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

Abstract

We present a record of seasonal and inter-annual oxygen ( δ 18O) and carbon ( δ 13C) isotope ratios from an 81-year-old stalagmite from Moondyne Cave, southwest Australia. The growth history of stalagmite MND-S1 is known since it grew on a cave boardwalk that was installed in 1911 and removed in 1992. This stalagmite provides an excellent test of speleothem climate proxies because the regional climate is strongly seasonal (wet winter/dry summer) and has experienced a 200 mm (20%) reduction of mean rainfall since the mid-1960s, and a 0.8 °C temperature rise since ∼ 1953. Seasonal variations in calcite δ 18O were measured in situ by high spatial resolution ion microprobe, whilst inter-annual variations of δ 18O and δ 13C were measured by conventional gas-source mass spectrometry. Comparison of the speleothem stable isotopes and instrumental temperature records reveals that δ 18O variations are too large to be driven by temperature alone, and are in the opposite sense. However, daily rainfall δ 18O measurements show that the mean seasonal range in δ 18O of rainfall in southwest Australia is large (2‰) and inversely correlated with rainfall amount. A rainfall driver for the speleothem δ 18O is confirmed by the detection of seasonal shifts of 0.7–1.5‰ in speleothem δ 18O that track rainfall δ 18O, smoothed by storage in the overlying limestone. The seasonal range in speleothem δ 18O is larger than any inter-annual and decadal variation observed in the record. The prominent annual cycles in speleothem δ 18O 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 δ 18O. 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 δ 18O and δ 13C 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.