(Table 1) Carbon and oxygen isotope ratios and depth of formation of authigenic carbonates at DSDP Sites 87-584, 57-438 and 77-439

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.

日本海沟附近深海阶地的438、439及584站位上新世至中新世硅藻质沉积物中的碳酸盐结核，由方解石、白云石与菱铁矿组成，且存在不同程度的化学置换。方解石的δ¹⁸O（d18O）值以千分比（‰）计，范围为-0.81‰至+5.15‰；白云石的δ¹⁸O值则介于+2.00‰至+7.29‰之间。方解石的δ¹³C（d13C）值波动范围极广，为-36.96‰至+11.45‰；白云石的δ¹³C值则介于-20.99‰至+4.75‰之间。菱铁矿的δ¹⁸O值为-1.90‰，δ¹³C值为+0.03‰。研究基于碳酸盐的δ¹⁸O值、孔隙水数据以及当前实测的地热梯度，利用"氧同位素测温法（oxygen isotope thermometer）"估算了这些碳酸盐沉淀时的埋藏深度。基于碳酸盐形成深度的估算结果，研究将碳酸盐自生作用划分为四个阶段：阶段I（海底以下0~20m，钙白云石）、阶段IIa（海底以下20~250m，镁方解石）、阶段IIb（海底以下250~530m，铁方解石）、阶段III（海底以下530~590m，铁白云石）以及阶段IV（海底以下590m至总钻深，菱铁矿）。自生碳酸盐的矿物学与化学特征的渐进变化，与孔隙水化学成分随深度增加的变化规律高度吻合。碳酸盐的δ¹³C值波动反映了CO₂来源的演变：阶段I与阶段IIa以氧化作用和硫酸盐还原作用为主；阶段IIb、III及IV则以甲烷发酵作用为主。