CARBON-ISOTOPE VARIABILITY IN TERRESTRIAL ENVIRONMENTS: EXAMPLES FROM THE PALEOCENE AND EOCENE SEDIMENTS OF THE BIGHORN BASIN, WYOMING (USA)

Isotopic values of bulk organic matter preserved in terrestrial sediments are widely used for correlating continental and marine sections under the assumption that both reservoirs are in isotopic equilibrium. Few studies have examined variability in carbon isotopic composition of bulk organic matter along a single stratigraphic horizon that captures different depositional settings, floral communities or microbial processing during pedogenesis. A wide diversity of biotic and physical processes affect organic matter source and preservation in lithologies ranging from lignites to oxidized paleosols, which could affect the stratigraphic pattern of carbon isotope composition and ultimately result in miscorrelation.

We document carbon isotope variability along five laterally extensive carbonaceous beds in the Paleocene Fort Union and Eocene Willwood formations of the Bighorn Basin, Wyoming (USA). Isotope values vary 0.2 to 0.9‰ VPDB (95% confidence intervals) within single beds across lateral distances of 100s to 1000s of m. Weight percent of organic carbon varies from 0.6 to 54.8%. Observed isotopic differences likely reflect systematic offsets caused by differences in floral inputs, preferential loss of certain organic compounds, or microbial processing during pedogenesis.

In spite of observed variations in the carbon isotopic composition of organic matter within each bed, there is a pronounced long-term change in mean carbon isotope values from 60 Ma (-25.0‰) to 52.6 Ma (27.6‰), which approximately tracks isotopic records from marine carbonate. Comparisons of terrestrial organic isotopic records with estimated carbon isotope values of atmospheric carbon dioxide inferred from marine foraminifera allows us to compare expected and measured values. Our findings suggest modest caution in using carbon isotopes for comparisons across multiple terrestrial sections due to subpermil-scale intrabed isotopic variability. Nonetheless, our data from an array of terrestrial facies yield composite carbon isotope values that track the predicted exogenous carbon pool.