STABLE ISOTOPIC RATIOS OF ORGANIC CARBON FROM THE UPPER CRETACEOUS AND EARLY PALEOCENE IN BIG BEND NATIONAL PARK, TEXAS

In an attempt to detect paleoclimate changes across the K-T boundary in Big Bend National Park, organic carbon samples were collected from four sections of the Javelina and Black Peaks Formations. Two sources of organic carbon (mudstones and fossil wood) were obtained from fluvial floodplain deposits spanning the late Cretaceous and early Paleocene. The isotopic composition of organic carbon (wrt V-PDB) from the mudstones and fossil woods is consistent with C₃ vegetation. Cretaceous mudstone d¹³C values range from - 22.4 to - 33.8‰, and Paleocene values range from -20.7 to -27.2‰. Cretaceous wood d¹³C values range from -20.0 to -27.0‰, and Paleocene values range from -20.5 to -28.3‰. The range of d¹³C values from mudstones and fossil woods differ, but mean values are comparable. Average d¹³C values for Cretaceous and Paleocene mudstones are - 25.5 and -24.7‰, respectively. Cretaceous and Paleocene woods average -24.3 and -26.2‰, respectively. Stratigraphic variation in d¹³C values may be due to differences between the original arborescent and understory vegetation, or variation from different plant parts of local or allochthonous origin contributed to the alluvial mudstones. Average d¹³C values of Cretaceous conifer (-23.5‰) and angiosperm (-25.1‰) woods differ by 1.6‰. Paleocene woods are rare and represent a smaller sample size with only one collected conifer sample. However, Cretaceous woods as a group have higher d¹³C values than Paleocene woods. Isotopic profiles of organic d¹³C indicate no abrupt excursion that coincides with the K-T boundary, but variation throughout the entire section could reflect temporal variation in vegetation type, source of organic matter, or changes in atmospheric pCO₂. Estimated atmospheric pCO₂ levels based on d¹³C for carbonate and organic matter for the Late Cretaceous (1520 ppmV) are five times that for the Paleocene (370 ppmV). Although this corresponds with paleosol data indicating a cooler wetter climate for the Paleocene, the isotopic variations do not coincide with changes in alluvial sediment mineralogy. Thus far, correlations amongst isotopic excursions in the studied sections have not emerged.