(Table 2) Mineralogy, stable isotopes, and mineral percentages of the carbonate fraction of samples from the South China Sea

Chemoherm carbonates, as well as numerous other types of methane seep carbonates, were discovered in 2004 along the passive margin of the northern South China Sea. Lithologically, the carbonates are micritic containing peloids, clasts and clam fragments. Some are highly brecciated with aragonite layers of varying thicknesses lining fractures and voids. Dissolution and replacement is common. Mineralogically, the carbonates are dominated by high magnesium calcites (HMC) and aragonite. Some HMCs with MgCO3 contents of between 30–38 mol%–extreme-HMC, occur in association with minor amounts of dolomite. All of the carbonates are strongly depleted in d13C, with a range from -35.7 to -57.5 per mil PDB and enriched in d18O (+ 4.0 to + 5.3 per mil PDB). Abundant microbial rods and filaments were recognized within the carbonate matrix as well as aragonite cements, likely fossils of chemosynthetic microbes involved in carbonate formation. The microbial structures are intimately associated with mineral grains. Some carbonate mineral grains resemble microbes. The isotope characteristics, the fabrics, the microbial structure, and the mineralogies are diagnostic of carbonates derived from anaerobic oxidation of methane mediated by microbes. From the succession of HMCs, extreme-HMC, and dolomite in layered tubular carbonates, combined with the presence of microbial structure and diagenetic fabric, we suggest that extreme-HMC may eventually transform into dolomites. Our results add to the worldwide record of seep carbonates and establish for the first time the exact locations and seafloor morphology where such carbonates formed in the South China Sea. Characteristics of the complex fabric demonstrate how seep carbonates may be used as archives recording multiple fluid regimes, dissolution, and early transformation events.2004年，研究人员在南海北部被动大陆边缘发现了化能生物丘碳酸盐（Chemoherm carbonates）以及诸多其他类型的甲烷渗漏碳酸盐（methane seep carbonates）。岩石学上，该类碳酸盐为微晶质结构，内含球粒（peloids）、岩屑（clasts）与贝壳碎屑；部分样品发生强烈角砾化，裂隙与孔隙内壁发育厚度不一的文石（aragonite）层，溶蚀与交代作用普遍存在。矿物学方面，该类碳酸盐以高镁方解石（high magnesium calcites, HMC）与文石为主，其中部分高镁方解石的MgCO₃含量介于30~38 mol%之间，被称为极端高镁方解石（extreme-HMC），它们常与少量白云石伴生。所有碳酸盐样品均具有显著的碳同位素负异常，δ¹³C值介于-35.7‰至-57.5‰ PDB标准之间，同时氧同位素呈正异常，δ¹⁸O值介于+4.0‰至+5.3‰ PDB标准之间。在碳酸盐基质与文石胶结物中，研究人员识别出大量微生物杆状与丝状结构，其大概率为参与碳酸盐形成过程的化能合成微生物（chemosynthetic microbes）化石；此类微生物结构与矿物颗粒紧密共生，部分碳酸盐矿物颗粒外观甚至酷似微生物。上述同位素特征、组构特征、微生物结构与矿物学特征，均指示该类碳酸盐形成于微生物介导的甲烷厌氧氧化过程（anaerobic oxidation of methane）。结合层状管状碳酸盐中高镁方解石、极端高镁方解石与白云石的发育序列，以及微生物结构与成岩组构的存在，本研究提出极端高镁方解石最终可转化为白云石。本研究结果丰富了全球甲烷渗漏碳酸盐的研究记录，并首次明确了南海北部此类碳酸盐的形成位置与海底形貌特征；其复杂组构特征揭示了甲烷渗漏碳酸盐可作为记录多流体体系、溶蚀作用与早期转化事件的地质档案的潜在应用价值。