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