230Thxs budgets in size-fractionated calcareous near-surface sediments from Walvis Ridge

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Kretschmer, S. , Geibert, W. , Rutgers v. d. Loeff, M. and Mollenhauer, G. (2007): 230Thxs budgets in size-fractionated calcareous near-surface sediments from Walvis Ridge , International Conference and 97th Annual Meeting of the Geologische Vereinigung e.V. (GV) The Oceans in the Earth System. Bremen, Germany October 1-5 .
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Deep-sea sediments are composed of particles settling vertically through the water column, but often contain significant portions of laterally advected material. This laterally supplied material contributes to total sediment accumulation. However its deposition does not correspond to particle fluxes through the overlying water column. With 230Thxs-normalized particle flux calculations it is possible to quantify advective sediment supply. Bottom currents, that transport resuspended sediment, are likely to sort sediment particles according to grain size and sinking velocity. Thus it is reasonable to assume that the laterally supplied portion of the sediment is predominantly composed of fine-grained particles. However 230Thxs-based flux calculations rely on radionuclide measurements of bulk sediment samples. On the other hand, particle separation may lead to a decoupling of the particle-associated proxy records, which might result in over- or underestimation of the associated fluxes. To study the effects of lateral transport the objective of this study therefore is to perform particle flux estimations with grain size fractionated sediments. We present initial data from calcareous near-surface sediments from two sites at Walvis Ridge in water-depths of 2200 m and 2700 m. Both samples were grain size fractionated by sieving and settling using MilliQ water. Fractionated samples were acid-digested and radioisotopes of Uranium (234U, 235U, 238U) and Thorium (230Th, 232Th) were measured with an ICP-MS. 230Thxs data were calculated following standard procedures. The sediment is composed of 25 34 % clay-sized particles (<2 µm), 22-43 % silt (2 63 µm) and 21 53 % sand (>63 µm). In contrast, 50 61 % of the excess Thorium inventory is contributed solely by the clay fraction. The silt fraction contributes to 230Thxs in a range of 28 38 %, whereas only less than 12 % of 230Thxs is residing in the sand fraction. This confirms results from Thomson et al. (1993). In almost all grain size fractions the ratio 238U/232Th is exceeding the range of 0.5 1.1, indicating an uptake of authigenic Uranium to the sediment as a result of reducing conditions (François et al. 1993). These results highlight the potential of miscalculating 230Thxs-normalized fluxes of certain sediment consitutents, e.g., of foraminifera using bulk radioisotope measurements.

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