Journal of Oceanography, Vol. 53 (No. 6), pp. 553-562, 1997
Hideki Minami and Yoshihisa Kato
School of Marine Science and Technology, Tokai University, Shimizu, Shizuoka 424, Japan
(Received 11 October 1996; in revised form 30 April 1997; accepted 30 May 1997)
Abstract: This study investigates the diagenetic remobilization of As in sediments collected from two different regions. One is the Hidaka Trough under the eutrophic Oyashio water, and another is the continental slope to the Shikoku Basin under the oligotrophic Kuroshio water. The concentrations of total dissolved As (AsIII+ AsV) in pore waters from the Hidaka Trough and the continental slope off Shikoku increase with core depth. The average concentrations in the top 2 cm layer are several times higher than those in the overlying bottom water. Consequently, dissolved As diffuses upward across the sediment-water interface due to the concentration gradient. In contrast, in the Nankai Trough and Shikoku Basin, the concentrations of total dissolved As in the pore waters of the surface sediments are rather low relative to those in the bottom sea water. This fact suggests that the diffusion of dissolved As takes place in the opposite direction, viz., into the pore water. The total sedimentary As mean concentrations in the top 2 cm layer of each core from the Oyashio transect decrease with the depth of water at the sampling site (i.e., with distance from the coast). On the other hand, the concentration in the Kuroshio transect increases with the water depth, the maximum appearing in the middle of the continental slope, as with reducible Mn. Consequently, we infer that the behavior of As is influenced by the organic carbon content reaching the seafloor in the eutrophic Oyashio transect. In particular As in surfacial sediments is transported out of the continental margin toward the ocean floor through the near-benthic boundary layer with Mn and Fe oxides in the oligotrophic Kuroshio transect. In the Kuroshio transect, the behavior of As is influenced by the fate of both Fe and Mn oxides in sediments. Recycling and remobilization of As in the seafloor of the continental slope, trough, and abyssal basin are controlled by the flux and decomposition of organic matter. The formation and dissolution of metal oxides also play an important role in determining the behavior of As.