Reducing microenvironments promote incorporation of magnesium ions into authigenic carbonate forming at methane seeps: Constraints for dolomite formation

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, δ13Ccarbonate 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.