230Th in the eastern South Pacific Ocean: Boundary scavenging and bottom scavenging by metal oxides derived from hydrothermal vents

The vertical distribution of total 230Th was investigated in the eastern South Pacific Ocean. Among the stations studied, 230Th inventories in the depth range 1000‒2500 m generally decrease with increasing net primary productivity and export production, which implies that the spatial variability of particle flux controls the 230Th distribution. On the other hand, at equatorial stations, 230Th inventories in the depth range 0‒1000 m were similar to those found at stations in lower productivity regions, and the effect of lateral input of 230Th by equatorial zonal flow was considered. The large-scale bottom scavenging effect on 230Th in deep layers arising from hydrothermal activity in the vicinity of the East Pacific Rise was observed at stations in the Southwest Pacific Basin and the Peru Basin. Corresponding to the pattern seen in the map of iron and manganese oxide content of surface sediment, the observed 230Th depletion was higher in the Peru Basin than the Southwest Pacific Basin, suggesting that these oxides contribute to the observed bottom scavenging.