Stable carbon isotope ratios and the formation of dolomite in ODP Leg 175 sites

We examine the link between organic matter degradation, anaerobic methane oxidation (AMO), and sulfate depletion and explore how these processes potentially influence dolomitization. We determined rates and depths of AMO and dolomite formation for a variety of organic-rich sites along the west African Margin using data from Ocean Drilling Program (ODP) Leg 175. Rates of AMO are calculated from the diffusive fluxes of CH4 and SO4, and rates of dolomite formation are calculated from the diffusive flux of Mg. We find that the rates of dolomite formation are relatively constant regardless of the depth at which it is forming, indicating that the diffusive fluxes of Mg and Ca are not limiting. Based upon the calculated log IAP values, log K(sp) values for dolomite were found to narrowly range between -16.1 and -16.4. Dolomite formation is controlled in part by competition between AMO and methanogenesis, which controls the speciation of dissolved CO2. AMO increases the concentration of CO3[2-] through sulfate reduction, favoring dolomite formation, while methanogenesis increases the pCO2 of the pore waters, inhibiting dolomite formation. By regulating the pCO2 and alkalinity, methanogenesis and AMO can regulate the formation of dolomite in organic-rich marine sediments. In addition to providing a mechanistic link between AMO and dolomite formation, our findings provide a method by which the stability constant of dolomite can be calculated in modern sediments and allow prediction of regions and depth domains in which dolomite may be forming.

本研究探讨了有机质降解、厌氧甲烷氧化（Anaerobic Methane Oxidation, AMO）与硫酸盐消耗之间的关联，并探究上述过程如何潜在影响白云岩化作用。我们借助大洋钻探计划（Ocean Drilling Program, ODP）第175航次的数据，对西非陆缘多个富有机质沉积位点的厌氧甲烷氧化速率、形成深度及白云岩形成情况进行了测定。厌氧甲烷氧化速率通过甲烷（CH₄）与硫酸盐（SO₄）的扩散通量计算得出，而白云岩形成速率则通过镁（Mg）的扩散通量计算得到。研究发现，无论白云岩形成所处深度如何，其形成速率均相对恒定，这表明镁与钙（Ca）的扩散通量并非限制因素。基于计算得到的离子活度积（log IAP）值，白云岩的溶度积常数（log K(sp)）取值范围狭窄，介于-16.1至-16.4之间。白云岩形成在一定程度上受厌氧甲烷氧化与甲烷生成作用之间的竞争调控，该竞争过程控制了溶解态二氧化碳的赋存形态。厌氧甲烷氧化通过硫酸盐还原作用提升碳酸根（CO₃²⁻）浓度，从而有利于白云岩形成；而甲烷生成作用则升高孔隙水的二氧化碳分压（pCO₂），抑制白云岩形成。通过调控孔隙水的二氧化碳分压与碱度，甲烷生成作用与厌氧甲烷氧化可共同调控富有机质海洋沉积物中的白云岩形成过程。本研究不仅揭示了厌氧甲烷氧化与白云岩形成之间的机制性关联，还提出了一种可用于计算现代沉积物中白云石稳定常数的方法，并能够预测白云岩可能形成的区域与深度域。