A NEW RECORD OF ENVIRONMENTAL CHANGE DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM

The Paleocene-Eocene Thermal Maximum (PETM) was a global episode of warming that occurred near the Paleocene-Eocene boundary (~55Ma). The PETM is recognized geochemically by a negative carbon isotope excursion (CIE) where d¹³C values from organic carbon are less than –26‰. The Hanna Formation in the Hanna Basin, south-central Wyoming, is a highly organic and richly fossiliferous unit deposited from the middle Paleocene into the early Eocene, bracketing the PETM. The section is exceptionally thick due to depositional rates that were approximately ten times the rate of other similarly-aged sections. In the summer of 2007, 46 coal and rock samples containing organic carbon were collected during the measurement of about 1000 meters of Hanna Formation known to contain the Paleocene-Eocene boundary on the basis of palynology and mammalian biostratigraphy. Some samples yielded an average d¹³C of about –25‰, indicative of pre- or post-CIE values. Others samples averaged about –28‰ indicative of the CIE associated with the PETM. Freshwater mollusk fossils were also collected throughout the section with many still preserving a nearly intact original aragonitic shell. Serial isotopic analysis of shells collected from before, during, and after the PETM will be compared to assess the environmental differences during during each time division and whether or not environmental conditions returned to pre-PETM conditions after the warming episode was over.