Control of Pa/Th ratio by particulate chemical composition in the ocean

Abstract

Controlled laboratory experiments were carried out using pure sorbents to examine the interactions of ²³⁴Th (or ²³³Pa) with particle surfaces and whether there is a fractionation between ²³⁴Th and ²³³Pa by particulate chemical composition. Our results show that the adsorption of ²³³Pa and ²³⁴Th on particle surfaces is highly selective and compound specific, resulting in a fractionation of ²³³Pa/²³⁴Th ratio, especially on SiO₂ (K_d ratio, K_d‐233Pa/K_d‐234Th, = 10) and CaCO₃ (K_d ratio, K_d‐233Pa/K_d‐234Th, = 0.01) surfaces. These new results agree well with previously published field observations that showed an increase of ²³¹Pa/²³⁰Th ratio (or K_d of ²³¹Pa) with increasing opal abundance and decreasing carbonate content in oceanic particles. In addition, they highlight the complications of using ²³¹Pa/²³⁰Th ratio as a proxy and have implications for both oceanography and trace element geochemistry although more studies are needed to further examine the influence of natural organic matter on the fractionation of Th/Pa ratios.