90Sr in seawater of the East China Sea: Inventory, new potential source, and environmental implications

Abstract

90Sr is useful for tracing water mass movement in oceans. We collected a suit of seawater samples from the East China Sea (ECS) in the May 2011 to investigate the spatial and vertical distribution of 90Sr and to understand its transportation and fate. To understand the sources and transportation of 90Sr more clearly, published 137Cs data from the same cruise were used to obtain the 90Sr/137Cs activity ratios. The results showed that 90Sr activities were controlled by the circulation system of the ECS, with high values in coastal regions and low values in oceanic waters. The plume with a high 90Sr/137Cs ratio showed that in late spring, the Changjiang Diluted Water could flow southeastward and extend to 126–127° E, which is farther than the previously known value of 124° E. The high 90Sr/137Cs ratios (1.35 ± 0.62) and a long effective half-life of 90Sr (20.0 ± 0.3 y) in the ECS surface water revealed that 90Sr is surplus in comparison with 137Cs. However, historical variations in the 90Sr/137Cs ratio seem to preclude the simple explanation that riverine input causes a 90Sr surplus in the ECS. Groundwater discharge with a high 90Sr but very low 137Cs may be a new potential source. However, it is difficult to quantify the contribution of groundwater discharge at present, and more detailed studies are required in this regard. Additionally, we compiled 90Sr and 137Cs water column inventory data in the western North Pacific and found that the cumulative fallout onto the ocean was different from that on land in the 20–40° N band.