The late Pleistocene and Holocene glaciation history of sub-Antarctic South Georgia

The ice cap of the sub-Antarctic island South Georgia is influenced by the Antarctic Circumpolar Current and is hence more sensitive to changing climate than the significantly larger and more isolated Antarctic ice sheets. Furthermore, the sediment deposits in fjords and glacially eroded troughs around the island have superbly archived glacier behavior, environmental and climatic changes since the late Pleistocene. This makes South Georgia an attractive target to study past climate variability in the Southern Hemisphere. Nevertheless, the ice sheet’s extents and dynamics during the Last Glacial Maximum (LGM), the Antarctic Cold Reversal (ACR), and the Holocene deglaciation phase are still poorly understood. Although several studies on land and in marine near-shore areas of South Georgia have addressed this, only few studies are based on marine sediment cores from the continental shelf. In this study, we use ten gravity cores from three different troughs on the southern and northwestern shelf to further investigate the climatic and glaciological evolution of South Georgia during and since the LGM.

Multi-proxy sedimentological analyses carried out in this study include core logging, XRF geochemical profiling, XRD analyses on bulk sediment and clay fraction, measurements of physical properties, magnetic susceptibility, grain size distribution and shear strength. For the Drygalski Trough on the southern shelf, lithofacies description reveals the deposition of stratified, predominantly sandy diamicton and greenish-grey massive to laminated, sometimes bioturbated mud with variable amounts of clasts. First radiocarbon ages from benthic foraminifera constrain the deposition of the diamicton, interpreted as waterlain till, on the outer shelf to the LGM. Inferred linear sedimentation rates attest to low sediment input on the outer shelf during the LGM (34 cm/ka) and the Holocene (23-32 cm/ka). In contrast, a higher sedimentation rate (114 cm/ka) between 14.7 and 13.7 cal ka BP is likely associated with enhanced erosion due to a possible re-advance of South Georgia’s glaciers during the ACR’s colder and wetter climate. For island-proximal cores, sedimentation rates are generally higher than on the outer shelf with rates of 80-2300 cm/ka during the Mid- to Late Holocene. This stronger fluctuation of sedimentation rates is due to higher temporal resolution of the dated sediments compared to the outer shelf. Grain-size distribution on the outer shelf shows a gravel content of 1-28 wt% in the diamicton facies from the LGM and 1-5 wt% in a sediment interval dated to 16.8 cal ka BP. This sediment interval is also characterized by a high content of pebbles, likely reflecting an increased input of IRD. The overlying ACR and Holocene show a low gravel content of 0-0.7 wt%. The diamicton suggests that ice-proximal conditions prevailed on the outer shelf during the LGM and therefore supports the theory of a shelf-wide glaciation. The combination of a low-resolution sediment core from the outer shelf and island-proximal high-resolution sediment cores has the potential to give new insights into South Georgia’s climate history from the Late Pleistocene to the Late Holocene.