CARBON ISOTOPIC COMPOSITION OF PLANT-DERIVED ORGANIC MATTER FROM A TROPICAL SEDIMENTARY SEQUENCE AS A RECORDER OF LATE CRETACEOUS-EARLY PALEOGENE CHANGES IN THE CARBON CYCLE

The dynamics associated with the carbon cycle and the linkage between the oceans, the atmosphere, and land plants provide an opportunity to correlate marine and terrestrial sedimentary sequences using stable isotopes of carbon. However, these correlations are difficult due to chronostratigraphic uncertainties and the isotope overprint created by microbial degradation and digenesis. In contrast, geochemical biomarkers are unaffected by these two processes, thereby allowing a reliable estimate of the plant isotopic signal. To evaluate the possibility of using carbon isotope ratios as a chronostratigraphic tool, we are comparing the Paleocene-Eocene marine carbon-isotope record with that of a terrestrial sequence from Colombia. Sediments of the terrestrial rock units were deposited in a transitional setting dominated by mudstones, coals, and small lenses of sandstones (Catatumbo and Barco Formations) and in a mixture of deltaic and fluvial conditions (Cuervos Formation). The stratigraphic control was based on palynological zones for the region. Isotope results for the studied terrestrial sequence show the same three carbon-isotope excursions found in the marine carbonate record. Terrestrial isotope values decreased from -24‰ to -26.5‰ in the oldest event. This shift is commonly associated with the recovery of the primary production after the K-T extinction event. A positive shift is present in the late Paleocene, with values increasing from -26.5‰ to -23.8‰. This event appears to be related to the burial of large amounts of organic matter. The third excursion is found near the Paleocene-Eocene boundary, with values changing from -23.8‰ to -26.5‰. The shift is commonly interpreted to result from the release of large amounts of methane. Our preliminary results highlight the possibility of correlating marine and terrestrial sequences using carbon isotopes.