The mineral phase of primary marine fibrous carbonate is supposed to be mainly controlled by the Mg/Ca of seawater during precipitation. However, fibrous Mg calcite was identified from seep carbonates in the Black Sea, although the high Mg/Ca of the Black Sea seawater should promote the precipitation of aragonite. The unexpected formation of fibrous Mg calcite indicates additional factors other than Mg/Ca. In order to clarify the factors, we studied seep carbonates from the Black Sea. Points of interest were selected by observation under a microscope. Main compositions of the points were measured by an energy-dispersive X-ray spectroscope. Mineral structure and spectroscopy characteristics were analysed by X-ray diffraction and Raman, respectively, and converted to MgCO₃ contents according to empirical formulas. Carbon and oxygen isotope compositions of fibrous carbonate minerals were measured by an isotope ratio mass spectrometer. The results suggest that the fibrous Mg calcite from the Black Sea were formed by sulphide-driven anaerobic oxidation of methane under reducing conditions, which contributes to the understanding of the formation of Neoproterozoic fibrous dolomite.

Funding information:K. C. Wong Education Foundation: http://dx.doi.org/10.13039/501100012692DAAD: http://dx.doi.org/10.13039/501100001655Guangzhou Elite Project (No. JY201223)Open Access Publication Fund of Universität Hamburg: http://dx.doi.org/10.13039/501100005711