelf where compelling evidence for significant WAIS advance during the LGM exists. In the Weddell Sea, this apparent diachronous behavior of the EAIS and WAIS is consistent with results from onshore studies. [Copyright &y& Elsevier]

Abstract: Three Cape Roberts Project drillholes (CRP-1, -2/2A and -3) penetrated, in aggregate, a 1500 m succession of glacial marine Oligocene–Lower Miocene strata on the margins of Victoria Land Basin, the upper 1000+ m of which consists of a stack of more than 50 unconformity-bounded stratigraphic sequences. The benthic foraminiferal biostratigraphy, ecology, and lithofacies relationships of three contiguous sequences (Sequences 9, 10, and 11; 130.27–306.65 mbsf) from CRP-2/2A, were first determined and then tested against Motif A, a sedimentation model developed for the drillhole succession. These sequences apparently represent relatively complete glacial–deglacial cycles which span an interval of ∼450 kyr straddling the Oligocene–Miocene boundary (∼24 Ma). Sedimentological and paleontological data suggest shoreface–inner shelf to outer shelf basin margin settings with deposition under the influence of near-shore traction currents, subglacial ice contact, ice-rafting, meltwater, and turbidity associated with ice margin advance and retreat, along with sea level variations in the order of 50 m. Stratigraphic distribution data for benthic foraminifera are at an average sampling interval of ∼1 m (=∼1000 to 3000 years), which offers stratigraphic resolution at less than orbital cycle frequencies in all major lithofacies within the three sequences. Taxa recovered represent a restricted immigrant group that repeatedly penetrated the shallow coastal waters during deglacial phases of each cycle and originated in the much more diverse assemblages of West Antarctic Rift System basins. Both eurytopic and stenotopic taxa are recognized. The former occur in all lithofacies and appear tolerant to a wide range of environmental parameters at and near the seafloor. Representative genera include, Trochammina, Cassidulinoides, Fursenkoina, Eponides, Epistominella, Cibicides, Nonionella, Melonis, Pullenia, Elphidium, Ammoelphidiella, and Cribroelphidium. Stenotopic taxa are less common, occur mainly in fine-grained lithofacies, represent maximum oceanicity, and coincide with maximum retreat of coastal glaciers and sea level highstands. Representative genera include, Quinqueloculina, Pyrgo, Triloculina, Lenticulina, Marginulina, Vaginulinopsis, Nodosaria, Pseudonodosaria, Pyrulinoides, Oolina, Fissurina, Parafissurina, Rosalina, Anomalinoides, and Hanzawaia. Planktic foraminifera are absent. Our foraminiferal data provide strong support for the Motif A model, and allow interpretation of intra-model facies and events. Motif A is an idealized glacial stratigraphic sequence construct derived from analysis of the entire Upper Oligocene–Lower Miocene sequence stack. In this model, Lowstand System Tract (LST) diamictites form the base of the sequence. Diamictites with microfaunas are considered glaciomarine, those without are more likely ice-proximal or subglacial. Fine-grained sandstone, muddy sandstone, and mudstone of mid-sequence aggradational Transgressive System Tract (TST) and Highstand System Tract (HST) strata reflect high stands of sea level, inshore penetration of open marine waters, and peak recession of ice margins, and normally contain the most diverse and abundant microfaunas (up to 17 spp/sample). Sediments of the overlying Regressive System Tract (RST) display low and upwardly-declining foraminiferal diversity in response to glacier ice advance, sediment progradation across the basin margin, and falling sea level. The time-variable Glacial Surface of Erosion (GSE) or sequence boundary terminates the Regressive System Tract (RST) and demarcates the base of the succeeding stratigraphic sequence. [Copyright &y& Elsevier]

