HIGH-MAGNESIUM CALCITE AND DOLOMITE PRECIPITATION ASSOCIATED WITH DEEP SEA METHANE SEEPS

It is difficult for dolomite to precipitate directly in modern seawater. However, massive high-magnesium calcites (HMC) and dolomites were discovered on the seafloor along the continental margins, such as Gulf of Mexico, Blake Ridge, Hydrate Ridge and South China Sea, etc. The carbonates are closely related with methane seep sites, where methane is oxidized by sulfate mediated by sulfate reducing bacteria (SRB) and Archaea, releasing bicarbonates and sulfides. This process is known as the Anaerobic Oxidation of Methane (AOM). Previous papers consider that the formation of HMC and dolomite is triggered by AOM that decreases the concentration of sulfate, and the dolomite precipitation may also be mediated by microbes. But, experiments indicate that sulfate is not the barriers of HMC and dolomite precipitation, and the mechanism of microbes promoting dolomite deposition is still unclear.

We investigated authigenic carbonates from South China Sea, using X-ray diffraction (XRD), Rietveld method, transmission electron microscopy (TEM), and the carbon isotope analyses. The samples are composed of Mg-calcite, protodolomite, aragonite, and silicates with various contents. The carbonate minerals are anhedral to sub-euhedral. The Mg content varies in a range among different samples. Based on fast Fourier transformation, the structure of carbonate minerals is not homogeneous even within single micro-crystals. Combining δ¹³C with MgCO₃ mole percentage of the carbonate components, we can find an obvious negative curve relationship, indicating that more Mg in carbonates is coupled with more intense AOM that results in depleted δ¹³C in Ca-Mg-carbonates. We propose that the AOM-related sulfide and extracellular polymeric substances (EPS) that adsorbed on carbonate surface through hydrogen bonds are the catalysts for promoting surface Mg-H₂O dehydration and the dolomite and HMC precipitation. The unevenly distributed catalyst may result in the heterogeneity of Mg content and structure ordering in the methane seep carbonates.