EVALUATION OF BULK ORGANIC CARBON ISOTOPE RECORDS FROM EARLY PALEOGENE STRATA IN THE HANNA BASIN (WYOMING U.S.A.) SPANNING THE PALEOCENE-EOCENE THERMAL MAXIMUM

The early Paleogene was dominated by a global greenhouse climatic state punctuated by abrupt, transient warming events known as 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 identified at many locations in marine strata, the PETM has only been definitively identified in a few terrestrial locations. This study focuses on carbon isotope stratigraphy from the Hanna Basin of south-central Wyoming to identify the PETM. Two stratigraphic sections were measured, one near the basin center (Hanna Draw) and one towards the northeast margin (The Breaks). The sections were 285 m and 488 m thick, respectively, and carbon isotope samples were obtained from a variety of lithofacies including coal layers, fissile organic-rich shales, gray siltstones/claystones, and sandstones. In total 404 samples were analyzed. δ¹³C values have an average of -27.3‰ ± 1.5‰ (1σ) in the Hanna Draw section and an average of -26.8‰ ± 1.1‰ (1σ) in The Breaks section. The amount of organic carbon (%C) in the samples varies with lithology sampled and averages 5.7% ± 12.7% (1σ) in Hanna Draw and 2.9% ± 4.7% (1σ) in The Breaks. A correlation does not appear to exist between δ¹³C values 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 subtle changes in local vegetation patterns, water stress conditions, degradation of local organic matter, input of allochthonous carbon, or taphonomic differences up-section.