Carbon Isotope Variability Across the Hanna Basin

SMSU Event Center BC

The early Paleogene was dominated by a global greenhouse climatic state punctuated by abrupt warming events (hyperthermals). These hyperthermals are associated with perturbations in the global carbon cycle and are manifested in stable carbon isotope records from both marine and nonmarine strata as negative isotope excursions. The Paleocene-Eocene Thermal Maximum (PETM) is the largest of these events and occurred at ~56 Ma. Although well known in marine strata, the PETM has only been identified in a few terrestrial locations. This study focuses on the Hanna Basin in Wyoming. Two stratigraphic sections were measured, one near the basin center (Hanna Draw) and one towards the northeast margin (The Breaks). Carbon isotope samples were obtained from a variety of lithofacies including coal, organic-rich shale, siltstone/claystone, and sandstone. δ13C values have an average of -27.3‰ in Hanna Draw and an average of -26.8‰ in The Breaks. The percent of organic carbon in the samples varies with lithology and averages 5.7% in Hanna Draw and 2.9% in The Breaks. No correlation is seen between δ13C and %C. In both sections a ~3.5‰ negative carbon isotope excursion corresponds to the first occurrence of Eocene pollen indicator taxa. We interpret this excursion as the PETM. However, significant δ13C variability exists over short stratigraphic distances not associated with the PETM. This variability does not appear to be related to the lithology sampled, but may record changes in local vegetation patterns, water stress conditions, degradation of local organic matter, input of allochthonous carbon, or taphonomy differences up-section.