Alpha-emitting thorium isotopes in northwest Pacific deep waters

The vertical distributions of 232Th, 230Th, 228Th and 227Th were determined in seawaters deeper than 3900 m in the western North Pacific by employing moored manganese-impregnated fiber method. 232Th shows an increase in its concentration toward the bottom, indicating the existence of a source of Th at the seafloor. Based on a one-dimensional vertical mixing model and the assumption that 228Th is supported by 228Ra, the apparent vertical diffusion coefficient, K was estimated to be from 20 to 60 cm 2/s with a mean value of 34 cm 2/s for the bottom 2000 m. A reversible scavenging model described earlier was successfully applied to 230Th and 232Th profiles, if the K values consistent with the above estimates are used. Thus, the apparent vertical diffusion coefficient in the bottom water may indeed be at least one order of magnitude greater than the value of 1 cm 2/s for the deep water between 1 and 4 km based on radiocarbon profiles. Conjunction of the two different K values for the deep and bottom waters may be tested if an additional radioactive tracer with an appropriate half-life is available. The best candidate is 227Ac with a half-life of 22 years which seems to be supplied from sediments to the overlying water much like Ra isotopes as inferred from the vertical distribution of 227Th.