Journal of Oceanography, Vol. 59 (No. 5), pp. 739-750, 2003
Hisashi Narita1*, Ryota Abe1, Kazuhisa Tate2, Young-Ill Kim2, Koh Harada2 and Shizuo Tsunogai1
1Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
2Department of Chemistry, Faculty of Fisheries, Hokkaido University, Hakodate 041-0821, Japan
(Received 26 July 2002; in revised form 22 April 2003; accepted 23 April 2003)
Abstract: We report the role of particle composition and lateral particle movement that influences the oceanic distribution of 231Pa and 230Th. Settling particles were collected during sediment trap experiments. These and surface sediments were obtained from five stations along 38 to 44°N in the northwestern North Pacific. The high total mass flux and seasonal variations in the marginal area of the western North Pacific are controlled by the supply of lithogenic materials and primary productivity. The high content of the lithogenic material in the settling particles in this area contributes to 230Thex fluxes that exceed the local rate of supply. The lithogenic materials are important as a carrier of 230Th and contribute to the fractionation between 230Th and 231Pa in the ocean, as the 231Paex/230Thex ratio in the settling particles decreases with increasing 232Th concentration. The 231Paex/230Thex ratio in the settling particles collected in the abyssal basin decreases with water depth, which indicates that lateral transport of the lithogenic particles from the marginal area and/or shallower depth plays an important role in determining the 231Paex/230Thex ratio in a population of settling particles and remineralization. This indicates that lateral redistribution of particles and sediment focusing influence the 231Paex/230Thex ratios in surface sediments. Thus, the observations reported here mean that the use of the sediment 231Paex/230Thex ratio as a paleoproductivity proxy will be problematic in the northwestern North Pacific.