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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 substitution. δ 1 8 θ ranges from -0.81 to +5.15‰ for calcite and +2.00 to +7.29‰ for dolomite. δC values vary widely between -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‰ δC. 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 measured 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 calcite), Stage lib (250-530 m sub-bottom depth, ferroan calcite), Stage III (530-590 m sub-bottom depth, ferroan dolomite), 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 δC 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.

Chemistry, Carbon and Oxygen Isotope Ratios, and Origin of Deep-Sea Carbonates at Sites 438, 439, and 584: Inner Slope of the Japan Trench