USING PALEOSOL GEOCHEMISTRY TO RECONSTRUCT REGIONAL CLIMATE PATTERNS IN LATE CRETACEOUS MONTANA

Understanding past greenhouse climates is critical for refining climate model predictions of future warming, particularly as anthropogenic emissions drive unprecedented change. Earth’s ancient greenhouse periods, such as the Late Cretaceous, offer analogs for how the climate system responds to elevated CO₂ over geologic timescales. In north-central Montana, the Late Cretaceous (~76 Ma) Two Medicine and Judith River formations provide a natural laboratory for developing methods to characterize greenhouse paleoclimate conditions over a large region. The Two Medicine Formation has been interpreted as an inland, seasonal, semi-arid environment, whereas the lower elevation Judith River Formation is interpreted as a wetter coastal plain setting. The primary objectives of this study are to (1) develop new quantitative paleoclimate reconstructions for the Judith River Formation and (2) compare these new estimates to reconstructions from the coeval Two Medicine Formation. Previous research on the Two Medicine Formation indicates mean monthly temperatures exceeding 30°C and a Mean Annual Range in Temperatures (MART) similar to modern seasonal variability. Preliminary fieldwork in the Judith River suggests more spatial variability in paleoclimate conditions than previously identified. This study will employ a novel geochemical method for deriving paleotemperatures and paleoprecipitation. This method will complement the temperature and precipitation estimates derived from the Paleosol Weathering Index (PWI) and the Chemical Index of Alteration minus Potassium (CIA-K). This new data derived from samples obtained in the Judith River Formation will be integrated with the existing PWI and CIA-K data from the Two Medicine to reconstruct regional climate patterns in Late Cretaceous Montana. The results will yield a more accurate characterization of the Late Cretaceous greenhouse climate and enhance our understanding of Earth’s response to elevated CO₂ levels.