TRACE-METAL CONTAMINATION IN TOKYO BAY, JAPAN

Concentrations of seven trace metals (Cr, Ni, Cu, Zn, Ag, Cd and Pb) in the Tokyo Bay surface sediment were determined for samples collected at 42 points in 1984-1985 and at 7 points in 1989. Significant metal accumulations were observed in the northern half part of the bay. The distribution profile indicated that majority of these metals are principally discharged from rivers located at the north-west end of the bay. However, the trace metal concentrations in sediments were relatively low around the river mouth regions, whereas they were considerably high in the upper reaches of the rivers, showing a discontinuity along the estuaries. The stable carbon isotope ratio, deltaC-13, of the sedimentary organic matter in dicated that metals in the river/estuarine sediments were deposited together with terrestrial plants debris. Meanwhile, metals in the bay sediments were primarily associated with authigenic planktonic organic matter. Possible factors controlling such dissemination of trace metals onto the bay sediments were ascribed to both the formation of the bay water/open seawater front and the loop current within the semi-closed bay. The bay-water/open seawater front (thermohaline front) developed at the mouth of the bay hinders advection of both terrigeneous and bay-born materials to the open ocean. The landward residual flow of the open seawater as well as the loop current in the bay was thought to be responsible for transportation and accumulation of the trace metal-rich fine particles at around the northern central part of the bay.