STABLE CARBON ISOTOPIC COMPOSITION OF TERRESTRIAL ORGANIC MATTER FROM CONTINENTAL SEDIMENTS OF THE MID-CRETACEOUS POTOMAC GROUP, CHESAPEAKE BAY, MARYLAND

The mid-Cretaceous Potomac Group is comprised of coarse- and fine-grained siliciclastic sediment with subordinate amounts of nodular siderite deposited in a continental setting. Organic-rich layers containing abundant woody plant remains occur in thin- to medium-bedded, dark gray to brownish gray mudstones. The mudstones contain abundant vascular tissues including cuticles and wood fragments with independently variable C/N ratios and stable carbon isotope values. The absence of correlation of C/N ratios to stable carbon isotopes suggests negligible diagenetic alteration of the original isotopic signature. Stable carbon isotopic values (δ¹³C) for bulk terrestrial organic matter in mudstones of the Potomac Group range from –18.9 to –25.4 per mil, with an average value of -22.1 per mil. A positive excursion of 4 per mil is observed in the studied stratigraphic sections, but the average baseline δ¹³C values differ by 2 per mil. Variations of 2 to 3 per mil of stable carbon isotope values in individual wood fragments and vertically in the stratigraphic sections probably reflect variations in different plant species, canopy effects, vital effects (sunlight, salinity, etc.), and/or selective preservation of lignin vs. cellulose. Vascular plants in the Cretaceous are inferred to use the same carbon fixation pathway as modern C3-plants, but carbon isotopic compositions are significantly different from Quaternary values. Cretaceous plant detritus is more enriched in ¹³C (δ¹³C typically -20.5 to -23.0 per mil) than modern C3-plants (δ¹³C typically -24 to -30 per mil). This discrepancy between carbon isotope values of Cretaceous terrestrial organic matter and modern C3-plants is interpreted to result from higher pCO₂ and/or isotopically heavier CO₂ in the Cretaceous atmosphere. The average δ¹³C of –22.1 per mil for terrestrial organic matter from the Potomac Group is comparable to other published δ¹³C data for Cretaceous terrestrial organic matter from continental and transitional marine sediments in North America, Europe, and Australia. The consistency of these stable carbon isotope data from various global locations suggests an altered carbon cycle that affected the mid-Cretaceous oceans and atmosphere.