Paleoenvironmental evolution of on-shore ice-free areas around Maxwell Bay, King George Island, South Shetlands Islands

Abstract

Understanding the Holocene is particularly important for providing the context for recent ice sheet dynamics a i.e. understanding whether current ice sheet dynamics are unusual or part of Holocene natural variability (Bentley et al., 2014). Knowledge on the most recent millennia of Antarctic Ice Sheet history is vital for evaluating the response of the ice sheet to various forcing agents, such as sea-level rise, atmospheric and oceanographic temperature changes, and for constraining grounding-line retreat on Holocene to recent time scales (Bentley et al., 2014). The main objective of this thesis is to add new data to reconstruct the Holocene deglaciation history of King George Island, South Shetland Islands, northwest Antarctic Peninsula, by investigating morpho-sedimentary records of glacigenic and coastal landforms and associated sediments from the on-shore ice-free areas around Maxwell Bay (King George Island), namely Potter Peninsula and Fildes Peninsulas. In order to accomplish the thesis objectives, I used (i) cosmogenic exposure dating and radiocarbon dating for absolute chronological constraints; (ii) stratigraphy and sedimentology for relative chronological constraints and reconstruction of paleoenvironmental conditions; (iii) geomorphological mapping for spatial distribution of landsystems; (iv) and ground-penetrating radar (GPR) investigations for the study of internal sedimentary architecture of coastal landforms. Radiocarbon dating results yield new age constraints for the onset of deglaciation on Potter Peninsula, which occurred around at or before 7.8 ka cal BP instead of an earlier accepted age of 9.5 ka cal BP. I provide additional evidence for a short-lived glacier re-advance between 7.2 and 7.0 ka cal BP. This re-advance is likely linked to a glacier re-advance or still-stand documented on South Shetland Islands for that time period. Nevertheless, climatic conditions associated with this glacial re-advance remain unclear. In contrast, on Fildes Peninsula, exposure and radiocarbon dating indicate that glacial oscillations were minimal during the last 7 ka. I applied radiocarbon dating to remnants of mosses preserved in moraines. The moraines were formed close to the present glacier limit between 0.5 and 0.1 ka cal BP, during the last glacier re-advance in South Shetland Islands. This advance is linked to reductions in summer/annual insolation coupled with a shift to more intense Southern Hemisphere westerly winds in the Southern Ocean. Stronger, and possibly more poleward-shifted southern westerly winds produced more precipitation-laden storm fronts passing over the South Shetland Islands and thus, increased ice accumulation. The data also show that between 1.9 and 1.3 ka cal BP a climatic optimum was reached on Fildes and Potter Peninsula, which lasted until the last glacier readvance. GPR investigations and radiocarbon dating from a gravel spit system on Potter Peninsula document coastal progradation during the late phase of the last glacier re-advance, with a stable relative sea-level. Results also show an interruption of spit progradation that coincides with a proposed onset of accelerated isostatic rebound in reaction to glacier retreat subsequent to the last glacier re-advance. Spit growth resumed in the late 19th century after the rate of isostatic rebound decreased, and continues until today. The findings of this thesis support both, glacio-isostatic adjustment (GIA) models that show limited and those which show more pronounced ice-load changes on the South Shetland Islands during the late Holocene, suggesting that some GIA model parameters for the South Shetland Islands (e.g., lithospheric thickness, mantle viscosity) need to be better constrained. Furthermore, my findings have implications for regional paleoclimatic reconstructions and on ice sheet modeling for the Holocene of the northwest Antarctic Peninsula region.