HYDROLOGIC INFLUENCE ON THE RADIOCARBON VARIABILTY IN AN INDONESIAN SPELEOTHEM DURING THE YOUNGER DRYAS

Constraining the temporal variations in the radioactively dead carbon [i.e. dead carbon fraction (DCF)] that is incorporated into a speleothem is critical to reliable ¹⁴C_spel chronologies, and has important implications to radiocarbon calibration. Whilst some recent studies have highlighted the potential utility of speleothems to extend/improve the radiocarbon calibration curve, others have shown some potential complications associated with such an undertaking, because in certain settings the DCF (which is derived from the soil and bedrock) is sensitive to cave recharge and hence did not remain constant through time. To further assess the potential hydrological control on speleothem radiocarbon variability, we constructed a new high-resolution DCF record from an Indonesian speleothem that encompasses a previously documented pluvial event in Flores, Indonesia—namely the Younger Dryas (YD). Twenty ¹⁴C analyses, anchored to a U-Th age model constructed from ~60 U-Th dates, were conducted on pieces of calcite extracted at ~5-mm increments along a ~100-mm section of stalagmite LR06-B1. To better characterize the paleoclimate, high-resolution stable-isotope (δ¹⁸O, δ¹³C) and trace-element (Mg/Ca, Sr/Ca) measurements were also conducted along the same section of stalagmite.

Broad comparison of the DCF record with the hydrologically-controlled proxy data suggests that the sudden increase in rainfall at the onset of the YD was matched by an increase in the DCF, which remained at this level until the abrupt termination of the YD ~1,000 years later. In line with a previous interpretation of DCF variability for the same specimen, but during the late Holocene, we interpret the DCF during the YD 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 during the YD 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.