On the Antarctic contribution to Holocene sea-level
Creators: name: Goodwin, ID
Publisher: name: University Of Tasmania
Year: 2023

description: Holocene glacial fluctuations of Antarctica are investigated using a combined glacial geological and glaciological approach. An understanding of the timing, forcing and sea-level contribution of these glacial fluctuations is crucial to the interpretation of global eustatic sea-level changes during the Holocene and the present. As a baseline for the Holocene studies, the recent surface mass balance rate distribution of Antarctica and its controlling factors are determined with respect to case studies in the Wilkes Land and the Lambert Glacier drainage basins. The variation in surface mass balance rates and mean annual oxygen isotope values over the last 50 years are interpreted from stratigraphic analyses of several shallow firn cores from these drainage basins. The surface mass balance rate is calculated to vary with surface air temperature by 20%/¬¨‚àûC in the ice sheet interior, and by 25%/¬¨‚àûC in the coastal margins. This variation is in response to changes in atmospheric circulation, surface air temperature and sea ice extent. The Holocene climate pattern is analysed with respect to the oxygen isotope records from the deep ice cores and the oceanic sediment cores, and is defined into periods where temperature has deviated up to 1-2¬¨‚àûC from the long-term Holocene mean. The corresponding terrestrial and marine glacial geological evidence for Holocene glacial fluctuations in the south-western Pacific Ocean, the Indian Ocean, and the Ross Sea sectors of East Antarctica is re-examined together with a case study of the Holocene glacial history of the Windmill Islands and the Law Dome ice sheet, in East Antarctica. An interpretation of the ice expansion during the Last Glacial Maximum and the subsequent melting history of the Law Dome ice sheet during the Early-Mid Holocene, is made from field observations of raised shorelines and the chronostratigraphy of lake sediments and abandoned Adelie penguin rookeries on the Windmill Islands. The Mid-Late Holocene evolution of the Law Dome ice sheet margin is determined from the stratigraphy, structure, morphology, isotopic, solute and sediment composition of basal ice exposures and the Loken Moraines. The results from the Law Dome case study support the view that contraction of the outlet glaciers was established before 8,000 years B.P. with the outer Windmill Islands, adjacent to the Vanderford Glacier, ice free at this time. Contraction of the grounded ice margins lagged the outlet glaciers with the inner Windmill Islands deglaciated by 5,500 years B.P.. Subsequently, the ice margin continued to retreat to a position further inland than the present until 4,500 years B.P.. During this period the East Antarctic outlet glaciers were at their maximum Holocene expansion. Between 4,000 to 1,000 years B.P. it is concluded that the Law Dome and the East Antarctic grounded ice margins expanded with a partial contraction between 500 years B.P. to the present. Relative sea-level changes and variation in surface mass balance rates throughout the Holocene are tested by a sensitivity analysis of the Lambert Glacier/Amery Ice Shelf system, the Law Dome and Wilkes Land to determine the forcing mechanisms for the glacial fluctuations. The palaeo-surface mass balance rates are calculated for the Early Holocene Climatic Optimum, the Mid Holocene cool period, and the Late Holocene warm and cool phases, using the interpreted 20-25%/¬¨‚àûC variation in surface mass balance with air temperature. The sensitivity analyses confirm that the size of Antarctica is controlled by its dual response to sea-level and climate changes, with expansion initiated by relative sea-level lowering and nourished by a positive surface mass balance. The relative sensitivities are characterised by the outlet glaciers leading the grounded ice sheet margins and the alpine glaciers, both in the Early Holocene contraction and in the Mid-Late Holocene expansion. It is hypothesised that variation in palaeo-surface mass balance rates during the Holocene has partially offset the sea-level rise contribution of the post-glacial contraction of Antarctica. An equivalent sea-level curve for Antarctica is constructed from the differential surface mass balance contributions and from the post-glacial melting history. The validity of the hypothesis is tested by comparing the derived sea-level curve to the relative sea-level records on mid-oceanic islands. The maximum contribution from Antarctica to post-glacial sealevels occurred at 4,000 years B.P. when there was widespread retreat of the East Antarctic outlet glaciers, and the grounded ice sheet margins. This is synchronous with the geomorphological evidence for high sea-level stands on the mid-oceanic islands. Antarctica contributed a slight lowering to sea-level between 4,000 to 1,000 years B.P.. During this century the probable Antarctic sea-level contribution was calculated to have varied from a sea-level contribution of +0.25 mm a -1 prior to 1970 followed by a contribution of -0.15 mm a-1 since 1970. It is suggested that these contributions to Holocene sea-levels provide a better fit to the oceanic sea-level records than those produced by models based on the continuing exponential melting and retreat of the Antarctic Ice Sheet throughout the Holocene.