ALBERT

Description: The history of glaciations on Southern Hemisphere sub-polar islands is unclear. Debate surrounds the extent and timing of the last glacial advance and termination on sub-Antarctic South Georgia in particular. Using sea-floor geophysical data and marine sediment cores, we resolve the record of past glaciation offshore of South Georgia giving insight into glacier response to climate variability through the transition from the Last Glacial Maximum to Holocene. We show a widespread, coherent sea-bed imprint of shelf-wide ice-sheet advance and retreat in the form of glacially-carved cross-shelf troughs, suites of end and recessional moraines, as well as populations of streamlined bedforms. Glacial troughs began to infill with sediments after c. 18 ka B.P. consistent with interpretations of an extensive last glacial advance and early onset of a progressive, and potentially rapid, deglaciation to coastal limits. A fjord-mouth moraine formed during renewed glacier resurgence between c. 15,170 and 13,340 yrs ago. From the geometry of moraines in adjacent fjords, we infer that many of South Georgia’s glaciers advanced during this period of cooler, wetter climate, known as the Antarctic Cold Reversal, extending the geographic footprint of the cryospheric response to an Antarctic climate pattern into the Atlantic sector of the Southern Ocean. We conclude that the last glaciation of South Georgia was extensive, and the sensitivity of its glaciers to climate variability during the last termination more significant than implied by previous studies. Keywords: Sub-Antarctic; ice-cap reconstruction; multibeam bathymetry; sediment cores

Description: The West Antarctic Rift System is one of the largest continental rifts on Earth. Because it is obscured by the West Antarctic Ice Sheet, its evolution is still poorly understood. Here we present the first low-temperature thermochronology data from eastern Marie Byrd Land, an area that stretches ~ 1000 km along the rift system, in order to shed light on its development. Furthermore, we petrographically analysed glacially transported detritus deposited in the marine realm, offshore Marie Byrd Land, to augment the data available from the limited terrestrial exposures. Our data provide information about the subglacial geology, and the tectonic and morphologic history of the rift system. Dominant lithologies of coastal Marie Byrd Land are igneous rocks that intruded (presumably early Paleozoic) low-grade meta-sedimentary rocks. No evidence was found for un-metamorphosed sedimentary rocks exposed beneath the ice. According to the thermochronology data, rifting occurred in two episodes. The earlier occurred between ~ 100 and 60 Ma and led to widespread tectonic denudation and block faulting over large areas of Marie Byrd Land. The later episode started during the Early Oligocene and was confined to western Pine Island Bay area. This Oligocene tectonic activity may be linked kinematically to previously described rift structures reaching into Bellingshausen Sea and beneath Pine Island Glacier, all assumed to be of Cenozoic age. However, our data provide the first direct evidence for Cenozoic tectonic activity along the rift system outside the Ross Sea area. Furthermore, we tentatively suggest that uplift of the Marie Byrd Land dome only started at ~ 20 Ma; that is, nearly 10 Ma later than previously assumed. The Marie Byrd Land dome is the only extensive part of continental West Antarctica elevated above sea level. Since the formation of a continental ice sheet requires a significant area of emergent land, our data, although only based on few samples, imply that extensive glaciation of this part of West Antarctica may have only started since the early Miocene.

Description: 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.

Description: TheWest Antarctic Rift System is one of the largest continental rifts on Earth. Because theWest Antarctic Ice Sheet mostly covers it, its evolution is still poorly understood. Here we present the first low-temperature thermochronology data from eastern Marie Byrd Land, an area that stretches 1000 km along the rift system. Furthermore, we petrographically analysed glacially transported detritus deposited in the marine realm offshore Marie Byrd Land. Our data provide information about the subglacial geology, and the tectonic and morphologic history of the rift system. Dominant lithologies of coastal Marie Byrd Land are igneous rocks that intruded (presumably early Paleozoic) low-grade meta-sediments. No evidence was found for un-metamorphosed sedimentary rocks exposed beneath the ice. According to the thermochronology data, rifting occurred in two episodes. The earlier occurred between 100 and 60 Ma and led to widespread tectonic denudation and block faulting over large areas of Marie Byrd Land. The later started during the Early Oligocene and was confined to western Pine Island Bay area. This Oligocene structure may kinematically be linked to previously described rift structures reaching into Bellingshausen Sea and beneath Pine Island Glacier, which were also assumed to be of Cenozoic age. However, our data provide the first direct evidence for Cenozoic tectonic activity along the rift system outside the Ross Sea area. Furthermore, our data suggest that uplift of the Marie Byrd Land dome only started at �20 Ma; that is, nearly 10 Ma later than previously assumed. The Marie Byrd Land dome is the only extensive part of West Antarctica elevated above sea level. Since the formation of a continental ice sheet requires a significant area of emergent land, our data imply that initiation of extensive glaciation of this part of West Antarctica may only have started since the early Miocene.

