A Comparative Study of Cave System Calcium Isotope Ratios: Implications for Quantitative Reconstruction of Paleorainfall From Speleothems

Variations in speleothem calcium isotope ratios (δ⁴⁴Ca) are thought to be uniquely controlled by prior carbonate precipitation (PCP) above a drip site and, when calibrated with modern data, show promise as a semi-quantitative proxy for paleorainfall. However, few monitoring studies have focused on δ⁴⁴Ca in modern cave systems. We present a multi-year comparative study of δ⁴⁴Ca, carbon isotopes (δ¹³C), and trace elemental ratios from cave drip waters, modern calcite, and host rocks from two cave systems in California—White Moon Cave (WMC) and Lake Shasta Caverns (LSC). Drip water and calcite δ⁴⁴Ca from both caves indicate PCP-driven enrichment, and we used a simple Rayleigh fractionation model to quantify PCP variability over the monitoring period. Modern calcite trace element and δ⁴⁴Ca data positively correlate at WMC, but not at LSC, indicating a shared PCP control on these proxies at WMC but not at LSC. At both WMC and LSC, we observe an inverse relationship between PCP and rainfall amounts, though this relationship is variable across individual drip sites. Our modeled data suggest that WMC experiences ∼20% more PCP than LSC, consistent with the fact that WMC receives less annual rainfall. This work supports speleothem δ⁴⁴Ca as an independent constraint on PCP that can aid in the interpretation of other hydrologically sensitive proxies and provide quantitative estimates of paleorainfall. Additional, long-term monitoring studies from a variety of climate settings will be key for understanding δ⁴⁴Ca variability in cave systems more fully and better constraining the relationship between PCP and rainfall.