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Fractionation of 230Th, 231Pa, and 10Be induced by particle size and composition within an opal-rich sediment of the Atlantic Southern Ocean

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Kretschmer, S. , Geibert, W. , Rutgers v. d. Loeff, M. , Schnabel, C. , Xu, S. and Mollenhauer, G. (2011): Fractionation of 230Th, 231Pa, and 10Be induced by particle size and composition within an opal-rich sediment of the Atlantic Southern Ocean , Geochimica et Cosmochimica Acta, 75 (22), pp. 6971-6987 . doi: 10.1016/j.gca.2011.09.012
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This study centers on the question: How sensitive are 231Pa/230Th and 10Be/230Th to sediment composition and redistribution? The natural radionuclides 231Pa, 230Th and 10Be recorded in deep sea sediments are tracers for water mass advection and particle fluxes. We investigate the influence of oceanic particle composition on the element adsorption in order to improve our understanding of sedimentary isotope records. We present new data on particle size specific 231Pa and 10Be concentrations. An additional separation step, based on settling velocities, led to the isolation of a very opal-rich phase. We find that opal-rich particles contain the highest 231Pa and 10Be concentrations, and higher 231Pa/230Th and 10Be/230Th isotope ratios than opal-poor particles. The fractionation relative to 230Th induced by the adsorption to opal-rich particles is more pronounced for 231Pa than for 10Be. We conclude that bulk 231Pa/230Th in Southern Ocean sediments is most suitable as a proxy for past opal fluxes. The comparison between two neighboring cores with rapid and slow accumulation rates reveals that these isotope ratios are not influenced significantly by the intensity of sediment focusing at these two study sites. However, a simulation shows that particle sorting by selective removal of sediment (winnowing) could change the isotope ratios. Consequently, 231Pa/230Th should not be used as paleocirculation proxy in cases where a strong loss of opal-rich material due to bottom currents occurred.

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