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ABSTRACT: Chemical Composition of Interstitial Waters from the Japan Sea, ODP Leg 128

A. Sturz, M. Von Breymann, R. Dunbar

During ODP Leg 128, interstitial waters were recovered from Oki Ridge (Site 798) and Kita-Yamato Trough (Site 799) sediment, Sea of Japan. Interstitial water chemical composition reflects diagenetic processes. Evidence indicating organic matter degradation processes includes sulfate depletion, high ammonium concentrations, and shallow maxima of dissolved phosphate. Rapid alkalinity increases in the uppermost sections of the sediments are accompanied by decreases in dissolved calcium, reflecting inorganic calcite precipitation. Authigenic dolomitization results in changes in slopes of the Mg/Ca molar ratios with depth. The
opal-A/opal-CT transition is documented by the concentration depth profiles of dissolved silica and lithium. Dolomitization precedes the opal-A/opal-CT transition at both sites. Kita-Yamato Trough sediments show an abrupt change in the compositional character of the pore fluids below 435 mbsf, which coincides with the occurrence of low porosity and high bulk density layers composed of dolomite and opal-CT. These layers impede to some extent diffusional communication with the overlying interstitial waters. The interstitial waters in sediments below 435 mbsf have chloride concentrations of 504-515 mM, significantly lower than that of modern day Japan Sea water (540 mM). The presence of low chloride waters within Miocene age sediments may indicate: (1) diagenetic reactions that involve the elease of exchangeable and structural bound water from clay minerals and/or opal-A, (2) Miocene connate brackish lake water, (3) phase separation of hydrothermal fluids associated with rifting, (4) potential effects of clay membrane filtration in a high pressure zone.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990