INFLUENCE OF ORGANIC PROVENANCE AND PRESERVATION ON THE CARBON ISOTOPE VARIATIONS OF DISPERSED ORGANICS IN ANCIENT FLOODPLAIN SEDIMENTS

Insufficient understanding of the portfolio of controls influencing bulk carbon isotopic variations in dispersed organic matter from floodplain sediments (δ¹³C_DOM) limits the potential of this tool for paleoenvironmental reconstruction. We sampled sedimentary rocks spanning the Paleocene Eocene Thermal Maximum (PETM) in two well-studied locations – Polecat Bench (PB) and Highway 16 (HW16) – in the Bighorn Basin. At both sites δ¹³C_DOM records of the carbon isotope excursion (CIE) differs from the δ¹³C from other substrates because rapid isotopic fluctuations and individual ‘outlier’ δ¹³C_DOM values are superimposed on the general pattern of isotopic change through the CIE. For each location we separated organo-mineral fractions (MOM), concentrated macerals from 20 stratigraphic levels and analyzed the carbon isotope ratio (δ¹³C) of each fraction. At both sites the δ¹³C of the fine and coarse MOM differ significantly from each other and from δ¹³C_DOM. Concentration-weighted mixing of these isotopically distinct MOM explains high resolution δ¹³C_DOM fluctuations observed at both sites, but does not explain δ¹³C_DOM outlier values observed at HW16. We analyzed the thermal maturity and quantified the different types of macerals present in the POM. At PB, macerals display homogeneous thermal maturity, but the proportion of isotopically distinct vitrinite and liptinite varies between facies. At HW16, we identified two thermally and isotopically distinct populations of macerals interpreted as being autochthonous and recycled organic matter. Variation in the mixing of these fractions explains some of the isotopic anomalies observed in the δ¹³C_DOM record at HW16.