Distortion of carbon isotope excursion in bulk soil organic matter during the Paleocene-Eocene thermal maximum

The Paleocene-Eocene thermal maximum was a period of abrupt, transient global warming, fueled by a large release of ¹³Cdepleted carbon and marked globally by a negative carbon isotope excursion. While the carbon isotope excursion is often identified in the carbon isotope ratios of bulk soil organic matter (δ¹³C_org), these records can be biased by factors associated with production, degradation, and sources of sedimentary carbon input. To better understand these factors, we compared δ¹³C_org values from Paleocene-Eocene thermal maximum rocks in the southeastern Bighorn Basin, Wyoming, with those derived from leaf wax n-alkanes (δ¹³C_n-alk). While both δ¹³C_n-alk and δ¹³C_org records indicate an abrupt, negative shift in δ¹³C values, the carbon isotope excursions observed in bulk organic matter are smaller in magnitude and shorter in duration than those in n-alkanes. To explore these discrepancies, we modeled predicted total plant tissue carbon isotope (δ¹³C_TT) curves from the δ¹³C_n-alk record using enrichment factors determined in modern C3 plants. Measured δ¹³C_org values are enriched in ¹³C relative to predicted δ¹³C_TT, with greater enrichment during the Paleocene-Eocene thermal maximum than before or after. The greater ¹³C enrichment could reflect increased degradation of autochthonous organic matter, increased input of allochthonous fossil carbon enriched in ¹³C, or both. By comparing samples from organicrich and organic-poor depositional environments, we infer that microbial degradation rates doubled during the Paleocene-Eocene thermal maximum, and we calculate that fossil carbon input increased ~28%-63%. This approach to untangling the controls on the isotopic composition of bulk soil carbon is an important development that will inform not only future studies of global carbon cycle dynamics during the Paleocene-Eocene thermal maximum hyperthermal event, but also any study that seeks to correlate or estimate duration and magnitude of past events using soil organic carbon.