Application of the speleothem calcium isotope paleo-rainfall proxy to the 8.2 kyr event in coastal California

Abstract

I apply the speleothem δ44/40Ca rainfall proxy to a fast growing stalagmite from White Moon Cave (WMC) on the California coast that grew during the 8.2 kyr event. Variability in speleothem δ44/40Ca is controlled by rainfall-driven variations in the amount of prior calcite precipitation that occurs along the groundwater flow path above a speleothem. Recent work indicates that speleothem δ44/40Ca data can be combined with a simple one-box Rayleigh fractionation model to generate quantitative records of rainfall if it is calibrated with cave drip water and modern calcite δ44/40Ca data, as well as a modern rainfall tie point. Here we aim to further develop this promising new proxy by applying it in a novel geologic and climatic setting and to use it to investigate precipitation dynamics over the 8.2 kyr event. High frequency variation in the WMC δ44/40Ca record from ~8226 and 8041 cal. years BP supports previous interpretations of WMC δ13C and trace element (Mg/Ca, P/Ca) data that suggest the 8.2 kyr event was characterized by higher variability in rainfall patterns punctuated by increases in infiltration above the cave. The WMC δ44/40Ca record does not display the sustained positive δ44/40Ca excursion observable in a contemporaneous speleothem record from Heshang Cave, China. This difference may reflect differences in geology, hydrology, and mean climate between the two sites and/or variable climate response to the 8.2 kyr event on either side of the Pacific Ocean. The resolvable decadal-scale variations in the WMC δ44/40Ca record suggest that this new proxy can yield useful climate information when applied in arid environments and to caves developed in fracture-dominated marble bedrock. However, using the current WMC dataset we are unable to use the Rayleigh fractionation model to generate a quantitative rainfall record that accurately reflects the evolution of groundwater δ44/40Ca during PCP. This may be due to the considerable variability in WMC drip water and host rock δ44/40Ca data or spatial variability in Ca isotope cycling within the cave system. I will further test the latter possibility by adding δ44/40Ca from two new drip sites that are deeper in the cave and closer to where the speleothem was sampled.