Thorium-232 in the eastern Caribbean Sea

Abstract

The radiogenic Th isotopes (²³⁴Th, ²³⁰Th and ²²⁸Th) are produced in situ in the ocean from their dissolved progenitors (²³⁸U, ²³⁴U and ²²⁸Ra, respectively). These parent–daughter pairs have extensively been used to study the rate and mechanism of chemical scavenging, a process that is believed to be important in controlling the distributions and concentrations of many trace metals, such as Be, Al, Cr, Mn, Fe, Co, Cu, Pb and the rare earths^1–12. However, it is difficult to make useful direct comparisons between the distributions of the radiogenic Th tracers and those of trace metals, because their input pathways to the ocean are very different. To provide a link, one needs to determine the water-column distribution of the dominant isotope ²³²Th, which is non-radiogenic and is thus delivered to the ocean by the same fluvial and aeolian pathways as most trace metals. However, with the exception of the mar-bottom measurements recently reported by Nozaki and Horibe¹³, little is known of the concentration level of ²³²Th or its distribution pattern in the ocean. We report here the first profile showing the complete distribution of ²³²Th in an oceanic water column from the Venezuelan Basin. The data were obtained by a sensitive neutron activation analysis method newly developed for this project. The profile shows relatively high concentrations in the upper 200 m with a maximum of 87 d.p.m./10⁶ kg near the base of the mixed layer. Average concentrations in deeper water (2.7 d.p.m./10⁶ kg) are uniform and a typically low compared with open-ocean deep waters, perhaps because of unusually rapid scavenging in the restricted basin. The source of the ²³²Th in the surface waters is not known, but rivers appear to be more likely candidates than the atmosphere.