Trace elements: from sources to cave drip water, south-eastern Australia

Abstract

Speleothem trace element time series are constructed from the infiltrating drip water geochemistry, and hence are examined under contrasting rainfall conditions associated with El Niño and La Niña phases of ENSO. The aim was to identify suitable inorganic element proxies for palaeoclimate interpretation in speleothem records from Harrie Wood Cave, Yarrangobilly. The drip water chemical composition at the stalactite tip reflects a contribution from different endmembers and processes; therefore it is necessary to study the different sources, pathways and processes that occur as water migrates through the atmosphere-soil-karst system. Here we present high resolution aerosol, rainfall and drip water 18O and inorganic drip-water datasets. Analysis of aerosol samples above the caves reveals the atmosphere supplies a suite of elements from automobile emissions, windblown soil, smoke, secondary sulfate and aged sea salt sources. The bedrock and aerosols were identified as contributory sources of solutes to the drip water. The clay-rich soil zone was recognised as a sink for inorganic elements, and a secondary source for Zn. In soil, a number of processes were demonstrated to modify the chemical composition of the resultant drip waters. The drip water chemistry is driven by the long-term gradient in the cumulative water balance. The flow paths feeding the drip sites were shown to be fracture flow, from a ventilated well-mixed pocket within the epikarst storage reservoir. Dilution and reduced prior calcite precipitation (PCP) controlled the drip-water chemistry during the La Niña/wet years whereas enhanced PCP was observed during the El Niño and dry periods. Mg and Sr show particular promise as paleoclimate proxies for drought and flood events, while with further research Na, K and Zn may also be reliably used. These findings will be applied in a modern speleothem record. © Author(s