Stable isotope geochemistry of upper cretaceous and paleocene strata in Big Bend National Park, Texas
Schmidt, David R.
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Fluvial deposits in the Javelina and Black Peaks Formations in Big Bend National Park provide a stratigraphic sequence spanning the Cretaceous-Tertiary (K-T) system boundary. The stable carbon and oxygen isotopic compositions of carbonate and organic matter from these deposits provide information on environmental change associated with the K-T boundary. Carbonate samples are analyzed in groups based on their morphology, crystal fabric, and facies association. The groups comprise pedogenic or ground-water carbonates, lacustrine or surface-water carbonates, early diagenetic carbonates, and late diagenetic carbonates. Although the isotopic compositions of all groups overlap to some degree, and indicate that isotopic exchange may have occurred, distinct isotopic compositions are recognized for each group. The pedogenic and lacustrine carbonates precipitated under similar conditions, and have Î´18O values that span a range of about 5â€°, suggesting precipitation from surface waters at varied temperature, or from waters concentrated to varied degree by evaporation. Stratigraphic excursions (Â± 2â€°) in carbonate Î´18O values could reflect dramatic but brief temperature change (â€œgreenhouseâ€ events) or periods of more or less intense evaporation. The pedogenic carbonates are slightly enriched isotopically relative to lacustrine carbonates and charophyte oogonia; and some contain inclusions of barite crystals, suggesting that soil waters were subject to evaporation. Stratigraphic variation in Î´18O, and the more negative values of Paleocene pedogenic carbonates compared to Cretaceous, could also be due in part to the coastal effect on meteoric waters resulting from sea level change. Both pedogenic and lacustrine carbonates have Î´13C values indicative of precipitation from solutions in equilibrium with CO2 derived from decomposition of terrestrial plant material. The diagenetic carbonates likely precipitated under early shallow burial conditions and later during deep burial methanogenesis or hydrothermal conditions. Particulate sedimentary organic carbon from alluvial mudstones and intact organic carbon extracted from fossil woods have isotopic compositions compatible with C3 vegetation. Carbon from alluvial mudstone and fossil wood differs in composition slightly, due perhaps to original differences between arborescent and understory vegetation, or varied contributions from different plant parts of local or allochthonous origin. Average Î´13C values of conifer and angiosperm woods differ by 2â€°, and Cretaceous woods as a group are isotopically heavier than Paleocene woods. No abrupt excursion in organic Î´13C concides with the K-T boundary, but variation throughout the entire section could reflect temporal variation in vegetation type, source of organic matter, or atmospheric pCO2. Estimated atmospheric pCO2 levels for the Late Cretaceous (1520 ppmV) are five times that for the Paleocene (370 ppmV). Although this corresponds with cooler wetter conditions interpreted for the Paleocene on the basis of paleosols, the isotopic changes do not coincide with any change in alluvial sediment mineralogy, and it has not been possible to correlate isotopic excursions among the studied sections.