CARBON OXIDATION IN A GREENHOUSE WORLD: EXAMINING POLYCYCLIC AROMATIC HYDROCARBONS (PAHS) ACROSS THE PALEOCENE-EOCENE THERMAL MAXIMUM (PETM)

Projections for Earth’s future suggest that wildfire activity will increase with global warming, but the factors controlling fire are complex. The Paleocene-Eocene Thermal Maximum (PETM), a past geologically abrupt global warming event that had profound effects on vegetation and hydrologic patterns, serves as a unique analog for modern climate change. Here, we focus on three paleo-floodplain depositional sites in the Western USA, which had PETM climatic regimes likely favorable for fires (drier or more seasonal precipitation). Samples were selected from two Bighorn Basin Coring Project cores in the Bighorn Basin, Wyoming (Basin Substation and Polecat Bench), and from an outcrop section in the Piceance Basin, Colorado. Polycyclic aromatic hydrocarbons (PAHs), which are produced as aerosols during combustion, and charcoal were characterized to assess changes in fire occurrence across the PETM.

A diverse suite of PAHs were evaluated using gas chromatography-mass spectrometry (GC-MS) in selective ion monitoring (SIM) mode. Total PAH concentrations normalized to total organic carbon (TOC) were comparable at all three sites. In general the Paleocene had higher PAH concentrations than the Eocene, but there was no marked trend during the PETM. At Basin Substation, PAH concentrations actually decreased by an order of magnitude during the PETM interval, concurrent with a decline in TOC and charcoal. In addition, high molecular weight (MW) PAHs tend to dominate in low TOC samples, suggesting loss of low MW PAHs. The decreased organic carbon preservation and potential loss of refractory carbon (i.e. PAHs and charcoal) at Basin Substation during the PETM could be due to higher rates of decay in a hotter climate. If so, higher decay rates might negate increased productivity during a hyperthermal event and hinder carbon sequestration. Since low MW PAHs are more susceptible to post-depositional processes relative to high MW PAHs, we use the relative proportion of high MW PAHs, TOC, and lithology to discern the signals of fire and of preservation. Despite climate conditions that tend to promote fire, there is no evidence for increased fires during the PETM. This suggests that other factors, such as changes in oxygen level or vegetation structure and connectivity, may have countered favorable temperature and moisture conditions.