Sedimentology of the southern Bellingshausen Sea

A major trough ('Belgica Trough') eroded by a palaeo-ice stream crosses the continental shelf of the southern Bellingshausen Sea (West Antarctica) and is associated with a trough mouth fan ('Belgica TMF') on the adjacent continental slope. Previous marine geophysical and geological studies investigated the bathymetry and geomorphology of Belgica Trough and Belgica TMF, erosional and depositional processes associated with bedform formation, and the temporal and spatial changes in clay mineral provenance of subglacial and glaciomarine sediments. Here, we present multi-proxy data from sediment cores recovered from the shelf and uppermost slope in the southern Bellingshausen Sea and reconstruct the ice-sheet history since the last glacial maximum (LGM) in this poorly studied area of West Antarctica. We combined new data (physical properties, sedimentary structures, geochemical and grain-size data) with published data (shear strength, clay mineral assemblages) to refine a previous facies classification for the sediments. The multi-proxy approach allowed us to distinguish four main facies types and to assign them to the following depositional settings: 1) subglacial, 2) proximal grounding-line, 3) distal sub-ice shelf/subsea ice, and 4) seasonal open-marine. In the seasonal open-marine facies we found evidence for episodic current-induced winnowing of near-seabed sediments on the middle to outer shelf and at the uppermost slope during the late Holocene. In addition, we obtained data on excess 210Pb activity at three core sites and 44 AMS 14C dates from the acid-insoluble fraction of organic matter (AIO) and calcareous (micro-)fossils, respectively, at 12 sites. These chronological data enabled us to reconstruct, for the first time, the timing of the last advance and retreat of the West Antarctic Ice Sheet (WAIS) and the Antarctic Peninsula Ice Sheet (APIS) in the southern Bellingshausen Sea. We used the down-core variability in sediment provenance inferred from clay mineral changes to identify the most reliable AIO 14C ages for ice-sheet retreat. The palaeo-ice stream advanced through Belgica Trough after ~36.0 corrected 14C ka before present (B.P.). It retreated from the outer shelf at ~25.5 ka B.P., the middle shelf at ~19.8 ka B.P., the inner shelf in Eltanin Bay at ~12.3 ka B.P., and the inner shelf in Ronne Entrance at ~6.3 ka B.P.. The retreat of the WAIS and APIS occurred slowly and stepwise, and may still be in progress. This dynamical ice-sheet behaviour has to be taken into account for the interpretation of recent and the prediction of future mass-balance changes in the study area. The glacial history of the southern Bellingshausen Sea is unique when compared to other regions in West Antarctica, but some open questions regarding its chronology need to be addressed by future work.

Sedimentary record of the western Amundsen Sea Embayment

The Amundsen Sea Embayment (ASE) drains approximately 35% of the West Antarctic Ice Sheet (WAIS) and is one of the most rapidly changing parts of the cryosphere. In order to predict future ice-sheet behaviour, modellers require long-term records of ice-sheet melting to constrain and build confidence in their simulations. Here, we present detailed marine geological and radiocarbon data along three palaeo-ice stream tributary troughs in the western ASE to establish vital information on the timing of deglaciation of the WAIS since the Last Glacial Maximum (LGM). We have undertaken multi-proxy analyses of the cores (core description, shear strength, x-radiographs, magnetic susceptibility, wet bulk density, total organic carbon/nitrogen, carbonate content and clay mineral analyses) in order to: (1) characterise the sedimentological facies and depositional environments; and (2) identify the horizon(s) in each core that would yield the most reliable age for deglaciation. In accordance with previous studies we identify three key facies, which offer the most reliable stratigraphies for dating deglaciation by recording the transition from a grounded ice sheet to open marine environments. These facies are: i) subglacial, ii) proximal grounding-line, and iii) seasonal open-marine. In addition, we incorporate ages from other facies (e.g., glaciomarine diamictons deposited at some distance from the grounding line, such as glaciogenic debris flows and iceberg rafted diamictons and turbates) into our deglacial model. In total, we have dated 78 samples (mainly the acid insoluble organic (AIO) fraction, but also calcareous foraminifers), which include 63 downcore and 15 surface samples. Through careful sample selection prior to dating, we have established a robust deglacial chronology for this sector of the WAIS. Our data show that deglaciation of the western ASE was probably underway as early as 22,351 calibrated years before present (cal 44 yr BP), reaching the mid-shelf by 13,837 cal yr BP and the inner shelf to within c.10-12 km of the present ice shelf front between 12,618 and 10,072 cal yr BP. The deglacial steps in the western ASE broadly coincide with the rapid rises in sea-level associated with global meltwater pulses 1a and 1b, although given the potential dating uncertainty, additional, more precise ages are required before these findings can be fully substantiated. Finally, we show that the rate of ice-sheet retreat increased across the deep (up to1,600 m) basins of the inner shelf, highlighting the importance of reverse slope and pinning points in accelerated phases of deglaciation.

