A comparative study of cave system calcium isotope ratios: Implications
for quantitative reconstruction of paleorainfall from speleothems
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 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. 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.