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Abstract
Speleothem trace-element records are a tool for understanding the long-term natural recurrence of persistent drought and flood events, which in south-eastern Australia are strongly influenced by El Niño-Southern Oscillation (ENSO). Speleothem trace-element records are sensitive to site-specific sources and processes; hence a full understanding of these is required as a prerequisite for examining speleothems. This thesis examined geochemistry and bi-monthly drip water hydrochemistry in Harrie Wood Cave, Yarrangobilly over the last 10-years, which captured contrasting hydroclimate conditions of ENSO phases. This research presents the first high-resolution rainfall and drip water δ18O, aerosol, soil, bedrock and inorganic drip-water geochemical datasets from three drip sites, to develop a comprehensive understanding of sources and processes that influence solute variability, to identify suitable palaeoclimate proxies for interpreting speleothem trace-element records. This research identified that the main contributors of aerosols to this montane study site were: automobile, secondary sulfate, smoke, soil and aged sea salt. This research also established that long-distance transport of bushfire smoke emissions and aeolian Na and soil from Australia’s desert interior to the study site increased during below-average rainfall and El Niño conditions. Using hydrochemical mass balance modelling, the parent bedrock was identified as the main source of solutes to the drip water, followed by aerosols. The clay-rich soil zone was recognised as a sink for elements, and a secondary source for Zn. In soil, cation exchange, K-fixation and colloidal mobilisation were demonstrated to modify the chemical composition of the resultant drip waters. The hydrochemistry of the drip waters were shown to be driven by variation in the site cumulative water balance due to ENSO. The flow paths feeding the drip sites were also demonstrated to be fracture flow, from a ventilated well-mixed pocket within an unsaturated zone water store. Distinctive processes, and therefore drip water hydrochemistry, occurred during ENSO events. Enhanced prior calcite precipitation increased water Mg/Ca and Sr/Ca ratios during the El Niño and Mg and Sr were proposed as palaeoclimate proxies. Na, K and Zn were also identified as potential palaeoclimate proxies and with validation in speleothems may also be reliably used.