A COMPARATIVE STUDY OF CAVE SYSTEM CALCIUM ISOTOPE RATIOS: IMPLICATIONS FOR QUANTITATIVE RECONSTRUCTIONS OF PALEORAINFALL (Invited Presentation)

The development of quantitative records of past rainfall is an important goal in the field of speleothem paleoclimatology. However, most traditionally-employed speleothem proxies respond to several complex climatic and environmental influences and are typically limited to qualitative interpretations of paleoclimate change. 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 quantitative proxy for paleorainfall. Yet 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 water, modern calcite, and host rock samples from two cave systems in California - White Moon Cave (WMC) and Lake Shasta Caverns (LSC). Modern calcite δ⁴⁴Ca tends to correlate with trace element ratios at WMC, but not at LSC, suggesting that PCP is not always a straightforward and shared control for these proxies. Drip water and calcite δ⁴⁴Ca from both caves indicate PCP-driven enrichment, and we use a simple Rayleigh fractionation model to quantify PCP variability over the monitoring period. Modern calcite δ⁴⁴Ca from deeper sites at WMC display a larger PCP signal than shallower sites, indicating that longer flow paths allow for more PCP under the same hydroclimate conditions. 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. We compare the monitoring data to δ⁴⁴Ca data from a WMC stalagmite that grew from 6,900 to 8,600 yr. cal. BP (WMC1) and find that coastal California likely experienced greater wet and dry extremes during the early and middle Holocene than during recent decades.

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, but also points to the importance of additional, long-term monitoring studies for understanding δ⁴⁴Ca variability in cave systems more fully and better constraining the relationship between PCP and rainfall.