Beschreibung: Past studies of South Georgia’s climatic history were constrained to land-based sedimentary records, such as peat bogs and coastal lakes, or to terrestrial geomorphology, such as terminal moraines. Hence, the current state of knowledge on past climatic changes in South Georgia is characterised by a complete absence of records from sedimentary marine archives in the fjords or coastal embayments of the region. This study comprises detailed examination of one of the first marine sediment cores recovered on its northeastern shelf in Royal Bay Glacial Trough. Alongside the analysis of new acoustic sub-bottom data, it is the first work to deliver extensive insight into South Georgia’s post-glacial climatic history from a marine perspective. The glacial troughs on the South Georgia shelf radiate from the coast towards the shelf edge and represent major sediment traps as they form the only key large-scale depressions in the shelf bathymetry. Sedimentary records, covering a period since at least the Last Glacial Maximum, are thought likely to be recorded in most of them. The sediment core of this study covers sedimentation dated from a maximum of 15,346 ±492 cal. yr BP until the present day. Physical core parameters indicate a major change in climatic conditions around 14,000 cal. yr BP, the time of the Antarctic Cold Reversal. Holocene climate variabilities are also recorded in the trough infill. The acoustic data show a major change in sedimentation and a pronounced unconformity at the core site, which appears to have had a widespread effect over a large area of the shelf. The origin of the unconformity remains unclear, though several hypotheses, including bottom-current erosion, glacial overriding and earthquake activity, are proposed and discussed. Another important finding at the core site is the presence of methane-derived authigenic carbonates. They form either as secondary precipitates in the subsurface or syndepositional at the seafloor as individual minerals or sediment cements. The authigenic carbonates are linked to the widespread occurrence of cold methane seeps on the shelf which are visible today as zones of acoustic blanking and wipe-out structures in the sub-bottom profiles. These seeps are evidence of major biogenic productivity and carbon drawdown on the South Georgia shelf and the subsequent decay of organic matter. This finding is an important consideration for future studies as authigenic carbonate production and methane release have an impact on the isotopic composition of carbonate shells of marine organisms. Thus, radiocarbon ages recovered from the South Georgia shelf need to be examined critically and might require additional corrections.

Beschreibung: Defining the extent of the Last Glacial Maximum Antarctic Ice Sheet and the timing of its subsequent retreat still remains poorly understood for numerous drainage sectors. New marine geoscientific field data from a formerly unstudied West Antarctic continental shelf sector reveal the last maximum extent of the ice sheet and its initial retreat. It is shown how modern continental shelf regions must have already been evacuated at a remarkably early stage, thereby validating a rather diachronous retreat pattern of the Antarctic Ice Sheet following its last maximum extent.

Beschreibung: High-resolution swath bathymetry data collected during several research cruises over the past two decades reveal a palaeo-ice stream trough (Abbot Glacial Trough) crossing the middle and outer shelf of the easternmost Amundsen Sea Embayment, east of the main Pine Island Trough. Regions of both fast palaeo-ice flow (within the central trough) and slow palaeo-ice flow (on adjacent seafloor highs referred to as inter-ice stream ridges) bear glacial landforms indicative of phases of grounding-line stabilization of the ice sheet. We associate a grounding-zone wedge situated within the outer Abbot Glacial Trough with a grounding-zone wedge in outer Pine Island Trough and suggest a synchronous grounding-line halt in both troughs. New sediment echosounder and sediment core data collected from outer Abbot Glacial Trough, between the seaward limit of the grounding-zone wedge and the shelf edge, reveal an up to 6 m-thick well stratified drape that is composed of unconsolidated glaciomarine sediments occasionally bearing calcareous microfossils. In order to decipher whether this unusually thick sediment drape might indicate sub-ice shelf and/or seasonal-open marine deposition throughout or since the Last Glacial Maximum, we used a multi-proxy approach to characterize its lithofacies and applied radiocarbon dating of calcareous microfossils. Here we present our initial results and discuss since when the outer shelf in the eastern Amundsen Sea has been free of grounded-ice. Such information will 1) improve ice sheet models that aim to reconstruct the flow and extent of the West Antarctic Ice Sheet during the Last Glacial Maximum, 2) help to quantify the ice volume of the West Antarctic Ice Sheet during this time, and 3) prove or reject the possibility that Antarctic benthic biota endured glacial periods in outer shelf refugia.

