Clumped Isotopes Record a Glacial-Interglacial Shift in Seasonality of Soil Carbonate Accumulation in the San Luis Valley, Southern Rocky Mountains, USA

Clumped isotope paleothermometry using pedogenic carbonates is a powerful tool for investigating past climate changes. However, location-specific seasonal patterns of precipitation and soil moisture cause systematic biases in the temperatures they record, hampering comparison of data across large areas or differing climate states. To account for biases, more systematic studies of carbonate forming processes are needed. We measured modern soil temperatures within the San Luis Valley of the Rocky Mountains and compared them to paleotemperatures determined using clumped isotopes. For Holocene-age samples, clumped isotope results indicate carbonate accumulated at a range of temperatures with site averages similar to the annual mean. Paleotemperatures for late Pleistocene-age samples (ranging 19-72 ka in age) yielded site averages only 2 degrees C lower, despite evidence that annual temperatures during glacial periods were 5-9 degrees C colder than modern. We use a 1D numerical model of soil physics to support the idea that differences in hydrologic conditions in interglacial versus glacial periods promote differences in the seasonal distribution of soil carbonate accumulation. Model simulations of modern (Holocene) conditions suggest that soil drying under low soil pCO2 favors year-round carbonate accumulation in this region but peaking during post-monsoon soil drying. During a "glacial" simulation with lowered temperatures and added snowpack, more carbonate accumulation shifted to the summer season. These experiments show that changing hydrologic regimes could change the seasonality of carbonate accumulation, which in this study blunts the use of clumped isotopes to quantify glacial-interglacial temperature changes. This highlights the importance of understanding seasonal biases of climate proxies for accurate paleoenvironmental reconstruction. Reconstructing the amount of temperature change associated with past climate changes for individual regions is important for understanding their climate vulnerability. Carbonate minerals developed naturally in desert soils record past temperatures in the numbers of their rare isotopes, called clumped isotopes. However, the temperature recorded in soil minerals is linked to the time of year they form, which varies greatly from winter to summer, so understanding the timing is key to interpreting past climate. We measured underground temperatures in the southern Rocky Mountains, compared them to mineral temperatures from young soils, and found that they record mean annual soil temperature. In contrast, temperatures recorded by soil minerals during the last ice age were only 2 degrees C colder than young soil temperatures, despite evidence that ice age air temperatures were 5-9 degrees C colder. We performed numerical modeling to predict the seasonal timing of soil carbonate accumulation under interglacial and glacial climate states and found that carbonate likely forms year-round during interglacial states but forms during the summertime under glacial conditions due to delayed melting of snow under colder temperatures. This lowers the difference between glacial and interglacial temperatures, which is important to account for when quantifying past climate change for the region. Clumped isotope temperatures for soil carbonate are biased to different seasons in different regions and time periods depending on climate In the San Luis Valley, USA, monitoring, modeling, and isotope results suggest carbonate accumulation throughout the year in the Holocene In the glacial late Pleistocene, clumped isotopes and soil modeling indicate longer snow cover shifted carbonate accumulation to the summer