Thorium speciation in seawater

Author Posting. © The Authors, 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Marine Chemistry 100 (2006): 250-268, doi:10.1016/j.marchem.2005.10.024.Since the 1960’s, thorium isotopes occupy a special place in the oceanographer’s toolbox as tracers for determining rates and mechanisms of oceanic scavenging, particle dynamics, and carbon fluxes. Due to their unique and constant production rates from soluble parent nuclides of uranium and radium, their disequilibrium can be used to calculate rates and time scales of sinking particles. In addition, by ratio-ing particulate 234Th (as well, in principle, other Thnuclides) to carbon (and other elements), and linking this ratio to the parent-daughter disequilibrium in the water column, it is possible to calculate fluxes of carbon and other elements. Most of these applications are possible with little knowledge of the dissolved chemical properties of thorium, other than its oxidation state (IV) and tendency to strongly sorb to surfaces, i.e., its “particle- or surface-activity”. However, the use of any tracer is hindered by a lack of knowledge of its chemical properties. Recent observations in the variability of carbon to 234Th ratios in different particle types, as well as of associations of Th(IV) with various marine organic biomolecules has led to the need for a review of current knowledge and what future endeavors should be taken to understand the marine chemistry of thorium.The writing of this paper was supported, in parts by NSF (OCE-0351559; OCE-0350758, and OCE 0354757)

A seasonal survey of carbohydrates and uronic acids in the Trinity River, Texas

TextDue to their potential significance as indicators of ecological health, the biogeochemical cycling of carbohydrates and uronic acids was investigated in the Trinity River Texas, during 2000â 2001. Concentrations of dissolved organic carbon (DOC), total carbohydrates (TCHO), polysaccharides (PCHO), monosaccharides (MCHO), uronic acids (URA), as well as of oxygen, suspended particulate matter, nutrients and trace metals (Cu, Pb, Cd) were assessed at various stages of discharge. TCHO/DOC ratios, as well as nutrient and hydrogen ion concentrations, were inversely related to temperature, which suggests that biological processes in Lake Livingston, the largest freshwater reservoir along the Trinity River, are not only regulating nutrient concentrations but also the preferential degradation of carbohydrates over that of bulk DOC. However, uronic acids were selectively preserved during this temperature controlled biological process, as is evident from the positive correlation of URA/TCHO ratios and temperature. Thus, uronic acids are more refractory compounds than bulk TCHO. Significant correlations between TCHO and dissolved Cu, Pb and Cd suggest that their pathways and cycles are linked through common sources or removal processes.Laboratory for Oceanographic and Environmental Research (LOER), Department of Marine Sciences, Texas A&M University at GalvestonCenter for Research in Water Resource

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

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

Coupling between 210Pbex and organic matter in sediments of a nutrient-enriched lake: an example from Lake Chenhai, China

Sediment cores were collected from deep-water areas of Lake Chenghai, China in June 1997. The vertical profile of 137Cs activity gives reliable geochronological results. The results also indicate that sediment accumulation rates in deep-water areas of Lake Chenghai were relatively constant in recent decades, averaging 0.43 g cm-2 y-1, despite a variable organic carbon influx. 210Pbeq (=226Ra) activity was relatively constant also, with an average value of 54.3 ± 3.2 Bq kg-1. Vertical profiles of 210Pbex (=210Pbtotal - 226Ra) decreased exponentially, resulting in somewhat lower sediment accumulation rates (0.3 g cm-2 y-1). These lower rates are likely less reliable, as the relatively large fluctuations in 210Pbex activities correlate closely to the organic carbon (Corg) content of the sediments. For example, the vertical profile of 210Pbex activity displays peaks at mass depths of 3.7-4.7 g cm-2 (10-12 cm) and 10-11 g cm-2(25-28 cm), similar to the maxima in the vertical profile of Corg. This phenomenon must be related to the delivery of particulate organic matter (POM) from the water to the sediments, or to watershed soil erosion. Since the mean atomic ratios of Horg/Corg and Corg/Norg in Lake Chenghai sediments are 5.5 and 7.0, respectively, indicating that POM was predominantly derived from the remains of authigenic algae, this eliminates watershed erosion rates as a primary control on lake sedimentation rates as resolved by 210Pbex. Sedimentation fluxes (F(Corg)) of particulate organic carbon since 1970 varied between 60 to 160 g m-2 y-1, and appeared to closely influence variations in 210Pbex concentrations. For example, sedimentation fluxes of 210Pbex (F(210Pbex)) showed maxima in the years 1972-1974 and 1986-1989, likely reflecting historical variations of lake biological productivity or carbon preservatio