Description: The history of glaciations on Southern Hemisphere sub-polar islands is unclear. Debate surrounds the extent and timing of the last glacial advance and termination on sub-Antarctic South Georgia in particular. Here, using sea-floor geophysical data and marine sediment cores, we resolve the record of glaciation offshore of South Georgia through the transition from the Last Glacial Maximum to Holocene. We show a sea-bed landform imprint of a shelf-wide last glacial advance and progressive deglaciation. Renewed glacier resurgence in the fjords between c. 15,170 and 13,340 yr ago coincided with a period of cooler, wetter climate known as the Antarctic Cold Reversal, revealing a cryospheric response to an Antarctic climate pattern extending into the Atlantic sector of the Southern Ocean. We conclude that the last glaciation of South Georgia was extensive, and the sensitivity of its glaciers to climate variability during the last termination more significant than implied by previous studies.

Description: There has been a long history of debate over the extent and timing of past glaciations on the small but climatically-sensitive islands of the sub-Antarctic. The largest of these islands, South Georgia, has been the focus of contention with two conflicting models proposed for the peak of the last glaciation (the Last Glacial Maximum, c. 21 k yrs B.P.): one suggesting an ice cap of restricted extent that saw tidewater outlets limited to the coastal basins of radial fjords; the other proposing an extensive shelf-wide ice cap based on well-preserved though as yet un-dated or un-sampled sea-bed geomorphology. It has been suggested that these two models form comparative end-members of 'South American‘ (restricted) and 'Antarctic‘ (extensive) modes of glaciation, and thus establishing whether the pattern of glaciation on sub- Antarctic islands follows one of these modes, or is in itself entirely different, can provide potentially important insight into past climate forcing of an otherwise poorly-constrained Southern Ocean region. The extent of island glaciation can also provide valuable far-field constraint for Antarctic ice sheet models, and forms vital information for biologists seeking to understand benthic marine communities around South Georgia, whose evolution and structure are intimately linked to the long-term history of ice advance and retreat. Here we investigate the geological record offshore of South Georgia to improve understanding of the extent and timing of past glaciation. Marine geophysical data from new and previous research cruises have been compiled and analysed to advance knowledge of palaeo-ice cap dynamics on the island‘s submerged sea bed and subsurface. We have also sought an independent verification of the glacial history of the island by linking new geomorphological analyses with dated marine sedimentary records recovered by coring, for the first time, on the continental shelf. We present results of new landform mapping on the sea-bed of the South Georgia block. Mapped moraine belts demarcate a minimum of three shelf limits of former glaciation: (1) a shelf edge maximum, (2) an outer basin/fjord mouth limit, and (3) an inner basin moraine position. In addition, numerous spatially-discontinuous moraines must represent further intermediary ice-marginal extents. Geomorphological observations are consistent with the larger configurations of the palaeo-ice cap being highly dynamic, reflected in the pattern, geometry, and sedimentary sequences of cross-shelf glacial troughs, extensive depositional moraine systems, subglacially-generated bedforms, and meltwater-carved channels mapped from multibeam bathymetric survey and sub-bottom echo-sounder data. Reconstructed limits suggest a complex offshore glacial history, and provide hitherto unrivalled insight into the spatial configurations of past sub-Antarctic ice caps. A key question is whether the most prominent moraines were formed during successive Cenozoic glaciations or during retreat and readvance(s) since the last glaciation. We present preliminary chronological investigations of marine gravity cores from the Cumberland Bay and Royal Bay areas of the north-eastern shelf that provide tests of the restricted vs extensive ice-cap hypotheses. We show additional sedimentological and physical properties data from the suite of sediment cores that give insight into fjord and shelf depositional environments, patterns of Holocene glacier behaviour, and contribute more broadly towards a better characterisation of South Georgia‘s long-term marine environmental history.