The Bighorn Basin Coring Project will collect ~200 meters of core from each of three sites (Polecat Bench, Gilmore Hill, Basin Substation) in the Bighorn Basin of Wyoming to better constrain the causes and effects of Paleogene hyperthermal events. Hyperthermals are abrupt, large-magnitude global warming events associated with extreme perturbations to the Earth's carbon cycle. Two events in particular will be targeted - the Paleocene-Eocene Thermal Maximum (PETM, ETM1) and the Elmo (ETM2) event. The PETM is the best documented hyperthermal and involved the release of 2,000-6,800 gigatons (Gt) of carbon from an unknown reservoir over less than 10 thousand years (ky) causing the earth to warm by 5-9ºC for a period of 100-200 ky. For comparison, the entire fossil fuel resource base on earth today is estimated to be ~5,000 Gt. Elmo is a smaller amplitude event that occurs after the PETM and just below the Chron C24N/C24R polarity reversal in marine records but remains poorly documented in continental sections. By applying a variety of sedimentological, geochemical, and palynological methods to the proposed cores we hope to better understand what caused these events, study the biogeochemical and ecological feedbacks that operated during them, and reveal precisely how they impacted continental environments. The Bighorn Basin preserves the most expanded stratigraphic records of the PETM and Elmo yet known, so these cores will provide an opportunity to study hyperthermals at unprecedented resolution.
Our results suggest that during the Paleocene-Eocene Thermal Maximum (PETM) organic carbon was readily degraded and not preserved well in soils and floodplain deposits. At all three sites in the western USA (Basin Substation, WY; Polecat Bench, WY; Piceance Basin, CO) total organic carbon decreased significantly from the Paleocene to the PETM. The decreased organic carbon preservation and potential loss of refractory carbon (i.e. polycyclic aromatic hydrocarbons, or PAHs, and charcoal) during the PETM suggest more agressive decay in soils, and were likely due to the hotter climate. We propose that higher carbon burn-down, due to accelerated decay rates, outpaced terrestrial productivity during the hyperthermal event, and that this hindered soil carbon sequestration and enhanced the atmospheric greenhouse. Despite hot and dry climate conditions that tend to promote fire, we found no clear evidence for increased fires at the onset or throughout the PETM hyperthermal event. The abundance of fire compounds, such as PAHs, do not increase with the onset of or during the PETM. Similarly, charcoal in the Bighorn Basin cores is consistent with PAH data. This suggests that the PETM climate had profound effects on vegetation and hydrologic patterns, but ultimately not on wildfire activity. As part of the Bighorn Basin Coring Project team, we developed protocols for testing drilling fluids and for overall clean conditions in order to minimize potential contamination with trace organic compounds during sample recovery. LacCore has integrated our approach into their protocols, and our methods are now regularly implemented in continental coring projects around the world.
