The influence of particle composition and particle flux on scavenging of Th, Pa and Be in the ocean

We have examined the relative affinity of Th, Pa and Be for sorption from seawater onto particles of variable composition (opal, carbonate, lithogenic particles and organic carbon). Nuclide concentrations in particles collected from time-series sediment traps were normalized by the dissolved nuclide concentration in the overlying water column in order to compute partition coefficients under conditions spanning a wide range of particle flux and particle composition. Our results suggest that the affinity of particles for Pa and Be increases with their increasing opal content and decreasing carbonate content, while the affinity of particles for Th increases with increasing carbonate content, and decreases with increasing opal content. We find no correlation between the aluminosilicate content of particles and their affinity for scavenging of any of these elements. Extrapolating to a pure CaCO₃ end member, the partition coefficient for Th (9.0×10⁶ g g⁻¹) is ∼40 times greater than for Pa, and roughly 100 times greater than for Be, whereas for a pure opal end member, the partition coefficient for Th (3.9×10⁵ g g⁻¹) is slightly less than that for Pa and Be. Partition coefficients decrease with increasing particle flux in open-ocean settings, but not in an ocean-margin region. This kinetic effect reflects the increasing contribution of unaltered surface material reaching the sediment traps as particle flux increases. The degree of fractionation between Pa and Th and between Be and Th depends on the opal:carbonate rain ratio. These results challenge the use of sedimentary ²³¹Pa/²³⁰Th and ¹⁰Be/²³⁰Th ratios as simple proxies of particle flux. However, the strong dependence of nuclide scavenging on the opal:carbonate rain ratio may provide a needed tool for reconstructing past changes in planktonic community composition.