A TWO-STEP APPROACH TO QUANTITATIVE TEMPERATURE ESTIMATES IN THE SIERRA NEVADA

Stalagmite records of stable isotope (δ¹⁸O_cc) variability from across the Southwestern United States change in-step with Northern Hemisphere temperature over the last deglaciation. The interpretations of δ¹⁸O_cc records, however, is complicated due to the multiple influences on δ¹⁸O_cc. These influences include seasonality of precipitation, regional temperature change, and variability in precipitation source, among others. To isolate the influence of temperature in δ¹⁸O_cc records, we must develop quantitative and independent records of temperature. Here, we use a multi-proxy approach in a previously studied stalagmite, ML-1, from western Sierra Nevada. We employ high-resolution 3-dimensional neutron computed tomography and x-ray computed tomography to map fluid inclusion distribution in the stalagmite. Water-rich regions in ML-1 occur contemporaneously with the lowest values of δ¹⁸O_cc. We also use ML-1 fluid inclusions to determine a noble gas temperature for the Last Glacial Maximum (18.8 ± 0.3 ka), indicating a 5.2 ± 1.7 (2σ) °C cooling relative to today. Noble gas concentrations were measured using a progressive step-crushing technique originally developed for understanding noble gas abundances in basalt. The progressive step-crushing technique and a robust water calibration reduces uncertainty in the resulting NGT relative to previous studies that utilize pre-crushing techniques. The temperature from the Last Glacial Maximum is used to correct for temperature variability in δ¹⁸O_cc. Variability in δ¹⁸O_cc is ~2.0 ‰ on the centennial scale originally interpreted to reflect temperature and precipitation source shifts. The combined results from these two approaches illustrate that, in the western Sierra Nevada, temperature variability only accounts for a fraction of total δ¹⁸O_cc changes on the centennial scale and points to variability in precipitation source region as a major influence.