Calcium isotope ratios (δ44Ca) in coeval California stalagmites record hydroclimate shifts and reveal soil-to-cave carbon transformations during the last glacial period

Calcium isotopes (δ⁴⁴Ca) in speleothems are thought to solely record changes in prior carbonate precipitation (PCP) along the seepage water flowpath. This unique sensitivity makes  d⁴⁴Ca a useful tool for both reconstructing past hydroclimate and exploring the influence of PCP on other proxies where it can be one of several influences. Here we present δ⁴⁴Ca records for two partially coeval stalagmites from Lake Shasta Caverns (LSC) in northern California that grew between 37,000 and 14,000 years BP. Both δ⁴⁴Ca records display similar mean values and temporal variations, and significant positive correlations with δ¹³C (r = 0.74, 0.73) and δ¹⁸O (r =0.49, 0.77), suggesting PCP also influences these traditional stable isotope proxies. However, neither stalagmite displays significant correlations between d⁴⁴Ca and trace element proxies (Mg/Ca, Sr/Ca, Ba/Ca) indicating these do not solely reflect PCP at this site.

LSC sits on the boundary between two hydroclimate regimes in the northwestern and southwestern United States (US). Stalagmite δ⁴⁴Ca and δ¹³C suggest wetter conditions during warm Dansgaard-Oeschger interstadials, similar to paleoclimate archives from the Pacific northwest. However, LSC proxies also indicate wet conditions during colder Heinrich Stadials, similar to archives from the US southwest. Values for the fraction of Ca remaining in solution after PCP (f) calculated using a Rayleigh fractionation model for δ⁴⁴Ca calibrated with modern monitoring data indicate that 0 to ~60% of dissolved Ca is lost to PCP. We compare stalagmite f values with modern PCP rates and measured rainfall to generate quantitative estimates of past rainfall. However, unreasonable f values during the wettest intervals indicate that the calcite-water calcium isotopic fractionation factor may have varied in the past, particularly during intervals of faster stalagmite growth. Using calculated f values, we estimate the δ¹³C of dissolved inorganic carbon prior to PCP which agrees with modern dripwater values. Notably, these δ¹³C estimates are higher during wetter warm interstadials and cold Heinrich Stadials, when PCP is lowest. This suggests that during wet intervals, seepage water has little time to equilibrate with soil CO2 leading to lower carbonate saturation and less PCP, likely a result of sparse soils and steep terrane above LSC.