Browsing by Subject "Organic carbon"
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Item Stable isotope geochemistry of upper cretaceous and paleocene strata in Big Bend National Park, Texas(Texas Tech University, 2009-05) Schmidt, David R.; Lehman, Thomas; Karlsson, Hal; Barrick, Jim; Chatterjee, SankarFluvial 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 δ18O values that span a range of about 5‰, suggesting precipitation from surface waters at varied temperature, or from waters concentrated to varied degree by evaporation. Stratigraphic excursions (± 2‰) in carbonate δ18O values could reflect dramatic but brief temperature change (“greenhouse†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 δ18O, 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 δ13C 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 δ13C values of conifer and angiosperm woods differ by 2‰, and Cretaceous woods as a group are isotopically heavier than Paleocene woods. No abrupt excursion in organic δ13C 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.Item The use of δ]¹³C values of leporid teeth as indicators of past vegetation(2013-05) Wicks, Travis Zhi-Rong; Shanahan, Timothy M.; Bell, Christopher J., 1966-Records of change of [delta]13C values in vertebrate teeth offer an opportunity to gain insight into changes in past vegetation. Increasingly, teeth from small mammals are used for such purposes, but because their teeth grow very rapidly, seasonal changes in vegetation potentially provide a large source of variability in carbon isotope composition, complicating interpretations of small mammal tooth isotope data. To investigate the controls of seasonality on the stable isotope composition of fossil teeth, we constructed a Monte-Carlo-based model to simulate the effects of changes in the seasonal pattern of diet in leporid lagomorphs (rabbits and hares) on the distribution of [delta]¹³C values in random populations of leporid teeth from the Edwards Plateau in central Texas. Changes in mean-state, seasonal vegetation range, and relative season length manifest themselves in predictable ways in the median, standard deviation, and skewness of simulated tooth [delta]¹³C populations, provided sufficient numbers of teeth are analyzed. This Monte Carlo model was applied to the interpretation of a 20,000 year record of leporid tooth [delta]¹³C values from Hall's Cave on the Edwards Plateau in central Texas. Variations in the [delta]¹³C values of teeth deposited at the same time (standard deviation = 1.69%) are larger than changes in the mean vegetation composition reconstructed from bulk organic carbon [delta]¹³C, indicating the influence of short-term variability, making it difficult to assess changes in mean C3/C4 vegetation from the tooth [delta]¹³C data. However, populations of teeth from different climate intervals (e.g., the late Glacial, Younger Dryas, and the Holocene) display changes in the shape of the tooth [delta]¹³C distributions. Interpretation of these changes as shifts in seasonal vegetation patterns that are based upon results from our model are consistent with hypothesized climatic changes. An increase in the standard deviation of the tooth population between the late Glacial and the Younger Dryas -- Holocene is consistent with an increase in seasonality. Furthermore, a shift to more C3-dominated vegetation in the tooth [delta]¹³C distribution during the Younger Dryas is accompanied by a more skewed population -- indicative of not only wetter conditions but an increase in the duration in the C3 growing season. However, late Holocene changes in vegetation are not clear in the tooth data, despite the evidence from bulk organic carbon [delta]¹³C values for an increase in % C3 vegetation of 57%. Small mammal teeth can potentially provide unique insights into climate and vegetation on seasonal and longer timescales that complement other data, but should be interpreted with a careful consideration of local conditions, taxon ecology and physiology, and the dominant timescales of isotope variability.