Fractionation of ²³⁰Th, ²³¹Pa, and ¹⁰Be induced by particle size and composition within an opal-rich sediment of the Atlantic Southern Ocean
Kretschmer, Sven | Geibert, Walter | Rutgers van der Loeff, Michiel M. | Schnabel, Christoph | Xu, Sheng | Mollenhauer, Gesine
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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