Beschreibung: Marine and terrestrial geological and marine geophysical data that constrain deglaciation since the Last Glacial Maximum (LGM) of the sector of the West Antarctic Ice Sheet (WAIS) draining into the Amundsen Sea and Bellingshausen Sea have been collated and used as the basis for a set of time-slice reconstructions. The drainage basins in these sectors constitute a little more than one-quarter of the area of the WAIS, but account for about one-third of its surface accumulation. Their mass balance is becoming increasingly negative, and therefore they account for an even larger fraction of current WAIS discharge. If all of the ice in these sectors of the WAIS was discharged to the ocean, global sea level would rise by ca. 2 m. There is compelling evidence that grounding lines of palaeo-ice streams were at, or close to, the continental shelf edge along the Amundsen Sea and Bellingshausen Sea margins during the last glacial period. However, the few cosmogenic surface exposure ages and ice core data available from the interior of West Antarctica indicate that ice surface elevations there have changed little since the LGM. In the few areas from which cosmogenic surface exposure ages have been determined near the margin of the ice sheet, they generally suggest that there has been a gradual decrease in ice surface elevation since pre-Holocene times. Radiocarbon dates from glacimarine and the earliest seasonally open marine sediments in continental shelf cores that have been interpreted as providing approximate ages for post-LGM grounding-line retreat indicate different trajectories of palaeo-ice stream recession in the Amundsen Sea and Bellingshausen Sea embayments. The areas were probably subject to similar oceanic, atmospheric and eustatic forcing, in which case the differences are probably largely a consequence of how topographic and geological factors have affected ice flow, and of topographic influences on snow accumulation and warm water inflow across the continental shelf. Pauses in ice retreat are recorded where there are “bottle necks” in cross-shelf troughs in both embayments. The highest retreat rates presently constrained by radiocarbon dates from sediment cores are found where the grounding line retreated across deep basins on the inner shelf in the Amundsen Sea, which is consistent with the marine ice-sheet instability hypothesis. Deglacial ages from the Amundsen Sea Embayment (ASE) and Eltanin Bay (southern Bellingshausen Sea) indicate that the ice sheet had already retreated close to its modern limits by early Holocene time, which suggests that the rapid ice thinning, flow acceleration, and grounding line retreat observed in this sector over recent decades are unusual in the context of the past 10,000 years.

Beschreibung: Palaeo-ice stream beds that are exposed today on polar continental shelves provide unique archives of conditions at the base of ice sheets that are difficult to assess beneath their modern counterparts. During the last decade, several of these palaeo-ice stream beds have been studied in detail to reconstruct the extent of the West Antarctic Ice Sheet (WAIS) at the Last Glacial Maximum (LGM), the patterns of ice drainage, and the timing of grounding-line retreat during the last deglaciation. However, despite significant advances, such information still remains poorly constrained in numerous drainage sectors of the WAIS. In particular, the maximum extent of ice at the LGM remains ambiguous for key drainage basins of the ice sheet. Whether the WAIS extended to the shelf break in the entire Pacific sector, or it advanced, at least locally, only to a middle or outer shelf position, is a crucial piece of information required for reconstructing and modeling patterns of ice-sheet change from past to present. Here we present new marine geological and geophysical data that we collected on R/V “Polarstern” expedition ANT-XXVI/3 in early 2010 to investigate the extent, flow, and retreat of the WAIS from an especially poorly studied part of the West Antarctic shelf, offshore from the Hobbs Coast in the western Amundsen Sea. Here, a landward deepening palaeo-ice stream trough is incised into the shelf. The seafloor within the western-central part of the trough is characterized by a large grounding zone wedge (GZW), ~70 m thick and ~17 km long, which overlies a high of seaward dipping sedimentary strata. Directly seaward of the GZW a ~20 km wide 80±10 m deep relatively flat basin is mapped. The back-slope of the GZW is characterized by highly elongate streamlined bedforms suggesting fast palaeo-ice flow towards NW. In contrast, the outer shelf seafloor offshore from the GZW is predominantly smooth, at numerous locations scoured by icebergs and characterized by a distinct and ~2 m-deep subbottom reflector. As in other Antarctic shelf sectors, this subbottom reflector is likely to mark the top of a subglacial stiff till that is probably of LGM age, because a calcareous microfossil from the thin layer above the subbottom reflector provided a radiocarbon age of ~9.0 corr. ka BP at 98 cmbsf (centimeter below seafloor). A radiocarbon date from the inner shelf shows that the grounded ice here had retreated landward from the GZW remarkably early (before ~13.0 cal 14C yrs). This early deglaciation is in agreement with other minimum deglaciation ages from the Amundsen Sea embayment (e.g. Pine Island Bay and Dotson-Getz Trough). For the GZW we propose two possible formation scenarios: either (1) that it formed during a prolonged stillstand when the WAIS retreated from the outer shelf following the LGM, or (2) it marks the maximum grounding-line extent at the LGM. Although we rather propose, that grounded ice extended all the way or close to the shelf edge during the LGM, we cannot yet exclude the possibility that the GZW marks the LGM limit of grounded ice. Here we specifically point to the size and volume of the GZW, which are more comparable to grounding-line features marking LGM-positions around Antarctica (e.g. in Prydz Bay), and less similar to GZWs deposited during episodic ice-stream retreat (e.g. in Pine Island Trough and Marguerite Trough). In order to test these two formation scenarios for the GZW and constrain the timing and duration of its formation, we will try to obtain additional ages from seasonal-open marine units in cores seaward of the GZW. This will help to decide whether LGM-or older ice was grounded in the basin directly seaward of the GZW. We will present preliminary interpretations of these data, which will aim to resolve the extent of the WAIS in this sector, provide new information on GZW formation in this trough and, at the same time, contribute an update on the dynamics of West Antarctic palaeo-ice streams, which drained the former ice sheet.

