CLAY FABRIC OF HEMIPELAGITES RECORDING OXIC-ANOXIC BOTTOM WATER FLUCTUATION IN SEA OF JAPAN

The Late Quaternary deep-sea deposits of Sea of Japan are characterized by alternation of cm–m thick light and dark colored muddy deposits. In general, it is interpreted that the alternating beds reflect fluctuation of bottom water environments represented by oxic–anoxic conditions. This study focused on microscale characterization of the Late Quaternary hemipelagites off Noto Peninsula, Sea of Japan. Samples were obtained by a piston corer at 693 m water depth during YK10-07 cruise by R/V Yokosuka of JAMSTEC. Muddy deposits of 5–6 m core depth within a 10-m-long core are characterized by alternation of bioturbated light color intervals (10–20 cm thick) and less bioturbated dark color intervals (10–30 cm thick). A black color interval (40.5 cm thick) with lamination also develops in part. It is interpreted that the alternating beds have been resulted from fluctuations of oxic and suboxic bottom water conditions on the basis of the previous studies. Furthermore, the black color interval is interpreted to be formed in anoxic bottom water condition during the Last glacial maximum. In general, wet muddy deposits are necessary to dry for SEM observations. Thus, we adopted an alcohol-replacing method: (1) Wet samples were gradually dehydrated by using ethyl alcohol for 30 days. (2) The ethyl alcohol was replaced by t-butyl alcohol for 60 days. (3) Samples were frozen by using liquid nitrogen. (4) The frozen samples were vacuum dried in less than 5°C environment. The vertical sections of the dried samples were used for SEM observations. As a result, three types of clay fabric were distinguished: (1) Light-colored mud is characterized by random-oriented particles associated with biogenic fragments. (2) Dark-colored mud is characterized by random-oriented particles. (3) Black-colored mud is characterized by preferred-oriented particles parallel and sub-parallel to bedding plane. The difference of clay fabric among those samples attributes to oxic–anoxic conditions. Random-oriented clay particles of light and dark colored mud are interpreted to be formed from settling of flocculated-clay particles. In contrast, black-colored mud is interpreted to be formed from dispersed clay particles. Therefore, these clay fabrics are likely to be useful tools for distinguishing oxic–anoxic condition of paleo deep-sea environments.