On the importance of opal, carbonate, and lithogenic clays in scavenging and fractionating 230Th, 231Pa and 10Be in the ocean

The influence of particle composition on the scavenging of 230Th, 231Pa, and 10Be in the ocean is critically re-examined based on the particle-solution distribution coefficient (Kd) data reported by Chase et al. [Earth Planet. Sci. Lett. 204 (2002) 215–229]. Our re-examination shows that the Kds for these radionuclides are strongly influenced by the amount of lithogenic material (lithogenics) in particulates as reported previously by us, and that a strong correlation between the lithogenic and carbonate contents in open-ocean particulates results in an apparent, non-linear correlation between %CaCO3 and Kd for each of the three radionuclides. Using a three-end-member model, we estimate that the respective Kds (g/g) of 230Th, 231Pa, and 10Be in the open ocean are: 2.3×108, 1.2×107, and 3.8×106 for lithogenics; 1.0×106, 1.0×105, and 3.0×104 for carbonate; and 2.5×105, 1.2×106, and 7.5×105 for opal, suggesting that lithogenics have a much stronger affinity for any of the three nuclides than does carbonate or opal. Lithogenics and carbonate preferentially scavenge 230Th over 231Pa and 10Be, whereas opal slightly favors 231Pa and 10Be over 230Th. Relative to lithogenics, carbonate plays a very limited role in fractionating 230Th, 231Pa, and 10Be due to its orders-of-magnitude smaller Kd values for the three nuclides. Our results indicate that fractionations between Pa and Th and between Be and Th through particle scavenging are mainly determined by the opal-to-lithogenic ratio in particulates, rather than the opal-to-carbonate ratio suggested by Chase et al.