RESOLVING INCONSISTENCIES AMONG PETM CIES IN THE SE BIGHORN BASIN, WYOMING: AN ALLOCHTHONOUS CARBON HYPOTHESIS

The Paleocene-Eocene Thermal Maximum (PETM), a period of abrupt, transient, and widespread global warming fueled by the large release of isotopically light carbon, is recorded globally as a negative carbon isotope excursion (CIE). We measured stable carbon isotope ratios in bulk organic matter (δ¹³C_org) at four sections that span the PETM in the SE Bighorn Basin, WY and generated a composite carbon isotope record from n-alkanes (δ¹³C_n-alk). The n­-alkane curve shows an abrupt, negative shift in δ¹³C values, an extended CIE body, and a relatively rapid recovery to more positive δ¹³C values. Despite recording the abrupt, negative carbon isotope shift, the δ¹³C_org CIEs are smaller in magnitude and fail to sustain minimum excursion δ¹³C_org values for the same stratigraphic thickness.

We modeled predicted δ¹³C_org curves by applying enrichment factors based on modern C₃ plants (approx. as difference between δ¹³C_n-alk and δ¹³C_{total plant tissue}) to the δ¹³C_n-alk records. Anomaly values, the difference between measured and predicted δ¹³C_org values, were calculated and compared to weight percent carbon and grain size. There is no correlation between anomaly values and grain size or weight percent carbon before or after the CIE. During the CIE, however, anomaly values are greatest at high grain size and low weight percent carbon. We hypothesize that the anomaly is a result of allochthonous fossil carbon mixing with autochthonous PETM carbon. The presence of shark teeth, dinoflagellates, and Cretaceous zircons all suggest the introduction of Mesozoic material. Changes in the autochthonous: allochthonous carbon ratio could explain fluctuations in the δ¹³C_org records during the CIE. Before and after the CIE, the ratio of autochthonous: allochthonous carbon would be uncorrelated with isotopic composition because the δ¹³C values of Mesozoic and Cenozoic C₃ plants would have been similar. During the PETM, however, the isotopic composition of autochthonous organic carbon decreased dramatically and autochthonous and allochthonous carbon pools became isotopically distinct. A higher proportion of autochthonous PETM carbon would lead to more ¹³C-depleted δ¹³C_org values. The negative correlation between weight percent carbon and anomaly values could represent greater autochthonous input at higher weight percent carbon.