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Two trenches off Japan were explored during DSDP Leg 87. One is the Nankai Trough and the other is the Japan Trench; Site 582 is located on the floor of the former and Site 584 is situated on the deep-sea terrace of the latter. Cores from Site 582 and 584 consist mainly of hemipelagic sediments and diatomaceous silts and mudstone, respectively. In this report we analyze the chemistry of the interstitial water and sediments, as well as the sediment mineralogy. Sulfate reduction is accompanied by the production of secondary pyrite, which is rich in the sediment at both sites. Dissolved Ca concentration is relatively low and changes only slightly at both sites, probably because of the formation of carbonate with high alkalinity. Concentrations of dissolved Mg decrease with depth at Site 584. The dissolved Mg depletion probably results from the formation of Mg-rich carbonate and/or ion exchange and reaction between interstitial water and clay minerals. Higher Si/Al values are due to biogenic opal in the sediments and roughly correlate with higher values of interstitial water SiO2. Increases in dissolved Li concentrations may be related to its release from clay minerals, to advection that results from dewatering, and/or to fluid transport. INTRODUCTION The chemical composition of interstitial waters of marine sediments may be modified by many processes, particularly diagenetic reactions and diffusive and advective transport of dissolved constituents. The qualitative and quantitative estimation of the above-mentioned processes is difficult because of the complexity and variation of the reactants, the multitude of possible reactions, and the scatter of sediment physical properties data. Only Perry and others (1976) and Lawrence and others (1975) clarified the relation between interstitial water and sediments with respect to Mg, Ca, and oxygen isotope by means of the precise analysis of both phases. In addition, a few studies have been done on samples from active margins, where sediment accumulation rates are very high and many faults and fissures exist (Moore and Gieskes, 1980). DSDP Leg 87 offered an opportunity to study the diagenesis of terrigenous sediments along an active margin. This report centers on the interstitial water chemistry of Sites 582 and 584 (Fig. 1) and incorporates information regarding sediment chemistry and mineralogy. SAMPLING AND ANALYTICAL PROCEDURES Sampling Samples of interstitial water were collected at all of the Leg 87 sites by standard shipboard squeezing techniques, and several samples were collected by means of the IPOD in-hole sampler, a device that collects both water for chemical analysis in a stainless tubular coil and water for gas analysis in a connected copper coil. Kagami, H., Karig, D. E., Coulbourn, W. T., et al., Init. Repts. DSDP, 87: Washington (U.S. Govt. Printing Office). 2 Addresses: (Kawahata) Marine Geological Department, Geological Survey of Japan 11-3 Higashi Yatabe, Tsukuba, Ibaraki 305, Japan; (Fujioka and Ishizuka) Ocean Research Institute, University of Tokyo, Minamidai, 1-15-1, Nakanoku, Tokyo 164, Japan. 40°N —

Sediments and Interstitial Water at Sites 582 and 584, the Nankai Trough and the Japan Trench Landward Slope