Beschreibung: Palaeo-ice sheet beds that are exposed today on polar continental shelves provide unique archives of conditions at the base of ice sheets that are difficult to assess beneath their modern counterparts. During the last decade, several of these palaeo-ice sheet beds have been studied in detail to reconstruct the flow, extent, and retreat of the West Antarctic Ice Sheet (WAIS) at and since the Last Glacial Maximum (LGM), respectively. The focus of these investigations, however, lay on troughs eroded by palaeo-ice streams into the shelf. Multibeam swath bathymetry mapping of a mid-shelf bank between the troughs of the Pine Island-Thwaites and Cosgrove palaeo-ice streams on the eastern Amundsen Sea shelf has revealed well-preserved sub- and proglacial bedforms, including large-scale ribbed moraines, hill-hole pairs, terminal moraines, and crevasse-squeeze ridges. Together, these features form a landform assemblage that is entirely different from previously described glacial bedforms in the adjacent troughs and allows us to reconstruct ice flow and retreat dynamics in an inter-ice stream area. This research closes an important gap in the understanding of past WAIS behaviour in the eastern Amundsen Sea Embayment and will serve as a diagnostic tool in future studies on similar inter-ice stream ridge areas. Another geomorphological study of a previously unmapped area of the West Antarctic continental shelf conducted systematic mapping of the West Antarctic continental shelf in the western Amundsen Sea, offshore from the westernmost Getz Ice Shelf. Here, a landward deepening palaeo-ice stream trough is incised into the shelf. The seafloor within the western-central part of the trough is characterized by a large, ~70 m thick and ~17 km long grounding zone wedge (GZW). The back-slope of the GZW is characterized by highly elongate streamlined bedforms suggesting fast palaeo-ice flow towards NW. In contrast, the outer shelf seafloor offshore from the GZW is predominantly smooth and at numerous locations scoured by icebergs. The GZW marks either the maximum WAIS extent in this area at the LGM or a pause in ice-stream retreat during the last deglaciation. We will characterise and interpret the bedforms in these two areas and use them for reconstructing the extent, flow, bed conditions and retreat history of the WAIS since the LGM.

Beschreibung: An extensive submarine cold-seep area was discovered on the northern shelf of South Georgia during R/V Polarstern cruise ANT-XXIX/4 in spring 2013. Hydroacoustic surveys documented the presence of 133 gas bubble emissions, which were restricted to glacially-formed fjords and troughs. Video-based sea floor observations confirmed the sea floor origin of the gas emissions and spatially related microbial mats. Effective methane transport from these emissions into the hydrosphere was proven by relative enrichments of dissolved methane in near-bottom waters. Stable carbon isotopic signatures pointed to a predominant microbial methane formation, presumably based on high organic matter sedimentation in this region. Although known from many continental margins in the world's oceans, this is the first report of an active area of methane seepage in the Southern Ocean. Our finding of substantial methane emission related to a trough and fjord system, a topographical setting that exists commonly in glacially-affected areas, opens up the possibility that methane seepage is a more widespread phenomenon in polar and sub-polar regions than previously thought.