Comparison of high resolution sub-annual records of trace elements in a modern (1911–1992) speleothem with instrumental climate data from southwest Australia

Abstract

High spatial resolution measurements of Mg, P, U, Sr, Ba and Na using 193-nm excimer laser-ablation inductively coupled plasma mass spectrometry has revealed clearly resolvable annual cycles in a modern speleothem from southwestern Australia. The age of this stalagmite is established by the dates of emplacement and removal of the boardwalk where it grew (1911–1992). This chronological constraint allows for the first confident comparison between the instrumental climate record and speleothem trace element content. Eleven laser-ablation transects across a ∼0.5 mm wide section of the speleothem growth axis were measured. Cycles that could be matched between adjacent transects were stacked into a master record to minimise variability between tracks and permit greater confidence in comparing the trace elements to the instrumental climate record. P and U positively, and Mg inversely, mimic the sudden 20% decrease in annual rainfall experienced by this region since 1965. We argue that P from seasonal vegetation decay is transported to the speleothem before it is mineralised in the soil, owing to the low P retention capacity of southwest Australian soils. Both vegetation activity and the transport of vegetation-derived HPO 4 2− are sensitive to rainfall. Groundwater P concentration may also influence the transport of U through the strong affinity between phosphate and uranyl ions. Mg appears to be sensitive to groundwater residence time as this affects drip-water Mg/Ca composition by preferential loss of Ca during drier episodes when calcite precipitates before reaching the stalagmite. The effects of groundwater residence time may also be important for Sr on inter-annual scales. However, the behaviour of Sr on the annual cycle is opposite to Mg but compatible with Ba and Na, which are shown to depend on speleothem growth rate.