16. CHEMISTRY, CARBON AND OXYGEN ISOTOPE RATIOS, AND ORIGIN OF DEEP-SEA

Carbonate concretions in Pliocene to Miocene diatomaceous sediments at Sites 438, 439, and 584 on the deep-sea terrace near the Japan Trench are composed of calcite, dolomite, and siderite with varying degrees of chemical substitu-tion. δ 1 8 θ ranges from-0.81 to +5.15 ‰ for calcite and +2.00 to +7.29 ‰ for dolomite. δ13C values vary widely be-tween-36.96 and + 11.45 ‰ for calcite and between-20.99 and +4.75 ‰ for dolomite. Siderite has- 1.90 ‰ δ 1 8 θ and +0.03 ‰ δ13C. The depth of burial at which these carbonates precipitated is estimated using the "oxygen isotope thermometer " on the basis of δ 1 8 θ values of carbonates and the interstitial water, and the geothermal gradients mea-sured at present. Based on the estimated depth of formation of carbonates, four stages of carbonate authigenesis are defined, Stage I (0-20 m sub-bottom depth, calcian dolomite), Stage I la (20-250 m sub-bottom depth, magnesian cal-cite), Stage lib (250-530 m sub-bottom depth, ferroan calcite), Stage III (530-590 m sub-bottom depth, ferroan dolo-mite), and Stage IV (590 m to total depth, siderite). The progressive changes in mineralogy and chemistry of authigenic carbonates are well matched with the variation of the chemical composition of the interstitial water with increasing depth. The fluctuation of δ13C of carbonates reflects the changing origin of CO2; oxidation and sulfate reduction at Stages I and Ila; and methane fermentation at Stages lib, III, and IV