Reconstructing the glacial history of Marguerite Bay, Antarctic Peninsula: A combined geomorphological and sedimentological approach
Oakes Fretwell, Lisa
Anderson, John B.
Doctor of Philosophy
Geological and geophysical data were interpreted to reconstruct the glacial history of Marguerite Bay. Antarctic Peninsula, since the Last Glacial Maximum. An extensive paleo-subglacial meltwater drainage system, which drained the lower Peninsula since at least the Last Glacial Maximum, was mapped on the inner-shelf. A distinct organization of flow is seen across the inner-shelf, as relatively small-scale randomly-oriented linked-cavities feed into larger, linear channels and tunnel valleys. Meltwater discharge estimates suggest that production from steady-state processes (geothermal heat and frictional heat dissipation) were enhanced locally by the sudden up-glacier release of subglacially-stored meltwater. Such large instantaneous meltwater releases may have led to periods of rapid sliding, or intermittent ice-bed separation, within the system. Evidence of glacial erosion on bedrock highs, in the form of striations and grooves, suggests that ice flow remained relatively slow outside the main troughs. As sediment accumulated on the mid-shelf, free meltwater became incorporated into the porous sedimentary strata, thus enabling the facilitation of sediment deformation and relatively rapid ice flow. The presence of mega-scale glacial lineations on the outer-shelf, which clearly extend to the shelf-break where gullies erode downslope, further supports a relatively rapid ice flow, and suggests that maximum ice extent during the Last Glacial Maximum was at the shelf-break. Therefore, ice flow behavior in the bay and trough was primarily controlled by subglacial substrate, the occurrence of subglacial meltwater and local topography. Sediment facies were interpreted as subglacial, proximal-grounding zone, sub-ice shelf, fjord and ice-distal/open marine, based primarily on lithologic attributes, shear strength and down-core magnetic susceptibility response. AMS dating of specific glaciomarine sediment facies implies that grounded ice retreated diachronously across the outer-shelf, retreating initially within the trough (approximately 12,000 years ago), and later in the inter-trough area (approximately 8,000 years ago). A high-resolution Holocene paleoclimate record, reconstructed from sediments in Neny Fjord, suggests that deglaciation had occurred by 8,000 years ago. Radiocarbon dates suggest that glaciers had re-advanced by 4,500 years ago. Magnetic susceptibility response and sediment facies analyses suggest that climate appears to have remained relatively ice-proximal over the last 2,000 years, punctuated by at least three cool oscillations.