CONTINENTAL EXPRESSION OF POST-PETM HYPERTHERMALS IN THE BIGHORN BASIN, WY

The Cenozoic marine record of climate change includes several brief episodes of rapid and intense warming (“hyperthermals”) characterized by global negative carbon isotope excursions. The largest of these was the Paleocene-Eocene Thermal Maximum (PETM), an event that has also been identified in terrestrial settings and is associated with coincident changes in hydrology, sedimentation, and ecosystems. To date, however, there has been little research into how other hyperthermals are expressed in continental environments. Such documentation is critical for determining the underlying causes, and global consequences, of these events.

There are two negative carbon isotope excursions in marine carbonate (H1 and H2) that fall near the base of Chron C24n.3n. We carried out meter-scale sampling of paleosol nodules and bulk sediment for geochemical analysis from this same stratigraphic interval in the fossiliferous McCullough Peaks section of the Bighorn Basin (WY) to investigate the expression of these short-term events in a continental environment. Preliminary results indicate two intervals of significant geochemical, biological, and/or sedimentary change within the 150 meters of section that we sampled. A well-defined but transient ~-2‰ shift in the δ¹³C of carbonate nodules (δ¹³C_carb ) occurs over ~20 meters in the upper part of the study interval, overlapping with the base of Chron C24n.3n but lacking any other obvious geological expression. Lower in the section there is a ~60 meter interval of low, but variable, δ¹³C_carb values that coincides with a progressive but short-term increase in calcite, swelling clay, organic matter content, and a replacement of hematite by goethite. Within this interval, many beds are also characterized by the presence of kaolinite and depleted δ¹³C_org. In addition, this part of the section displays the onset of thick channel sandstone formation, and the largest early Eocene turnover in fossil mammals after the PETM (“Biohorizon B”). Based on chronostratigraphic correlation to marine records, we correlate the lower changes in our section to the H1 event (aka Elmo, ETM2) and the upper change to the H2 event. It appears that these hyperthermals, like the PETM, can be recognized in terrestrial settings although their expressions differ somewhat from the PETM and from each other.