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

Thorium-234 is a naturally occurring radionuclide that has been widely stud ied in ocean sciences. These studies use the disequilibrium between the sol uble parent uranium-238 (T-1/2 = 4.5(.)10(9) y), and its particle reactive daughter, Th-234 (T-1/2 = 24.1 d), to quantify the in-situ removal rates of 234Th On sinking particles. Here, we present additional experiments that t est a new 2-1 procedure in which 234Th is co-scavenged with a MnO2 precipit ate. Unlike other techniques, this method can be easily applied at-sea with an overall precision and accuracy of less than or equal to5%. Our experime nts have sought to elucidate the effects of delaying reagent addition and p recipitate filtration, differences in sample bottle types, and issues relat ed to sample backgrounds and Th-234 particulate sampling. Most of these exp eriments were conducted using water collected on repeated occupations of st ation ALOHA (22 degrees 45.0'N, 158 degrees 00.0'W), 100 km North of Oahu, Hawaii.