CARBON ISOTOPE COMPOSITION OF SOIL ORGANIC MATTER SPANNING THE PALEOCENE-EOCENE BOUNDARY: NEW RESULTS FROM BASIN SUBSTATION, BIGHORN BASIN, WY

Past warming events provide the ideal backdrop to understanding changes in carbon cycling and storage in terrestrial systems associated with greenhouse climate conditions. One of the largest and best-documented of these events is the Paleocene-Eocene thermal maximum (PETM) occurring ~55.5 Ma. The PETM is characterized by ~5-8°C of warming globally and a distinctive (>2.5‰) negative carbon isotope excursion (CIE) and recovery. It has recently been proposed that continental environments released additional ¹³C-depleted carbon in response to initial PETM warming. In order to investigate changes in continental paleoenvironments and organic carbon burial through the PETM we generated a high-resolution record of bulk sedimentary organic carbon concentration and δ¹³C values from sedimentary rocks cored at Basin Substation, Bighorn Basin, WY. Unlike prior studies that measured bulk sedimentary organic δ¹³C values on samples taken from outcrop, this new record is based on unweathered drill core sediments and is stratigraphically and temporally continuous. Initial results show a high level of sample-to-sample variability suggesting multiple preservational, source, and global carbon cycle controls on the preserved carbon isotope record. After correction for several of these factors the data yield a CIE of ~2‰ and show a drastic decrease in organic carbon preservation throughout the body of the PETM. These new results compliment other data from across the Bighorn Basin, which suggest a substantial change in critical zone conditions during the PETM, and are consistent with regional changes in carbon storage in terrestrial ecosystems during hyperthermal events.