REFINING CLAY MINERAL PALEOTHERMOMETRY: TOWARDS DEVELOPMENT OF A PROXY SYSTEM MODEL FOR CLAY OXYGEN AND HYDROGEN ISOTOPES

Clay mineral isotope paleothermometry is an underutilized tool to understanding Earth's climate system and landscape evolution. Few geologic records capture both oxygen and hydrogen stable isotope ratios in the same phase, making clays a unique potential archive. Status quo methods found in the literature assume constant factors, such as temperature and water isotopic compositions, and typically ignore seasonality, soil water evaporation and depth dependent temperature changes. In this work we propose first-order modifications to status quo approaches to address these factors and test them in a modeling framework using published data from various settings. We develop this model using both forward and inverse modeling approaches. Our forward model using modern soil pore water datasets reveals that neglecting evaporation and temperature variability may lead to significant underestimations of clay formation temperatures, especially in Mediterranean settings. Testing of these results is currently limited by the lack of modern weathering profile datasets in the literature with clay oxygen and hydrogen isotope measurements. Our inverse model results indicate that high-latitude Eocene clay formation temperatures from several localities were ~8°C warmer than modern. Further, Eocene river gravel sediments in the Sierra Nevada show evaporation-influenced trends, suggesting, using our refined approach, that previous paleoelevation estimates were underestimated. Our new framework demonstrates that explicit consideration of soil pore water evaporation and temperature variability is necessary when interpreting clay mineral isotope data in the context of temperature, hydroclimate and elevation reconstructions. Development of a proxy-system model for interpreting clay oxygen and hydrogen isotope variations is a necessary step for interpreting clay mineral datasets used for paleoclimate and tectonic reconstructions, and may serve as a new tool in studying modern critical zone weathering processes. While the isotope effects modelled here have been discussed qualitatively or schematically/graphically in previous work, this work provides the first modeling framework to refine this methodology based on modern and paleo observations across a diversity of climatological locations and pedogenic settings where clays are formed in the critical zone.