In situ measurement of seasonal δ18O variations and analysis of isotopic trends in a modern speleothem from southwest Australia

Comparison of the speleothem stable isotopes and instrumental temperature records reveals that δ18O variations are too large to be driven by temperature alone, and are in the opposite sense. However, daily rainfall δ18O measurements show that the mean seasonal range in δ18O of rainfall in southwest Australia is large (2‰) and inversely correlated with rainfall amount. A rainfall driver for the speleothem δ18O is confirmed by the detection of seasonal shifts of 0.7-1.5‰ in speleothem δ18O that track rainfall δ18O, smoothed by storage in the overlying limestone. The seasonal range in speleothem δ18O is larger than any inter-annual and decadal variation observed in the record. The prominent annual cycles in speleothem δ18O revealed by ion microprobe analysis indicate that subtle changes in the frequency of intense winter rainfall events, or possibly also moisture sources, could produce significant changes in mean speleothem δ18O. The ion microprobe results also raise the possibility that the masses of speleothem calcite deposited in winter and summer could vary as a function of the seasonal drip rate and carbonate saturation state of these waters. If this is the case, then small changes in the relative masses of calcite deposited in winter and summer could produce significant shifts in mean δ18O and δ13C that have a complex relation to climate. This finding should be generally applicable to the interpretation of long-term trends in speleothem geochemical records for shallow cave sites where seasonal variations in geochemical tracers are relatively large, including most of the sub-tropical monsoon belts and mid to high latitudes with distinct rainfall seasons.