Few Palaeogene and Neogene sediment cores from the Antarctic continental margin have been dated with sufficient precision to enable establishment of direct linkages between glacial events on the Antarctic continent and marked events in deep-sea δ18O records. As a result, much of our knowledge of the gradual, but stepwise, shift from ‘greenhouse’ climates of the Cretaceous to the ‘ice-house’ climates of the Quaternary is inferred from well-dated and more continuously deposited deep-sea sediments. In this study, we present new magnetostratigraphic results from the CIROS-1 drill core from McMurdo Sound, Antarctica, along with a reinterpretation of a published diatom biostratigraphic zonation that is constrained by correlation to a high-precision age model from the nearby CRP-2/2A drill core. Our results suggest that most of the upper 350 m of the CIROS-1 drill core represents rapid sediment accumulation during a short time interval spanning the Oligocene–Miocene boundary. Chronostratigraphic control is precise enough to enable correlation of this interval of glacimarine sedimentation with the Mi-1 deep-sea δ18O event, which confirms that the Mi-1 event was related to a major expansion of Antarctic ice. A major unconformity at 366 m in the CIROS-1 drill core, which is widely observed in regional seismic reflection studies, represents 9 Myr of missing time. This unconformity can be traced offshore into the Ross Sea using seismic stratigraphy and is interpreted to indicate significant East Antarctic ice sheet development during the Mi-1 glaciation. The stratigraphic expression of this ∼400-kyr glacial event is evidently multiphase and complex in the Victoria Land Basin, probably because it was punctuated by higher-frequency orbitally induced glacial oscillations. The presence of Nothofagidites pollen throughout the CIROS-1 drill core and the presence of a Nothofagus (Southern beech) leaf within the early Miocene portion of the core indicate that Antarctic mean summer temperatures did not decrease below 5°C throughout the Mi-1 glaciation. These temperatures are significantly warmer than present-day mean summer temperatures at sea level in McMurdo Sound. The persistence of a Nothofagus forest in coastal southern Victoria Land throughout this time interval suggests that the present state of deep refrigeration was not reached until some time after the Mi-1 glaciation. [Copyright &y& Elsevier]

The Lambert Graben is occupied by the world’s largest fjord system, through which flows the Lambert Glacier, the Amery Ice Shelf and their tributaries. Along the western margin of the graben, in the northern Prince Charles Mountains, remnants of uplifted Miocene and Pliocene strata of the glacigenic fjordal Pagodroma Group total more than 800 m in thickness. These sediments provide evidence for a dynamic East Antarctic ice sheet during the Neogene Period. Each of the four Pagodroma Group formations defined from this region rests unconformably on either Proterozoic or Permo-Triassic rocks. The unconformity surfaces represent parts of the walls and floors of Neogene fjords. For these surfaces to have been eroded, the ice must have been grounded out as far as the continental shelf in Prydz Bay. The Pagodroma Group was deposited by wet-based glaciers discharging into a fjordal setting and includes lithofacies that are quite different from those produced by modern Antarctic ice masses. The principal lithofacies are massive diamicts and soulder gravels, deposited both close to a calving, grounded glacier terminus and from icebergs. The few stratified diamicts are the product of more distal iceberg sedimentation. An ice-transported gravel lithofacies includes rockfall debris derived from palaeofjord walls and mixed with subglacially derived diamicts. Some lithofacies contain evidence of subaquatic slumping and gravity flowage. Volumetrically minor lithofacies include laminites, with some exposures exhibiting large ice-rafted clasts. The laminites represent less proximal, mainly ice-free fjordal sediments, resulting either from tidal-current sorting of suspended sediment originating from subaquatic glaciofluvial discharge, or from turbidity currents derived from unstable subaquatically deposited glacigenic sediment. The Pagodroma Group provides a record of multiple glaciation by dynamic, sliding glaciers carrying large amounts of both basal and supraglacial debris. The closest modern analogues, in terms of the thermal and dynamic characteristics of the Neogene Lambert Glacier, appear to be the fast-flowing tidewater glaciers of East Greenland. These glaciers originate from the interior ice sheet and discharge large volumes of icebergs; the resulting lithofacies are predominantly diamicts. [ABSTRACT FROM AUTHOR]