Decreased soil carbon in a warming world: Degraded pyrogenic carbon during the Paleocene-Eocene Thermal Maximum, Bighorn Basin, Wyoming

Global warming will likely perturb carbon storage and cycling throughout many components of the exogenic carbon cycle, but its net impact on the long-term fate of organic carbon stabilized in soils is unclear. Abrupt warming during the Paleocene-Eocene Thermal Maximum (PETM) profoundly altered vegetation and hydrologic patterns globally. To assess the consequences for soil carbon in a mid-latitude region we measured total organic carbon (%TOC), polycyclic aromatic hydrocarbons (PAHs), charcoal, and sporomorphs (pollen and spores) at two paleo-floodplain depositional sites in the Bighorn Basin, Wyoming, USA. At both sites %TOC, PAHs, charcoal, and sporomorphs declined during the PETM. The decline in pyrogenic carbon, which is more severe than the decline in %TOC, is consistent with isotopic and fossil evidence for degradation of labile organic compounds and preservation of highly refractory allochthonous organic carbon. The severe loss of less-labile contemporaneous PETM (autochthonous) soil carbon, illustrated by the fate of pyrogenic carbon, indicates intensified rates of organic matter decay during the PETM. Because of the highly degraded signature of organic matter in these PETM sections, it is difficult to discern if less pyrogenic carbon is in part a consequence of less fire occurrence during the PETM. We propose that in this mid-latitude region of the western USA increased soil carbon oxidation hindered soil carbon sequestration during this period of hotter climate with more seasonal precipitation.