Exploring Terrestrial Temperature Changes during the Early Eocene Hyperthermals

The Early Eocene is marked by a number of rapid global warming events called hyperthermals. These hyperthermals are associated with negative carbon isotope excursions (CIE) in both marine and terrestrial records. Multiple theories exist to explain the connection of these hyperthermals with the CIEs and each theory predicts different responses by the climate system. Characterizing the timing, duration and magnitude of temperature change that is associated with these hyperthermals is important for determining whether the hyperthermals are all driven by the same underlying climate dynamics or perhaps differ from one another in cause and climatic consequences. In the simplest case, all share a common underlying mechanism; this predicts that the associated temperature changes scale in a predictable way with the magnitude of the CIE (and perhaps exhibit other similarities, such as the relative amplitudes of marine and terrestrial temperature change). To our knowledge, however, the only hyperthermal with paleotemperature data from land is the Paleocene-Eocene Thermal Maximum (PETM). Here we present preliminary carbonate clumped isotope paleotemperature estimates for Early Eocene hyperthermal ETM2/H2 from paleosol carbonates from the Bighorn Basin in Wyoming, USA. We compare the results to existing clumped isotope paleotemperature estimates for the PETM in the Bighorn Basin. Temperatures recorded by paleosol carbonates (which likely reflect near-peak summer ground temperatures) prior to each CIE are ~30°C and increase to ~40-43°C during the apex of each CIE. Following both CIEs, temperatures drop back to pre-CIE values. In the case of ETM2/H2, temperatures begin to rise again immediately, possibly in association with a later hyperthermal, though further work needs to be done to establish this with certainty. These preliminary data suggest that both the absolute values and the magnitudes of temperature changes associated with the PETM and ETM2/H2 are similar; the magnitude of temperature changes (~10°C) also appears similar to that found in arctic sediments (Sluijs et al., 2009). If confirmed, this result suggests that the temperature change associated with the hyperthermals does not necessarily scale with the magnitude of the local CIEs from paleosol carbonate nodules (~6‰ for the PETM and ~4‰ for ETM2). Future work includes analyses of additional paleosol carbonates from higher in the section to definitively identify the H2 hyperthermal, and to analyze samples at higher resolution within each CIE to assess whether the warming precedes the onset of the CIE. Sluijs, A., Schouten, S., Donders, T., Schoon, P., Rohl, U., Reichart, G., Sangiorgi, F., Kim, J., Sinninghe Damste, J. 2009. Warm and wet conditions in the Arctic region during Eocene Thermal Maximum 2. Nature Geoscience.