Isotopic composition of diagenetic silica and associated pore waters of Japan Sea sediments

Over a broad region of the eastern Japan Sea, Neogene opaline diatomaceous sediments alter with depth to hard porcellanites and cherts composed of opal-CT and quartz. We examined the oxygen isotopic compositions of these diagenetic silica minerals at four widely spaced sites occupied during ODP Leg 127 in order to investigate the thermal history of the region. Formation temperatures computed from these isotopic data range from 22° to 68°C for opal-CT and from 44° to 92°C for diagenetic quartz, quite similar to temperature ranges estimated from the extrapolated modern gradients, 36°-43°C and 49°-64°C, respectively. At each site the isotopic temperature values cluster near the extrapolated ambient sediment temperatures. As a first approximation, the similarities suggest that the positions of the silica transformations in the basin are controlled by the present thermal regime. In detail, isotopic and ambient temperatures differ. If these differences are real, then they reflect variations in the thermal histories at these sites. At Sites 794 and 797 in the Yamato Basin, isotopic temperatures and gradients computed from these data are lower than or comparable to ambient temperatures and gradients. We suggest that the silica zones have roughly equilibrated with the modern gradients at these localities. At Site 795 in the Japan Basin, isotopic temperatures are also lower than ambient sediment temperatures at comparable depths, but the gradient computed from the isotopic temperatures is higher than the present measured gradient. For both scenarios to hold, the silica zones must have formed under initially high gradients during the early post-rift period at this locality. These zones were then rapidly buried and have yet to equilibrate with the modern lower gradient. At Site 796 on Okushiri Ridge, isotopic temperatures exceed present temperatures as expected for an area of recent uplift. The gradient computed from our isotopic data and the thickness of the opal-CT zone indicate a higher gradient than at present at this site, apparently reflecting higher heat fluxes during the early post-rift period or recent frictional heating from nearby reverse fault activity.