CARBONATE CLUMPED ISOTOPES AND IN SITU TEMPERATURE MONITORING FOR HOLOCENE SOILS IN THE SAN LUIS VALLEY, USA INDICATE SPRINGTIME CARBONATE FORMATION

Pedogenic carbonate horizons are abundant in semi-arid and arid regions worldwide and within the geologic record. They present a widely distributed archive of past environmental conditions, driven by global climate or tectonically-controlled elevation changes. Oxygen and carbon isotopes in calcite-rich nodules and clast rinds are widely-applied indicators of past soil water and CO₂ composition linked to changing precipitation and plant communities. The temperature of carbonate formation, however, provides key constraint on past water and CO₂ isotope values and can elucidate why they may have changed in the past. Clumped isotope thermometry can provide temperature constraints and additional climate information, if the carbonate forming system is well understood. We present preliminary clumped isotope (∆₄₇) temperatures for Holocene soil carbonates dated by ¹⁴C and U-Th disequilibrium, and compare those results to two years of in situ soil temperature data to better understand the mechanism and seasonality of carbonate formation in the San Luis Valley region of the southern Rocky Mountains. Five temperature-monitoring sites ranging in elevation (1940-2450 m) and latitude (36.2-37.9°N) were installed in a variety of settings (range front, valley center, and canyon). All sites have similar temperature variations at depths >60 cm in terms of both seasonal amplitude (~18°C) and absolute value on the same day (within <4°C). This suggests ∆₄₇ temperatures determined from soil carbonate that formed under similar conditions should be comparable at sites across the region. Temperatures based on ∆₄₇ measurements of Holocene (>1.8 to 11.0 ka BP) carbonate from multiple sites yield consistent temperatures of 10±4°C, which is similar to modern springtime soil temperatures at >60 cm depth. This season of formation matches previous results of isotopic modeling at sites further south along the Rio Grande corridor. Temperatures measured between March and May show multiple, abrupt warming and cooling cycles on weekly timescales caused by wetting and drying of the soil during spring precipitation events. This may drive carbonate precipitation under low pCO₂ conditions before increased plant respiration increases soil pCO₂ later in the season.