Lake sediment archives of late Holocene climate variability in Lutzow-Holm Bay, East Antarctica

Abstract

Large fluctuations in year-to-year climate variability have been observed at southern high latitudes over the last 60 years, however short instrumental records make the identification and interpretation of long-term trends difficult. In the Antarctic region, the need for a longer term perspective on climate variability can be addressed using natural archives including ice cores and lake and marine sediments. Lakes in coastal ice-free regions sit at the boundary of the continent and the oceans, and provide an opportunity to fill a spatial gap between the ice core records constrained to the interior of the continent, and the more extensively studied lower latitudes. This thesis presents records of environmental change spanning 3000 years inferred from the sediments of two lakes, Lake Hamagiku and Lake Naga, in the Lützow-Holm Bay region of East Antarctica. These records of past environmental change are supported by an investigation into the modern relationship between diatom assemblages and their habitats and lake water chemistry. Specific conductivity was found to be the primary factor explaining variations in diatom assemblage, consistent with previous studies. Diatom assemblages were also observed to differ significantly between the lake littoral region and the lake floor deeper than two metres water depth. These modern observations are used in the interpretation of the fossil diatom records to reflect changes in ice cover as a result of regional temperature variations, where longer ice-free conditions result in a greater relative abundance of the taxa inhabiting the lake floor region. Fossil diatom assemblages revealed a coherent and sustained shift in the relative abundance of key taxa at ~1800 cal. yr BP in both lakes, which is interpreted to reflect regional warming, and an associated increase in the duration of ice-free conditions at these sites. Diatom valve concentration, organic carbon and nitrogen concentrations and carbon and nitrogen stable isotope ratios suggest that the climatic shift at this period was not associated with changes in lake productivity, which is attributed to the limitations imposed by the low nutrient conditions of these lakes. Periodicities in the variability archived in the sediments of both lakes are identified, in the fluctuations in key diatom taxa and to a lesser extent in the organic geochemistry records. This shared periodicity is observed with a wavelength of ~250 years from 2500 to 1000 cal. yr BP, after which a periodicity of closer to 128 years is observed across the records. These periodicities are consistent with those reported from a range of Southern Hemisphere palaeoclimate records and reconstructions, influenced by the Southern Hemisphere westerly airflow, and also solar activity in the Southern Hemisphere. The results presented in this thesis are a valuable addition to our knowledge of Southern Hemisphere climate throughout the late Holocene.