Rainfall variability and temporal changes in the dead carbon fraction in an Indonesian speleothem

The number of speleothem-based paleoclimate records has increased significantly in recent years. To assess the potential hydrological control on speleothem radiocarbon variability, we constructed a high-resolution dead carbon fraction (DCF) record from a speleothem from Flores, Indonesia for two different periods, the Younger Dryas (YD) chronozone and the Last Millennium. A total of thirty-four 14C analyses were conducted on calcite extracted from U-Th dated stalagmite LR06-B1. To better characterise the paleoclimate and environmental changes, highresolution stable-isotope (δ18O, δ13C) and trace-element (Mg/Ca, Sr/Ca) measurements were also conducted along the same stalagmite sections. Broad comparison of the DCF record with the hydrologically-controlled proxy data suggests that rainfall increases were matched by DCF increases. In line with a previous interpretation of DCF variability for the same specimen, but during the interval 2.4-2.8 ka and the post-bomb period, we interpret the DCF during the YD and the Last Millennium to have been primarily controlled by limestone dissolution associated with changes in open- versus closed-system conditions, rather than other potential factors such as kinetic fractionation and/or variations in the age-spectrum of soil organic matter above the cave. It then follows that more abundant monsoon rainfall in Flores resulted in the soil-karst system being in a more closed state, which inhibited carbon isotope exchange between the karst-water dissolved inorganic carbon and soil-gas CO2, and ultimately led to a greater contribution of dead-carbon from the bedrock. Our results indicate that DCF in tropical speleothems can be used as a proxy of past rainfall and consequently monsoon variability