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

We present a record of seasonal and inter-annual oxygen (delta O-18) and carbon (delta C-13) 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 degrees C temperature rise since similar to 1953. Seasonal variations in calcite delta O-18 were measured in situ by high spatial resolution ion microprobe, whilst inter-annual variations of delta O-18 and delta C-13 were measured by conventional gas-source mass spectrometry. Comparison of the speleothem stable isotopes and instrumental temperature records reveals that delta O-18 variations are too large to be driven by temperature alone, and are in the opposite sense. However, daily rainfall delta O-18 measurements show that the mean seasonal range in delta O-18 of rainfall in southwest Australia is large (2 parts per thousand) and inversely correlated with rainfall amount. A rainfall driver for the speleothem delta O-18 is confirmed by the detection of seasonal shifts of 0.7-1.5 parts per thousand in speleothem delta O-18 that track rainfall delta O-18, smoothed by storage in the overlying limestone. The seasonal range in speleothem delta O-18 is larger than any interannual and decadal variation observed in the record. The prominent annual cycles in speleothem delta O-18 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 delta O-18. 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 delta O-18 and delta C-13 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. (c) 2005 Elsevier B.V. All rights reserved.