Anomalous large scavenging of Th-230 and Pa-231 controlled by particle composition in the northwestern North Pacific

Abstract

We report the role of particle composition and lateral particle movement that influences the oceanic distribution of Pa-231 and Th-230. Settling particles were collected during sediment trap experiments. These and surface sediments were obtained from five stations along 38 to 44degreesN 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 Th-230(ex) fluxes that exceed the local rate of supply. The lithogenic materials are important as a carrier of Th-230 and contribute to the fractionation between Th-230 and Pa-231 in the ocean, as the Pa-231(ex)/Th-230(ex) ratio in the settling particles decreases with increasing Th-232 concentration. The Pa-231(ex)/Th-230(ex) 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 Pa-231(ex)/Th-230(ex) ratio in a population of settling particles and remineralization. This indicates that lateral redistribution of particles and sediment focusing influence the Pa-231(ex)/Th-230(ex) ratios in surface sediments. Thus, the observations reported here mean that the use of the sediment Pa-231(ex)/Th-230(ex) ratio as a paleoproductivity proxy will be problematic in the northwestern North Pacific.