Grain size effects on 230Th xs inventories in opal-rich and carbonate-rich marine sediments

Abstract

Excess Thorium-230 ( 230Th xs) as a constant flux tracer is an essential tool for paleoceanographic studies, but its limitations for flux normalization are still a matter of debate. In regions of rapid sediment accumulation, it has been an open question if 230Th xs-normalized fluxes are biased by particle sorting effects during sediment redistribution. In order to study the sorting effect of sediment transport on 230Th xs, we analyzed the specific activity of 230Th xs in different particle size classes of carbonate-rich sediments from the South East Atlantic, and of opal-rich sediments from the Atlantic sector of the Southern Ocean. At both sites, we compare the 230Th xs distribution in neighboring high vs. low accumulation settings. Two grain-size fractionation methods are explored. We find that the 230Th xs distribution is strongly grain size dependent, and 50–90% of the total 230Th xs inventory is concentrated in fine material smaller than 10 µm, which is preferentially deposited at the high accumulation sites. This leads to an overestimation of the focusing factor Ψ, and consequently to an underestimation of the vertical flux rate at such sites. The distribution of authigenic uranium indicates that fine organic-rich material has also been re-deposited from lateral sources. If the particle sorting effect is considered in the flux calculations, it reduces the estimated extent of sediment focusing. In order to assess the maximum effect of particle sorting on Ψ, we present an extreme scenario, in which we assume a lateral sediment supply of only fine material (< 10 µm). In this case, the focusing factor of the opal-rich core would be reduced from Ψ = 5.9 to Ψ = 3.2. In a more likely scenario, allowing silt-sized material to be transported, Ψ is reduced from 5.9 to 5.0 if particle sorting is taken into consideration. The bias introduced by particle sorting is most important for strongly focused sediments. Comparing 230Th xs-normalized mass fluxes biased by sorting effects with uncorrected mass fluxes, we suggest that 230Th xs-normalization is still a valid tool to correct for lateral sediment redistribution. However, differences in focusing factors between core locations have to be evaluated carefully, taking the grain size distributions into consideration.