Sulphate and d34S in pore waters and δ¹³C, δ¹⁸O and mineral composition of carbonate nodules from ODP Leg 188 sites@en

Carbon and oxygen isotopic compositions of authigenic carbonate nodules or layers reflect the diagenetic conditions at the time of nodule growth. The shallowest samples of carbonate nodules and dissolved inorganic carbon of pore water samples beneath the sulfate reduction zone (0-160 meters below seafloor [mbsf]) at Site 1165 have extremely negative d13C values (-50 per mil and -62 per mil, respectively). These negative d13C values indicate nodule formation in association with anaerobic methane oxidation coupled with sulfate reduction. The 34S of residual sulfate at Site 1165 shows only minor 34S enrichment (+6 per mil), even with complete sulfate reduction. This small degree of apparent 34S enrichment is due to extreme \"open-system\" sulfate reduction, with sulfate abundantly resupplied by diffusion from overlying seawater. […]自生碳酸盐结核（authigenic carbonate nodules）或结核层的碳、氧同位素组成，可反映结核形成时期的成岩环境。1165站位（Site 1165）硫酸盐还原带下方、海底以下0~160米（mbsf）范围内的碳酸盐结核样品，及其孔隙水中的溶解无机碳（dissolved inorganic carbon）样品，其δ13C值均呈现极强负值（分别为-50‰与-62‰）。这类显著负偏的δ13C值，表明结核的形成与厌氧甲烷氧化（anaerobic methane oxidation）耦合硫酸盐还原作用紧密相关。1165站位的残余硫酸盐δ34S值仅表现出小幅富集（+6‰），即便此时硫酸盐已完全还原。这种微弱的表观δ34S富集现象，是极端‘开放体系（open-system）’硫酸盐还原作用的结果：上覆海水通过扩散作用持续大量补给硫酸盐。[…]