An isotopic biogeochemical study of the Green River oil shale (Piceance Creek Basin, Colorado)

The microbial ecology and, hence, related aspects of the paleoenvironment, of the Green River Lake system (Eocene, Wyoming, Utah, and Colorado) have been reconstructed through geochemical analyses. Specifically, concentrations and isotopic compositions of bulk carbon, sulfur, and nitrogen, as well as carbon-isotopic compositions and abundances of numerous individual hydrocarbons ("biomarkers") have been examined. The Green River Formation was deposited in two large alkaline, saline lakes occupying hydrologically closed basins. Utilizing core material from the Piceance Creek Basin, this study has focused on (1) the sources and cycling of nutrients for the producer community and (2) the pathways of carbon flow in the ecosystem.; Estimates of annual accumulation of organic carbon in the Green River lakes are low compared to measurements of productivity in modern saline lakes, suggesting that a great deal of the organic material was reworked prior to preservation. Carbon sulfur relationships indicate sulfate reduction was limited by available sulfate. Organic material enriched in {dollar}sp{lcub}15{rcub}{dollar}N indicates a complex and active nitrogen cycle in which nitrogenous intermediates were cycled through microbial redox reactions. Enrichment of {dollar}sp{lcub}13{rcub}{dollar}C in carbonates suggests extensive methanogenic activity in the sediments.; In order to examine effects of environmental changes, samples for isotopic analysis were taken at intervals of 60 cm through three different "sulfur cycles", intervals in which concentrations of total sedimentary S varied between {dollar}sim{dollar}0.2 and {dollar}sim{dollar}3.5%. Stratigraphic variations of {dollar}sp{lcub}13{rcub}{dollar}C abundance in individual n-alkanes indicated derivation from at least five distinct biological sources. Isotopic variations in numerous materials derived from polyisoprenoids and from di-, tri-, and tetraterpenoids indicate mixing of three components: products of primary photo-autotrophs and their immediate consumers, products of methanotrophic bacteria, and products of unknown bacteria. Independent variations in {dollar}sp{lcub}13{rcub}{dollar}C contents of a variety of photosynthetic products indicated the presence of a diverse producer community. Hopanoids produced at times of low sulfur concentration were particularly depleted in {dollar}sp{lcub}13{rcub}{dollar}C, indicating significant inputs from methanotrophic bacteria, and suggesting competition between methanogenic bacteria and sulfate-reducing bacteria for metabolic intermediates.; This study has been the first to combine detailed stratigraphic sampling with compound-specific isotopic analyses, and indicates that this approach can significantly enhance process-oriented detail in the reconstruction of ancient environments.