Browsing by Subject "Organic carbon"
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Item Organic and inorganic carbon in the recent sediments of the open Gulf, barrier island and bay environments, Mustang Island, Texas(1960) Jones, Darrell K., 1935-; Kornicker, Louis S., 1919-2018The sediment at both surface and one-foot depths in cores from the Gulf of Mexico, Mustang Island, and Corpus Christi Bay Texas, was analyzed by a wet combustion method to determine the per cent of carbon by dry weight in the organic fraction (organic carbon) and in the carbonate fraction (inorganic carbon). The statistical "t" test showed a significant difference (P = 0.05) between the mean per cent organic carbon content of the surface sediments from 1) Gulf (0.52%) and island (0.17%) and 2) island (0.17%) and bay (0.58%) and of one-foot sediments from 1) Gulf (0.29%) and island (0.11%) and 2) island (0.11%) and bay (0.44%). A significant difference was also found between the means of the per cent of carbonate carbon in surface sediments from 1) Gulf (0.50%) and island (0.09%) and 2) island (0.09%) and bay (0.68%) and in sediments from Gulf (0.32%) and bay (0.68%). A comparison of the means of the per cent of carbonate carbon in sediments at the surface (0.50%) and one-foot depths (0.32%) of all Gulf cores showed a significant difference at the 0.05 probability level. The organic carbon and inorganic carbon content of the sediment may be useful in distinguishing between Gulf and barrier island, and bay and barrier island sediments in the geologic record, but probably not for distinguishing between Gulf and bay environmentsItem The impact of salinity diffusion, poroelasticity, and organic carbon in sediment acoustics(2019-05) Venegas, Gabriel Ricardo; Wilson, Preston S.; Hamilton, Mark R; Haberman, Michael R; Salamone, Salvatore; Lee, Kevin M; Ballard, Megan STo optimize the use of sound in waters on the continental shelf for naval, commercial, and environmental monitoring applications, the acoustic properties of the ocean bottom must be well understood. The effects of 1) pore water salinity variability on acoustic reflectivity, 2) poroelasticity on geoacoustic inference, and 3) organic carbon on sediment properties were formerly-considered insignificant in sediment acoustics, but due to advancements in other areas of underwater acoustics systems and modeling, have now become significant. Three separate but related studies were conducted to begin to quantify these effects. 1) A high-frequency acoustic reflection experiment was performed on a water-clay interface, while varying the salinity of the water. Results demonstrated significant changes in reflectivity at high incident angles, as well as a transient effect explained by a new coupled salt diffusion/reflection model. Using the model, the effective diffusion coefficient of salt in clay was inferred from the experiment, and reflectivity was then simulated at lower frequencies and longer time-scales. From this modeling effort, at a given time-scale of fluctuation, a characteristic frequency was identified, below which the reflectivity should not be assumed temporally invariant. 2) A model geoacoustic inference procedure was performed on a layered waveguide consisting of water and water-saturated glass beads contained within a glass tube. The resonance frequencies of the system were measured and compared with simulations of the experiment. Within each simulation, various sediment acoustics models were used. The only model that allowed for self-consistency between the inference and an independent set of high-frequency sound speed measurements, was a model that accounted for poroelastic effects. 3) A sediment constituent that has great value to the planet and is ubiquitous in natural marine sediment, organic carbon, has been ignored in sediment acoustics models. To begin to explore this relationship, sediment cores were extracted from a T. testudinum seagrass meadow in the Lower Laguna Madre, Texas, USA. A strong correlation between organic carbon and the primary-wave modulus was identified using a custom-built automated broad-band core and resonance logger and an elemental analyzer. The sediment properties attained from the cores were compared, and a theory explaining the correlations was developed. The acoustic sensitivity to organic carbon in a seagrass meadow has demonstrated promise toward developing an acoustic tool to more rapidly quantify marine organic carbon stores, which is needed in climate science. However, a larger-scale study is required to determine its applicability across a broader range of seagrass meadows and sediment typesItem 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.