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. d18O ranges from -0.81 to +5.15 per mil for calcite and +2.00 to +7.29 per mil for dolomite. d13C values vary widely between -36.96 and + 11.45 per mil for calcite and between -20.99 and +4.75 per mil for dolomite. Siderite has -1.90 per mil d18O and +0.03 per mil d13C. The depth of burial at which these carbonates precipitated is estimated using the "oxygen isotope thermometer" on the basis of d18O 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 IIa (20-250 m sub-bottom depth, magnesian calcite), Stage IIb (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 d13C of carbonates reflects the changing origin of CO2; oxidation and sulfate reduction at Stages I and IIa; and methane fermentation at Stages IIb, III, and IV.

DEPTH, sediment [m] is depth of occurrence, 0 = vein.

Supplement to: Matsumoto, Ryo; Matsuhisa, Yukihiro (1986): Chemistry, carbon and oxygen isotope ratios, and origin of deep-sea carbonates at Sites 438, 439, and 584: Inner slope of the Japan Trench. In: Kagami, H; Karig, DE; Coulbourn, WT; et al. (eds.), Initial Reports of the Deep Sea Drilling Project, Washington (U.S. Govt. Printing Office), 87, 669-678