Calcium isotopes record changes in prior calcite precipitation (PCP) in coeval speleothems from Lake Shasta Caverns, CA during the Late Pleistocene.

Abstract

Traditional speleothem proxies, such as oxygen and carbon isotopes (δ18O, δ13C), and trace element to calcium ratios (i.e., Mg/Ca, Sr/Ca, P/Ca), can respond to multiple climatic and environmental processes, which often makes their signals challenging to interpret. Calcium isotope ratios (δ44/40Ca) are uniquely sensitive to a process called prior calcite precipitation (PCP). Two coeval δ44/40Ca speleothem records from Lake Shasta Caverns reveal dynamic changes in the amounts of PCP that occurred before speleothem precipitation, and these changes coincide with climatic shifts recorded during the Late Pleistocene. The δ44/40Ca records generated agree with traditional LSC speleothem proxy interpretations that indicate wetter-than-modern conditions during DO interstadials 2-7 and Heinrich Stadial 1 and drier-than-modern conditions during the Last Glacial Maximum. The similarity among the coeval δ44/40Ca records suggests that the same climatic and/or environmental processes influenced the preserved stalagmite signals, strengthening speleothem δ44/40Ca as a reliable hydroclimate proxy. The similarity between the LSC δ44/40Ca records with the LSC speleothem δ18O and δ13C records indicates that PCP influences these signals at LSC. Given that trace elemental ratios often serve as proxies for PCP, the lack of similarity between the δ44/40Ca records with the trace elemental ratio proxy records and monitoring data suggests that trace elements should be carefully evaluated as proxies for PCP in each cave system. The Rayleigh fractionation model allowed for the estimation of relative amounts of PCP preserved in the coeval speleothems. Many speleothem f values calculated were greater than 1, suggesting that δ44/40Ca may serve as a better proxy for aridity. Furthermore, calcium isotopes can be used to elucidate traditional speleothem proxies and show promise as a reliable hydroclimate speleothem proxy.