HOT SUMMERS ON LAND IN THE EARLY EOCENE SUBTROPICS

Terrestrial proxy records from the early Eocene (~55 to 50 Ma) can offer insights into how the earth system responds to extreme climate conditions. Here, we develop a terrestrial climate record from early Eocene paleosols in Big Bend National Park, Texas (~30 °N). We apply multiple proxies in this stack of fluvial overbank paleosols: the paleosol weathering index (PWI), which is a ratio of Na, Mg, K, and Ca within each B horizon, is used a semi-quantitative proxy for mean annual temperature, and carbonate clumped thermometry is used as a quantitative proxy for summer temperature. The PWI value measured in the Big Bend paleosols is beyond the calibration range established with modern samples, which suggests mean annual temperatures higher than 26 °C, with an unknown amount of error. Clumped isotope analysis of micritic carbonate from pedogenic nodules suggests that summer temperatures were approximately 36 °C. These high summer temperatures are likely higher than would be comfortable for plant growth, and may be difficult to reconcile with our observation of metasequoia trunks in the section. However, our summer temperature estimates are in general agreement with estimates of 35-40 °C that are predicted for the region by early Eocene climate models. Previous workers estimate summer temperatures in the Green River Basin, Wyoming (~41 °N) that are about 10 °C less than summer temperatures in Big Bend. This spatial pattern of summer temperatures also generally agrees with model predictions for meridional temperature gradients in continental interiors. These results confirm that climate models and proxy data appear to be converging upon a solution for greenhouse climates like the Eocene, although the implications for life and soil health under hothouse conditions remains to be explored.