COMPLEX CONTROLS ON LACUSTRINE ORGANIC MATTER BURIAL AND CARBON ISOTOPE RECORDS: THE GREEN RIVER FORMATION OF EOCENE LAKE UINTA, UTAH USA

The lacustrine Green River Formation is the largest oil shale in the US and potentially a significant archive of terrestrial environmental conditions during the Early Eocene Climactic Optimum (EECO). Analysis of Green River Formation rocks has identified cyclicity in several proxies on orbital timescales, including total organic carbon content and carbon isotopic composition. A model for interpreting how orbital configurations affected carbon burial in the Green River Formation would yield valuable insight into environmental conditions during the EECO. However, because bulk organic carbon records have many drivers, no such model is currently available.

To investigate the mechanisms underpinning bulk organic carbon records in the Green River Formation, we examined lipid biomarkers extracted from the Mahogany Zone in the eastern Uinta Basin, Utah, USA. Whereas δ¹³C_TOC from this interval is known to have several negative carbon isotope excursions, compound-specific carbon isotope analysis of GRF lipid biomarkers reveals that these excursions likely arises from changes in the proportions of organic matter sources, not changes in the isotopic composition of the sources. Furthermore, decoupling of the carbon isotopic stratigraphic patterns of lacustrine and terrestrial compounds indicates disequilibrium between atmospheric CO₂ and lacustrine total inorganic carbon. Partial decoupling of carbon isotopic trends occurs among the lacustrine-sourced compounds as a function of stratification, long-term carbon burial and shorter-term variations in surface water productivity. Finally, linkage between TOC, δ¹³C_TOC, and δ¹³C_biomarkers provides a framework in which to identify intervals where dilution with inorganic sediments influenced TOC. Our findings provide insight into mechanisms underpinning potential paleoclimate records from the GRF, but complicate correlation based on carbon isotope chemostratigraphy due to evidence for the importance of lake-atmosphere equilibration and stratification.