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 (K_d) data reported by Chase et al. [Earth Planet. Sci. Lett. 204 (2002) 215-229]. Our re-examination shows that the K_ds 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 K_d for each of the three radionuclides. Using a three-end-member model, we estimate that the respective K_ds (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 K_d 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.