Abstract

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.

Document Type

Article

Publication Date

10-2024

Notes/Citation Information

© 2024 The Author(s). Geochemistry, Geophysics, Geosystems published by Wiley Periodicals LLC on behalf of American Geophysical Union. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Digital Object Identifier (DOI)

https://doi.org/10.1029/2024GC011691

Funding Information 

Funding was provided by the National Science Foundation (AGS‐1554998 and AGS‐2202883) and the National Geographic Society (NGS‐39815) (to Jessica L. Oster), the Karst Waters Institute, the Cave Research Foundation, the Geological Society of America, and the National Speleological Society (to Cameron B. de Wet), and the National Science Foundation (EAR‐2202851) (to Elizabeth M. Griffith). We thank Mike Davis of the Western Cave Conservancy, Susan Petrie of the Peninsula Open Space Trust, and Dave Mundt and the Lake Shasta Caverns staff for their support in the field. We also thank reviewers Heather Stoll and Christopher Day for their helpful comments and suggestions.

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