ABSTRACT

Data obtained by recent sediment-trap experiments were compiled to discuss the mechanism for the removal of trace metals from the ocean. The vertical-change index (I_M), or the rate constants of first-order change with depth, were estimated. The results and the I_M values in km^-1 shown in parentheses are as follows. The total particulate matter flux(-0.05) is nearly constant with depth due to rapid sinking of its major components of tests of biogenic silica(-0.03) and calcium carbonate(-0.05) to the bottom. The organic fraction(-0.27) of settling particles reduces with depth together with components contained in soft tissue of organisms such as Cd(-0.23). The fluxes of the lithophile components of Al(+ 0.22), Fe(+0.17), and ²³²Th(+0.19) largely increase with depth due to the lateral transport of particles from the continental margin. The large index of Mn(+0.47) is due to the early diagenetic process in the surface sediment in addition to the simple resuspension. The flux of short-lived ²³⁴Th(-0.01) is saturated, but that of ²²⁸Th(-0.14) having no substantial source in the pelagic ocean is the smallest among radiogenic nuclides in seawater. The largest index is found for the long-lived radionuclides such as ²³⁰Th(+0.50) and ²³¹Pa(+0.56). The effect of radioactive decay is somewhat reflected in the indices of ²¹⁰Pb(+0.41) and ²¹⁰Po(+0.43). These results support the train-passengers model proposed by Tsunogai (1987) for the behavior of chemical elements in the ocean.