Soil carbon degradation during the Paleocene-Eocene Thermal Maximum in the Piceance Basin, USA

Abstract

The Paleocene-Eocene Thermal Maximum (PETM) warming event is marked by a negative carbon isotope excursion (CIE). The magnitude and shape of the CIE differs among carbon archives, but bulk organic carbon isotopes (δ13Corg) in terrestrial sections are often particularly variable and attenuated. Multiple lines of evidence from the Bighorn Basin, Wyoming, USA have revealed that extensive organic carbon degradation and increased refractory or allochthonous carbon inputs diminished the δ13Corg excursion magnitude. To test if this was a spatially widespread phenomenon and to understand possible underlying mechanisms, we examined the abundance of total organic carbon (%TOC) and polycyclic aromatic hydrocarbons (PAHs) as metrics of soil degradation at a neighboring terrestrial basin, the Piceance Basin in Colorado. Outcrop samples reveal both diminished %TOC and PAH concentrations across the Paleocene-Eocene boundary. Using PAHs as a proxy for intermediate refractory carbon, such as found in mineral soils, evidence from the Piceance Basin supports increased organic carbon degradation and greater proportions of refractory allochthonous carbon remaining. Correlations between %TOC and elemental oxides (e.g., Al2O3 and TiO2) suggest soil organic carbon was stabilized by its association with clay minerals. We hypothesize that decreased clay content in the soils (which reduced soil capacity to stabilize fresh carbon) and increased fluctuations in soil moisture (which destabilized older, refractory carbon), in conjunction with increased temperatures (which increased microbial decomposition rates), contributed to reduced soil organic matter preservation during the PETM. These mechanisms destabilized carbon on millennial timescales and, with sustained higher temperatures across the PETM (~150,000 years), increased soil carbon degradation persisted for tens of thousands of years.