A COMPARATIVE STUDY OF DRIP WATER AND MODERN CALCITE CA ISOTOPE RATIOS WITH RAINFALL, C ISOTOPE RATIOS, AND TRACE ELEMENT DATA FROM THREE US CAVE SYSTEMS

The development of quantitative records of past rainfall is an outstanding goal in the field of speleothem paleoclimatology and represents an essential step for benchmarking paleoclimate model simulations. However, most traditionally employed speleothem proxies, including δ¹⁸O, δ¹³C, and trace element to calcium ratios, respond to a number of complex climatic and environmental influences and typically provide only qualitative records of paleoclimate change. Variations in speleothem Ca ratios (δ⁴⁴Ca) are uniquely controlled by carbonate precipitation above a drip site (prior carbonate precipitation, or PCP), which can be modeled as a Rayleigh fractionation process and calibrated with modern rainfall data. Thus, speleothem δ⁴⁴Ca shows promise as a (semi) quantitative proxy for past changes in local effective rainfall rates. However, few cave monitoring studies have focused specifically on Ca isotope variability in cave systems and the ways that important factors, like host rock δ⁴⁴Ca variability and geology, epikarst thickness, ventilation, and seasonal rainfall distribution affect δ⁴⁴Ca signals in speleothems are not well understood.

We present a comparative study of δ⁴⁴Ca data and coeval measurements of δ¹³C and trace element ratios, established proxies for water infiltration, from cave drip waters, farmed calcite, and host rocks from three different cave systems - White Moon Cave (WMC) in coastal California, Lake Shasta Caverns (LSC) in northern California, and Blue Springs Cave (BSC) in east-central Tennessee. These cave systems are characterized by different hydroclimate, geology, flow path geometry, and seasonal infiltration characteristics. To assess the relationship between Ca isotope evolution and effective rainfall we also compare cave system δ⁴⁴Ca data with local rainfall rates and supplement these comparisons with drip rate information when possible.

The comparison of WMC, LSC, and BSC δ⁴⁴Ca, δ¹³C, and trace element data from sites with different flow path geometry and from caves in different geologic and climate settings allows independent assessment of these key factors. This work, and the direct comparison between δ⁴⁴Ca measurements and measured local rainfall rates in particular, aids in the refinement of speleothem δ⁴⁴Ca as a new, (semi) quantitative proxy for paleorainfall.