(Table 1) d18O of interstitial waters at DSDP Sites 87-582, 87-583, and 87-584

In the present paper, we report preliminary results on the 18O/16O ratios of the interstitial waters of the DSDP cores taken from subduction-related trenches near Japan: Sites 582 and 583 at the Nankai Trough off southwestern Japan, and Site 584 at the Japan Trench off northern Honshu, where thick piles of young sediments have accumulated. Special attention was paid to any differences in isotopic behavior of interstitial waters with different surrounding lithoiogy, the details of isotopic variation of interstitial waters in young, unconsolidated sediments, and the effects of sedimentary structural disturbance on interstitial waters.

3He/4He and 4He/20Ne ratios in pore water and gas in Pacific deep-sea sediments

We have measured the 3He/3He and 3He/20 Ne ratios of thirty-nine pore water and gas samples in deep-sea sediments collected at twelve sites on the Pacific Ocean bottom during the cruises of Deep Sea Drilling Project Legs 87, 89, 90 and 92. The 3He/4He and 4He/20Ne ratios vary from 0.000000215 to 0.00000165 and from 0.29 to 20, respectively. He in the sample is composed of four components: (1) atmospheric He dissolved in seawater; (2) atmospheric He with mantle-derived He in Pacific bottom water; (3) in situ radiogenic He in the sediment; and (4) crustal He in the basement rock. Assuming that the 20Ne contents are constant with the value of seawater, the depth variations in the 4He/20Ne ratios at five Sites, 583D, 594, 597A, 598A and 504B, may provide useful information on 4He flux at the ocean bottom. The estimated 4He fluxes vary from 2000 to 40000 atoms cm**-2 s**-1 and are one to three orders of magnitude less than those calculated from the excess He in deep ocean water. An overall similarity between the geographical distribution of the 3He/4He ratios and heat flow data is found in the study area, between the East Pacific Rise across the Pacific Ocean and the Japanese Islands. The tendency is well explained by a conventional sea-floor spreading model.