MONITORING A CAVE ENVIRONMENT IN THE SIERRA NEVADA — IMPLICATIONS FOR INTERPRETING STALAGMITE RECORDS FROM A SEMI-ARID CLIMATE

The southwestern United States, including California, is one of the world’s most climatically sensitive regions given its low (≤250 mm/year) seasonal precipitation and its inherently variable hydroclimate, subject to large magnitude modulation. To reconstruct California climate, cave calcite deposits (stalagmites) have been utilized as an archive of environmentally sensitive proxies, such as stable isotopes (d¹⁸O, d²H, d¹³C), trace elements (Mg, Ba, Sr), and Sr (⁸⁷Sr/⁸⁶Sr) and U (δ²³⁴U) isotopes. The interpretation of the climatic signal from stalagmite geochemistry requires monitoring of the cave and surface environment as well as documenting the chemical evolution of cave drip water and calcite under changing seasonal and longer-term conditions. This research shows the integrated results of four undergraduate student research projects undertaken as part of the broader impact component of a NSF-funded (Paleoclimate Perspectives and Climate Change Program) project to study several cave systems distributed along the western foothills of the Sierra Nevada. Two caves, California Caverns (CC; elevation) and Lilburn Cave (LL; elevation) have been monitored since May 2018. In both caves, pCO₂remains near atmospheric pCO₂throughout the year and relative humidity remains at 100%. The cave-air temperature in both caves varies diurnally by <0.2°C but represents average surface air temperatures at each location. Annual drip-water geochemical variability in both caves reveals the high potential for stalagmites from these caves to be sensitive to climatic fluctuations. Trace elements Mg and Ba of the dripwater respond to seasonal variability in precipitation amount indicating their sensitivity as precipitation proxies. Dripwater ⁸⁷Sr/⁸⁶Sr is evaluated to better understand the trace element response to precipitation amount. The stable isotopes (δ¹⁸O and δ²H) of dripwater vary with seasonal variability in precipitation that we hypothesize is related to precipitation type (snow vs. rainfall). Importantly, the difference between d¹⁸O and d²H values of the two sites — lower elevation CC and the higher elevation LL— captures the temperature-driven environmental lapse rate in the Sierra Nevada (~3.8°C/km; Wolfe, 1992). Monitoring in Sierra Nevada caves indicate that regional stalagmites have high potential to archive long-term variability in seasonality. Future stalagmite-based climate reconstructions from the study region will benefit from the monitoring framework presented here.

Wolfe, J.A. 1992. US Geol. Surv. Bull.