Grain-size, lithic grains, foraminifera-derived and dinocyst-derived data of sediment core MD99-2281

The last glacial period was punctuated by abrupt climatic events with extrema known as Heinrich and Dansgaard-Oeschger events. These millennial events have been the subject of many paleoreconstructions and model experiments in the past decades, but yet the hydrological processes involved remain elusive. In the present work, high-resolution analyses were conducted on the 12-42 ka BP section of core MD99-2281 retrieved southwest of the Faeroe Islands, and combined with analyses conducted in two previous studies (Zumaque et al., 2012; Caulle et al., 2013). Such a multiproxy approach, coupling micropaleontological, geochemical and sedimentological analyses, allows us to track surface, subsurface, and deep hydrological processes occurring during these rapid climatic changes. Records indicate that the coldest episodes of the studied period (Greenland stadials and Heinrich stadials) were characterized by a strong stratification of surface waters. This surface stratification seems to have played a key role in the dynamics of subsurface and deep-water masses. Indeed, periods of high surface stratification are marked by a coupling of subsurface and deep circulations which sharply weaken at the beginning of stadials, while surface conditions progressively deteriorate throughout these cold episodes; conversely, periods of decreasing surface stratification (Greenland interstadials) are characterized by a coupling of surface and deep hydrological processes, with progressively milder surface conditions and gradual intensification of the deep circulation, while the vigor of the subsurface northward Atlantic flow remains constantly high. Our results also reveal different and atypical hydrological signatures during Heinrich stadials (HSs): while HS1 and HS4 exhibit a "usual" scheme with reduced overturning circulation, a relatively active North Atlantic circulation seems to have prevailed during HS2, and HS3 seems to have experienced a re-intensification of this circulation during the middle of the event. Our findings thus bring valuable information to better understand hydrological processes occurring in a key area during the abrupt climatic shifts of the last glacial period.

Core scans and lithologic analysis of Subglacial Lake Whillans sediment cores

The hydrologic system beneath the Antarctic Ice Sheet is thought to influence both the dynamics and distribution of fast flowing ice streams, which discharge most of the ice lost by the ice sheet. Despite considerable interest in understanding this subglacial network and its affect on ice flow, in situ observations from the ice sheet bed are exceedingly rare. Here we describe the first sediment cores recovered from an active subglacial lake. The lake, known as Subglacial Lake Whillans, is part of a broader, dynamic hydrologic network beneath the Whillans Ice Stream in West Antarctica. Even though "floods" pass through the lake, the lake floor shows no evidence of erosion or deposition by flowing water. By inference, these floods must have insufficient energy to erode or transport significant volumes of sediment coarser than silt. Consequently, water flow beneath the region is probably incapable of incising continuous channels into the bed and instead follows preexisting subglacial topography and surface slope. Sediment on the lake floor consists of till deposited during intermittent grounding of the ice stream following flood events. The fabrics within the till are weaker than those thought to develop in thick deforming beds suggesting subglacial sediment fluxes across the ice plain are currently low and unlikely to have a large stabilizing effect on the ice stream's grounding zone.