The thorium isotope content of ocean water

²³²Th concentrations of surface and deep Pacific Ocean waters are 0.01–0.02 dpm/1000 kg (60 pgm/kg). The²³⁰Th activity is 0.03–0.13 dpm/1000 kg in surface waters and 0.3–2.7 dpm/1000 kg in deep waters. Chemical residence times based on in situ production from parent isotopes are about the same for²³⁰Th and²²⁸Th in surface waters (1–5 years) but are ten times greater for²³⁰Th in deep waters (10–100 years). Apparently there are additional sources of²³⁰Th into deep waters. At MANOP site S manganese nodule tops are enriched in Th isotopes by adsorption of Th from seawater and not by incorporation of Th-rich particulates.