The Relative Importance of Terrestrial Versus Marine Sediment Sources to the Nueces-Corpus Christi Estuary, Texas: An Isotopic Approach

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Marine Science


Determining sources of sediment to coastal systems is an important and complex problem that figures prominently in a myriad of geological, geomorphological, geochemical, and biological processes. Lithogenic (Ra-226, Ra-228, Tb-228, (230)n, M-211) and fallout ((CS)-C-137, Ph-210) isotopes were employed in conjunction with sedimentological methods to determine rates of sedimentation in the Nueces Delta and Nueces-Corpus Christi Estuary and to assess the relative importance of marine versus terrestrial sediment sources to the estuary. Similarity of lithogenic isotope ratios in surface sediments throughout the system precluded a numeric approach to discerning the importance of each of the two large scale sediment sources (terrestrial and marine). A stepwise, graphical examination of discrete lithogenic isotope activity concentrations shows more promise. Terrestrial, marine, and bay sediment means for Ra-226 versus Th-112, Ra-226 versus 230n, and Ra-228 versus Th-232 show that terrestrial and marine sediment sources have different signatures, despite having similar grain size distributions (sands), and that sediment deposited in Nueces and Corpus Christi Bays are indistinguishable from the terrestrial component. Supporting evidence is provided by thorium isotopes, Th-226 versus Th-232, Th-228 versus Th-232, and Th-228 versus Th-230. Nueces Delta sedimentation (0.09-0.53 g cm(-2) yr(-1)) shows a subtle gradient, with rates generally lower in the west and progressively higher moving cast, likely reflecting contrasts in land use and topography. Nueces Bay cores differ from those in Corpus Christi Bay in that sands comprise a larger percentage of their composition, and they are mixed over greater depth, most likely due to geographic and physiographic effects. Sediment accumulation rates consistently decrease over the first 20 km from the Nueces River and become constant after that, implying that the river is the most significant source of sediment to the estuary. The interpretation of sediment supply to this estuary as dominated by terrestrial inputs is based on three complimentary sets of data: sediment grain size distributions, discrete lithogenic isotope data (Ra versus Th and Th versus Th), and sediment accumulation rates for both Nueces and Corpus Christi Bays.

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Estuaries and Coasts





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