An Investigation into the Potential of Pliocene Speleothems from the Nullarbor Plain, Southwest Australia, to Preserve Interpretable Palaeoclimate Signals
AuthorSellman, Safana Louise
AffiliationSchool of Earth Sciences
Document TypePhD thesis
Access StatusOpen Access
© 2019 Safana Louise Sellman
The Pliocene has been identified as a key time interval from which estimates of future climate scenarios can be made. At present there is a significant paucity of Pliocene climate data from the Southern Hemisphere, and moreover, from terrestrial sources therein. This thesis aims to address both of these scarcities through applications of both traditional and novel techniques of palaeoclimate analysis to speleothems from the Nullarbor Plain, southwest Australia. This thesis focuses on two primary stalagmites, as well as several additional speleothems, that grew during the Pliocene as revealed by U-Pb dating. It provides one of the first detailed studies of speleothems of a greater antiquity than the 500 kyr dating limit previously imposed by the U-Th method. This study has provided invaluable insights into the climate of the Nullarbor Plain during the Pliocene, through the application of conventional stable isotope analyses, trace element analyses, fluid-inclusion analyses, clumped-isotope analyses, and modern precipitation isotope analyses. For speleothems of such antiquity, a multi-proxy approach grounded in a site-specific modern isotope study, has provided the means for delineating the key drivers of the geochemical variations present. The use of both traditional and novel speleothem proxies provides key information regarding both temperature and precipitation dynamics in the Nullarbor region during the Pliocene. Conventional stable isotope and trace element records from the speleothems reflect changes in precipitation above the cave and the overlying vegetation dynamics. Modern precipitation analyses enable a comprehensive understanding of the influences on isotopic variation in current precipitation, with applications to the interpretation of the speleothem data. This study indicates that during the Pliocene, precipitation was significantly higher in the Nullarbor region, suggestive of an increase in moisture sourced from NCBs and the Southern Ocean. The increased precipitation resulted in significantly higher vegetation cover compared to present, as supported by both the carbon isotope and trace element signals. These conclusions are in agreement with both modelled estimates of vegetation and precipitation for the Nullarbor Plain, and also with studies of speleothem-derived pollen data from the region. Palaeotemperature estimates were derived from combined fluid-inclusion and clumped-isotope analyses; indicating a temperature range of 18.8 'C +/-1.8 'C to 21.4 'C +/-1.3 'C. While the individual methods provided several additional estimates, the uncertainties associated with each limit the reliability of the absolute temperatures. However, they signify significant temperature variations within the Pliocene, indicating that despite being a period of overall global warmth in comparison to the present day, the Nullarbor region experienced significant fluctuations in average temperatures. This study has identified several areas for focus in future research in order to further develop the novel application of speleothem research to the more distant past, while providing unique and important information regarding the climate dynamics of the Nullarbor Plain during the Pliocene.
KeywordsPalaeoclimate; Speleothems; Pliocene; Palaeotemperature; Stable isotopes; Trace elements; U-Pb dating; Fluid-inclusions; Clumped-isotopes; Modern precipitation
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