In part composed of Mg calcite and dolomite with a nearly continuous spectrum of MgCO₃ contents, carbonates forming at marine methane seeps are ideal candidates to study the formation of early diagenetic dolomite at surface conditions. Laboratory experiments, modelling and the co-variation of mineralogical and geochemical attributes of seep carbonates suggest that sulphide – locally released from sulphate-driven anaerobic oxidation of methane – drives catalytic dolomite formation at seeps. Direct comparison of the concentration of dissolved sulphide with the MgCO₃ content of seep carbonate could test this hypothesis. Although the concentration of sulphide during precipitation of carbonate cannot be determined, sedimentary fabrics probably had an effect on local sulphide concentration. Carbonates from the seabed of the Shenhu seepage area of the South China Sea reveal longitudinal, winding and branched fabrics with a width of 400 to 700 μm, interpreted to represent burrows piercing semi-consolidated sediment. The abandoned burrows were infilled with fine-grained sediment, which was subsequently cemented by microcrystalline dolomite and Mg calcite. The degree of cation ordering of dolomite in burrow infills and host sediment is similar, indicating virtually coeval dolomite formation. The Mg/Ca mole ratios of carbonate minerals are lower in burrow infills than in the surrounding sedimentary matrix. Similarly, δ¹³C_carbonate values tend to be higher for burrow infills (from −41 to −33‰) than for the matrix (from −43 to −38‰), suggesting stronger seawater influence on pore waters in the burrows. More reducing microenvironments of the sedimentary matrix supposedly came along with higher concentrations of dissolved sulphide, which allowed more Mg²⁺ ions to enter the crystal lattice and led to more catalytic dolomite formation in the host sediment. The distribution of MgCO₃ contents and δ¹³C values in the authigenic Shenhu carbonates reinforces the hypothesis that sulphide release by sulphate-driven anaerobic oxidation of methane is a key factor in the formation of dolomite at seeps.

中文翻译：

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减少微环境促进镁离子掺入甲烷渗漏处的自生碳酸盐形成：白云岩形成的限制

部分由镁方解石和白云石组成，具有几乎连续的 MgCO ₃含量谱，在海洋甲烷渗漏处形成的碳酸盐是研究地表条件下早期成岩白云岩形成的理想候选者。实验室实验、建模以及渗漏碳酸盐的矿物学和地球化学属性的共变表明，硫化物——从硫酸盐驱动的甲烷厌氧氧化局部释放——推动了渗漏处催化白云石的形成。溶解硫化物浓度与 MgCO _{3 的}直接比较渗碳酸盐的含量可以检验这一假设。虽然无法确定碳酸盐沉淀过程中硫化物的浓度，但沉积结构可能对当地的硫化物浓度有影响。南海神狐渗流区海底碳酸盐岩呈纵向、曲折、分枝状，宽度为400～700 μm，解释为洞穴穿透半固结沉积物。废弃的洞穴被细粒沉积物填充，随后被微晶白云石和镁方解石胶结。洞穴填充物和宿主沉积物中白云岩的阳离子排序程度相似，表明实际上是同时期的白云岩形成。洞穴填充物中碳酸盐矿物的 Mg/Ca 摩尔比低于周围沉积基质。相似地，洞穴填充物的¹³ C_碳酸盐值（从 -41 到 -33 ‰）往往高于基质（从 -43 到 -38 ‰），这表明海水对洞穴中孔隙水的影响更强。据推测，沉积基质的还原微环境越多，溶解的硫化物浓度越高，这使得更多的 Mg ²⁺离子进入晶格，并导致在宿主沉积物中形成更多的催化白云石。自生神狐碳酸盐岩中 MgCO ₃含量和 δ ¹³ C 值的分布强化了以下假设：硫酸盐驱动的甲烷厌氧氧化释放硫化物是渗流处白云石形成的关键因素。