(Table 1) Oxygen isotoic composition of interstitial watres from ODP Sites 128-798 and 128-799

Oxygen isotope compositions of the interstitial waters have been measured for 21 samples taken from the depth intervals of 1.5 to 398.9 mbsf at Site 798 (Oki Ridge) and 16.5 to 435.6 mbsf at Site 799 (Kita-Yamato Trough) in Japan Sea. The d18O values decrease with depth from -0.49 to -3.38 per mil (SMOW) at Site 798 and from -0.71 to -4.36 per mil (SMOW) at Site 799 corresponding to an average depletion gradient of -0.8 per mil per 100 m. Material balance calculations reveal that the d18O-variations at Sites 798 and 799 were principally controlled by low-temperature alteration of basement basalt and andesite, resulting in negative shifts in pore water d18O values, and by the polymorphic transformations of biogenic opal-A to opal-CT and opal-CT to microquartz, which tend to increase d18O of interstitial waters. Carbonate diagenesis and ash alteration also caused weakly negative shifts in pore water d18O values.