Isotope Sclerochronology of Freshwater Mollusks: A New Look at Terrestrial Environmental Change during the Paleocene-Eocene Thermal Maximum in Wyoming

The Paleocene-Eocene boundary (~55Ma) is marked by a sudden, global episode of warming known as the Paleocene-Eocene Thermal Maximum (PETM). The PETM is recognized globally, in both terrestrial and marine systems, by a sudden negative carbon isotope excursion (CIE) where δ¹³C values decrease by about ~2.5–8‰. The nearly 3000m-thick Hanna Formation, in the Hanna Basin of south-central Wyoming, is comprised of highly organic and richly fossiliferous terrestrial sediments that were deposited from late early Paleocene into the earliest Eocene. The Paleocene-Eocene boundary is recognized in the Hanna Formation on the basis of a rapid 2.5‰ decrease in isotopic values of dispersed organic carbon at about 2550m above the local base of the formation. Fossil mollusks are common throughout the Hanna Formation, especially higher in the section where the Paleocene-Eocene boundary is found. We present here preliminary results about trends of environmental change in the Western Interior of North America during the PETM. We analyzed isotopes of carbon and oxygen from samples collected serially along the growth axis of fossil freshwater mollusks (both gastropods and pelecypods) collected from parts of the Hanna Formation deposited during the PETM. We considered (1) the mean values for carbon and oxygen from each individual shell, (2) correlation coefficients between carbon and oxygen within a single shell, and (3) the overall pattern of intrashell variability in order to make environmental determinations. Our results indicate that early in the PETM, conditions in the Hanna Basin were subtropical. The environment shifted toward more temperate conditions as the PETM continued. Further work is planned to improve our interpretations and to compare trends during the PETM with pre- and post-CIE environmental conditions.