Application of the Speleothem Calcium Isotope Paleo-Rainfall Proxy to the 8.2 Ka Event in Coastal California

Variations in speleothem oxygen and carbon isotopes (δ¹⁸O and δ¹³C) suggest that the 8.2 ka event was characterized by more frequent or intense winter storms on the central California coast. However, the complex controls on these isotope systems preclude more quantitative assessments of how rainfall changed during this event. Radiogenic and non-traditional stable isotope systems can provide information about water-rock interactions and epikarst processes that compliment and improve the interpretations of traditional stable isotope records. For example, δ^44/40Ca is a hydrologically sensitive proxy that has not previously been applied to paleoclimate records from western North America.

We present new δ^44/40Ca data from a White Moon Cave (WMC) speleothem that grew from 8600 to 6900 cal yr BP. We use a one-box Rayleigh fractionation model developed for Heshang Cave, China and calibrated using modern δ^44/40Ca data for WMC to investigate Ca isotope cycling within the cave system and determine a semi-quantitative precipitation record over the 8.2 ka event. The WMC δ^44/40Ca record shows decadal-scale fluctuations similar in magnitude to the δ^44/40Ca record from Heshang Cave despite the more arid climate and potentially short residence time of water in the fractured marble bedrock at WMC. As with δ¹³C, stalagmite δ^44/40Ca shows higher frequency fluctuations over the 8.2 ka event relative to the rest of the record, suggesting highly variable rainfall above the cave during this interval. This finding is consistent with the interpretation from WMC stable isotope and trace element records that the 8.2 ka event resulted in more frequent or intense winter storms in coastal CA. However, these data differ from the δ^44/40Ca record at Heshang Cave, which indicates a sustained shift and prolonged aridity in central China during the 8.2 ka event. The differences between these two records may reflect a combination of variable climatic response to the 8.2 ka event forcing in these regions and differences in geology and mean climate that influence fluid flow through the epikarst. However, the resolvable, decadal-scale variations in the WMC δ^44/40Ca record over the 8.2 ka event suggest that this proxy can yield useful climatic information when applied in arid environments and to caves developed in fracture dominated marble bedrock.