Determination of particulate and dissolved ²²⁸Th in seawater using a delayed coincidence counter

The application of thorium-228 towards understanding particle dynamics in the open ocean is limited because of its lownatural abundance in seawater and associated sampling and analytical challenges. Herewe describe a fast and nondestructivemethod for measuring both dissolved and particulate ²²⁸Th activities in the open ocean using Radium Delayed-Coincidence Counters (RaDeCC). Particulate and dissolved samples were collected from the upper 1000mof the Sargasso Sea water column during the US GEOTRACES intercalibration cruise using large volume in situ pumps equipped with Quartz microfiber filters and MnO₂ impregnated cartridges. Samples were directly counted on the RaDeCC systemusing a custommachined filter sample holder and a commercially available cartridge holder followed by traditional alpha counting. The two methods were found to be in good agreement (r² = 0.95). We also applied this method to particulate and dissolved samples collected from a US GEOTRACES station near the Cape Verde islands. The particulate and dissolved ²²⁸Th distributions were consistent with the expected pattern of upper ocean scavenging and removal of ²²⁸Th with activities varying between 0.02 and 0.06 dpm 100 L^-1 (particulate) and 0.04 and 0.65 dpm 100 L^-1 (dissolved) over the upper 1000 m of the water column.