AmericanChemicalSociety.com

The early Eocene (~48-55 Ma) was punctuated by multiple events associated with atmospheric CO₂ and global temperature increase with unprecedented and as of yet unexplained rapidity.  Relative temporal spacing of three such events, termed hypothermals – the Paleocene-Eocene Thermal Maximum (PETM), Eocene Thermal Maximum 2 (ETM2, aka "Elmo") and Eocene Thermal Maximum 3, (ETM3, aka “X” event) – identified by large (1‰ - 2.5‰) negative carbon isotope excursions, have lead to the hypothesis that Eocene hyperthermals are paced by million-year-scale orbital cyclicity.  Terrestrial records through the PETM indicate an enhanced continental response to hyperthermal carbon cycle perturbations, resulting in both greater carbon isotope excursions (3‰ - 6‰), and a larger increase in mean annual temperatures than measured from the marine record. We compiled a spliced continuous record of Fischer assay data from drilled cores of the Green River Formation that spans both an East-West and a North-South transect of the Uinta Basin of Utah.  Detailed work on two cores demonstrate that Fischer assay measurements covary with total organic carbon and bulk carbon, allowing us to use Fisher assay results as a representative carbon cycling proxy throughout the stack. We identified a fourth early Eocene carbon cycle perturbation associated with an anomalous accumulation of organic matter. Orbital tuning of the early Eocene record from the Uinta basin indicates a link between the timing of the hyperthermals and the 1.2 Myr obliquity cycle, which has implications of the mechanisms that caused global change during the hyperthermals.

Our preliminary geochemical work on Uinta Basin samples demonstrates that during discrete intervals of time, the organic matter contains high concentrations of aryl isoprenoidal biomarkers indicating that green sulfur bacteria were a principal component of the plankton. In this way, the ecosystem differed fundamentally from organic matter constituting modern large lakes. This is consistent with the anoxic conditions inferred to be associated with deposition of organic rich shale, but is unusual in that green sulfur bacteria require sulfide, and are thus rare in freshwater settings. Their presence here reflects a small but significant input of sulfate, perhaps via groundwater, but regardless of mechanism, it provides an opportunity to explore changes in water column stratification and anoxia, and the possibility that changes in carbon cycling and methane production play a key role in the onset and abrupt ending of Eocene hyperthermals.

The orbital cyclicity of the oil-shale yield records promise considerable predictability in exploration, suggesting intervals that while at the basin margin might be relatively low-grade, could improve dramatically towards the center of the basin.