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Distribution of 230Th, 10Be and 231Pa in sediment particle classes

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Kretschmer, S. , Geibert, W. , Schnabel, C. , Rutgers v. d. Loeff, M. and Mollenhauer, G. (2008): Distribution of 230Th, 10Be and 231Pa in sediment particle classes , 18. Annual V.M. Goldschmidt Conference, July 13th-18th 2008, Vancouver, Canada. .
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The 230Thxs-normalization method is a widely used tool for the calculation of vertical fluxes to marine sediments, correcting for the influence of lateral sediment transport. However, the strong particle surface reactivity of thorium may lead to a grain size specific distribution of 230Th. Lateral transport during particle settling might have a sorting effect on particles, thus affecting the sedimentary 230Th signal as well. In this study, the specific distribution pattern of 230Th within different particle classes is investigated in carbonate-rich sediments and opal-rich sediments from the Atlantic Ocean. Sediments were split into distinct particle classes by wet-sieving and settling. The results show a clear preference of 230Th for the smallest particle sizes. Both in carbonate-rich and opal-rich sediment, most of the 230Th is bound within the particle size class <20µm accounting for 60-77% and 90% of the total 230Th inventory, respectively, whereas this size fraction was only 45% and 72% of sediment mass, respectivley. In the opal-rich material, 230Th does not show any preference for opal or non-opaline material.We also compare similar neighbouring deep-sea sediment cores (PS1768-8 and PS1769-1). While the difference in sedimentation rates is a factor of 2-3, the grain-size distribution in the two cores, as well as the 230Th-distribution between the grain sizes, does not differ significantly. These results indicate that syndepositional sediment redistribution at the sea floor may be a process that does not necessarily have a major impact on the grain size distribution. In addition, results from 10Be and 231Pa measurements of the same size-fractionated sediment samples will be presented. In contrast to the 230Th, it is expected to find these isotopes preferentially adsorbed on biogenic substances (opal, carbonate). The isotope ratios 10Be/230Th and 231Pa/230Th are used as palaeoproductivity and palaeocirculation proxies. If 230Th, 10Be and 231Pa are bound to different particle types, lateral transport involving particle sorting might affect the isotope ratios.

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