Oceanic 232Th: A reconnaissance and implications of global distribution from manganese nodules

Abstract

232Th concentrations in seawater reported in the literature since 1961 are compiled and evaluated, and new data are presented. In general, recent data are lower than old ones by 1 to 2 orders of magnitude, with nearly all post-1980 data lower than 0.1 dpm/10 3−1 and pre-1980 values higher than 0.1 dpm/10 3−1. We believe this is caused by improvements in sampling and analytical techniques. Reliable profiles of 232Th in coastal and open ocean waters have been measured in recent years, some by the newly developed neutron activation analysis and mass spectrometry techniques. The data indicate external sources of 232Th to the surface and its scavenging throughout the water column. We suggest that aeolian input dominates in the open ocean; but, fluvial input may be more important in coastal regions if mechanisms exist for recycling Th from estuarine sediments. Some information on the global distribution pattern and marine geochemistry of 232Th in seawater can be inferred from initial 230Th/ 232Th activity ratios observed on surfaces of manganese nodules and crusts. The rather distinct interocean variation of the 230Th/ 230Th activity ratio in the central gyre region (averaging ~ 160 in the South Pacific, ~ 60 in the North Pacific and ~ 17 in the Atlantic) may reflect higher aeolian input of materials (including 232Th) to the Atlantic and to the northern hemisphere. Superimposed on the large-scale global trend are some small-scale variations which are indicative of the following effects: (1) abrupt change of water depth, and hence 230Th concentration, at regions of irregular seafloor topography; (2) higher 232Th input at ocean boundaries; and (3) enhanced supply of 232Th from hydrothermal vents. It is still unclear whether seafloor sediments act as a net source or sink of dissolved 232Th (and 230Th) in bottom waters. Pore water study should be undertaken to address this important issue. This task can be accomplished by the mass spectrometric technique.