Evidence for Holocene changes in Australian-Indonesian monsoon rainfall from stalagmite trace element and stable isotope ratios

Trace element and stable isotope ratios from an active stalagmite (LR06-B1) recovered from Liang Luar Cave on the island of Flores (eastern Indonesia) are used to reconstruct the position of the austral summer inter-tropical convergence zone and Australian-Indonesian summer monsoon variability during the Holocene. Uranium-series dating of the stalagmite shows that it commenced growth 12,640 years ago, with hiatuses spanning 8,560 to 6,420 and 3,670 to 2,780 years ago. Stalagmite Mg/Ca and Sr/Ca ratios correlate significantly with one another, and with δ¹⁸O and δ¹³C, throughout the record. This suggests that the Mg/Ca and Sr/Ca ratios are dominated by prior calcite precipitation, a process whereby degassing in the vadose zone during periods of low recharge causes deposition of calcite and disproportionate loss of Ca²⁺ ions (relative to Mg²⁺ and Sr²⁺) 'upstream' of the stalagmite. The degree of initial ²³⁴U/²³⁸U disequilibrium also appears to have been controlled by recharge to the overlying aquifer. Together with the Mg/Ca, Sr/Ca, and δ¹⁸O values, the initial uranium isotope activity ratios ([²³⁴U/²³⁸U]_I) imply a generally drier early Holocene, coincident with a lower sea level and lower Southern Hemisphere summer insolation. Comparison of speleothem δ¹⁸O time-series from Flores and Borneo shows that they vary in unison for much of the Holocene. However, there is a significant decrease in the Borneo δ¹⁸O record ~6,000 to 4,000 years ago that does not occur in the Flores record. This anomaly may be related to a change in the Australian-Indonesian summer monsoon circulation in response to a protracted positive phase of the Indian Ocean Dipole. Under this scenario, stronger upwelling off of western Indonesia would, based on present-day effects, result in reduced summer convective activity over Flores and a subsequent northward shift of the intertropical convergence zone.