230Th and 232Th distributions in mid-latitudes of the North Pacific Ocean: Effect of bottom scavenging

Vertical distributions of total Th-230 and dissolved Th-232 were determined in the mid-latitudes of the North Pacific Ocean (8 degrees N, 95 degrees W-40 degrees N, 160 degrees E). The observed Th-232 concentrations are some of the lowest values reported for oceans. Because of low biogenic particle flux, high Th-230 activity is exhibited in the central gyre. Most vertical distributions of Th-230 in the Pacific Ocean have been described well using the reversible-scavenging model. However, we observed the depletion of total Th-230 compared with a reversible scavenging model prediction for the deep layers in the mid-latitudes of the North Pacific Ocean. Considering the time-scale of the scavenging residence time of Th, horizontal eddy diffusion from the higher particle flux region could not have a major influence on Th-230 depletion in deep waters. The observed depletion is partly explained by lateral advection, adopting the scavenging-mixing model. Additionally, a bottom scavenging process above the seafloor and a diffusion process are considered to explain the Th-230 deficit. Taking into account the time-scale of the scavenging residence time of Th in deep waters ( < 100 yr), the vertical diffusion process could reach 800-1600 m above the seafloor. The enhanced vertical eddy diffusion derived from the complex topography of the western Pacific Ocean might promote bottom scavenging and transport of the low Th-230 in deep water to the upper layers. (C) 2012 Elsevier B.V. All rights reserved.