Microbial mediation of dolomite formation : geochemical and microbial investigations in the Coorong region of South Australia
Dolomite is shown to precipitate in laboratory experiments that simulate the microbiogeochemical conditions prevailing during the later stages of evaporation in the ephemeral, hypersaline, dolomitic lakes of the Coorong region, South Australia, where microbially-dominated ecosystems support intensive bacterial sulphate reduction. Analyses of numerous lake- and pore-water samples from Coorong lakes document rapid geochemical changes with depth and time. Extremely high sulphate and magnesium ion concentrations occur in lake waters and decline rapidly with depth in pore waters through the sulphate reduction zone. Ultimately sulphate ions are totally consumed but magnesium ions are replenished, presumably from desiccated cyanobacterial sheaths. Carbonate concentrations in pore waters reach levels up to 100 times those of normal seawater. Most-probable-number counts show that large populations of sulphate-reducing bacteria are present in sediment cores, while sulphur isotope analysis of residual lakewater sulphate indicates that microbial fractionation takes place in all the study lakes. Microbes from the lakes were cultured in the laboratory under anoxic conditions and viable populations were injected into vials containing a sterilised granular substrate immersed in a simulated lakewater solution. Falls in the levels of sulphate ion concentration and rising pH in selected vials were interpreted as due to active bacterial sulphate reduction accompanied by increased concentrations of carbonate. After 1 month, subspherical nano-grains of dolomite were precipitated. This study proposes that bacterial sulphate reduction overcomes kinetic constraints on dolomite formation by removing the sulphate ions and releasing magnesium and calcium ions from neutral ion pairs, and by generating elevated carbonate concentrations in a hypersaline and strongly electrolytic solution. Microbiogeochemical and isotopic data therefore demonstrates that bacterial sulphate reduction controls dolomite precipitation in both the laboratory experiments and the lake sediments. It is proposed that dolomite formation through bacterial sulphate reduction provides a process analogue that is applicable to thick platformal dolostones of the past, where benthic microbial communities were the dominant colonisers of the shallow marine environment.