Testing a new small-volume technique for determining ²³⁴Th in seawater

Abstract

Thorium-234 is a naturally occurring radionuclide that has been widely studied in ocean sciences. These studies use the disequilibrium between the soluble parent uranium-238 (T _1/2 = 4.5·10⁹ y), and its particle reactive daughter, ²³⁴Th (T _1/2 = 24.1 d), to quantify the in-situ removal rates of ²³⁴Th on sinking particles. Here, we present additional experiments that test a new 2-l procedure in which ²³⁴Th is co-scavenged with a MnO₂ precipitate. Unlike other techniques, this method can be easily applied at-sea with an overall precision and accuracy of ≤5%. Our experiments have sought to elucidate the effects of delaying reagent addition and precipitate filtration, differences in sample bottle types, and issues related to sample backgrounds and ²³⁴Th particulate sampling. Most of these experiments were conducted using water collected on repeated occupations of station ALOHA (22°45.0'N, 158°00.0'W), 100 km North of Oahu, Hawaii.