School of Earth Sciences - Theses
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ItemNovel proxies study of long-term climate variability and karst evolution using speleothems from southern Australia( 2021)This research program aimed to produce robust and reliable speleothem chronologies for the Naracoorte Caves in South Australia which could then be used in studies of palaeoclimate and karst evolution. The thesis is structured around three sub-themes and the main findings of each are outlined below. The first component evaluates the utility of speleothem age frequency distributions for palaeoclimate assessment. Age distributions of this type can be used as proxies for past climate change, where the peaks in age density are linked to a controlling climatic parameter (e.g., temperature or precipitation). There remain, however, considerable gaps in our understanding of how best to use speleothem age frequency distributions in this way. To address these issues, a synthetic age dataset was modelled by randomly generating U-Th ages based on a known climatic forcing, followed by sampling from this dataset under varying conditions. The model shows that periodic Quaternary climate fluctuations can be recovered from age frequency distributions with a minimum sample size of 120–150 radiometric ages. This study provides a much-needed statistical framework for the use of age frequency distributions relevant to speleothem palaeoclimate studies — and one which is also beneficial for the radiocarbon and zircon dating communities. The second theme concerns the antiquity of the Naracoorte Cave Complex (NCC) in southern Australia. Caves are unique archives of past environmental and climatic conditions and may also act as important fossil repositories, as is the case for the World Heritage listed NCC. In these circumstances, understanding the timing of initial cave development and opening can shed light on the potential antiquity of the fossil deposits (and thus guide excavation), but these geomorphological processes remain challenging to constrain. This study places robust temporal constraints on the onset of cave and entrance development of the NCC by utilising an extensive campaign of U-Th and U-Pb dating of speleothems. Additionally, speleothem charcoal and pollen concentrations were used as novel indicators of cave openness. The key finding is that caves can be twice as old as their surface expression. These techniques provide important new tools for a range of disciplines interested in the timing and extent of cave opening, e.g., palaeontology, palaeoanthropology and archaeology. The final theme concerns the palaeoclimatic history of semi-arid southern Australia. Changes in the hydroclimate during the Quaternary remain poorly constrained for the southern Australian semi-arid subtropics. In this study, changes in southern Australia’s hydroclimate were reconstructed for the Late Quaternary using an age frequency distribution and pollen-based climate reconstruction from U-Th dated speleothems. This study represents the largest geochronological dataset from a single cave province in the Southern Hemisphere. Collectively, these reconstructions demonstrate orbitally-paced speleothem growth within the 100-ka period linked to changes in moisture availability that consistently lag maximum interglacial temperatures by ~25 ka over the last three glacial-interglacial cycles. The results show that times of highest moisture availability occurred during parts of the glacials, rather than interglacials periods, which implies that, in the semi-arid subtropics, temperature and moisture availability were strongly decoupled.