INFLUENCE OF MACERAL PROVENANCES AND TYPES ON THE CARBON ISOTOPE VARIATIONS OF DISPERSED ORGANIC 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. In this work, we study organic matter provenance as a factor influencing δ¹³C_DOM variations. We sampled floodplain sediments deposited during the Paleocene Eocene Thermal Maximum (PETM) in two well-studied locations – Polecat Bench and Cabin Fork – in the Bighorn Basin. At Polecat Bench δ¹³C_DOM records a carbon isotope excursion (CIE) of lower amplitude than the CIE recorded in δ¹³C of pedogenic carbonates both records are well-correlated. At Cabin Fork, the CIE is difficult to identify in δ¹³C_DOM because the record includes large isotopic fluctuations which correlate poorly the CIE recorded in δ¹³C of n-alkanes at the same location. For each location we concentrated macerals from 20 stratigraphic levels using heavy liquid floatation and analyzed the δ¹³C of the floated organic fraction. We also measured vitrinite reflectance (R_o) and liptinite fluorescence to qualify the thermal signature of macerals and quantify the different types of maceral present in the floated organic fraction. We show that thermal signatures of macerals is relatively homogeneous within and between strata at Polecat Bench, but at Cabin Fork thermally distinct populations of macerals can be identified within and between strata. We conclude that dispersed organic matter in the PETM interval at Cabin Fork floodplain sediments includes at least two distinct fractions: 1) authigenic PETM organic matter displaying CIE δ¹³C values and 2) fluvially deposited detrital organic matter composed of either older recalcitrant vitrinite and/or organics bound to mineral surfaces, both of which record pre-PETM organic carbon isotope values. We further demonstrate that second order variations of δ¹³C_DOM in both sections can be explained by differences in the relative proportion of isotopically distinct vitrinite and liptinite in different strata. We concluded that both sources and types of macerals are important factors to consider when interpreting δ¹³C_DOM variations for paleoenvironmental reconstruction.