CATALYTIC ROLES OF MICROBES IN SEDIMENTARY DOLOMITE PRECIPITATION

It has been more than 220 years since dolomite was first recognized by Deodat de Dolomieu. Sedimentary dolomite is generally abundant in the geological record, yet scarce in Holocene and modern sediments. Sedimentary dolostones and dolomite-cemented sandstones can be potential hydrocarbon reservoirs, because most hydrocarbon reservoirs in the US are related to dolomite. Previous dolomitization models address the role of hydrologic circulation in supplying the necessary Mg(II). The main factor inhibiting dolomite formation and disordered dolomite nucleation / crystallization is the strong hydration of aqueous Mg(II) ion. Recent studies show that microbes may play an important role in sedimentary dolomite formation. It was proposed that sulfate-reducers that reduce sulfate may promote dolomite formation. However, exact roles of the microbes in dolomite precipitation is not clear. Our recent results show that sulfate will not inhibit dolomite precipitation in presence of catalysts like hydrogen sulfide, polysaccharides, and microbial EPS (dominated by polysaccharides) (Zhang et al., 2012a, b; 2013). Dissolved sulfide can also promote crystallization of disordered dolomite and high-Mg calcite. Adsorbed hydrogen sulfide and polysaccharides form hydrogen bond with the surface carbonate anions, and weaken bonding strength between water molecule and surface Mg(II). Sedimentary carbonate with dolomite and limestone layers (or micro-lamina) may indicate microbial biomass buried together with the initial calcite and high-Mg calcite affect dolomitization of the carbonate sediments.

Zhang, F., Xu, H., Konishi, H., Shelobolina, E. S., and Roden, E. E. (2012a) Polysaccharide-catalyzed nucleation and growth of disordered dolomite: A potential precursor of sedimentary dolomite. American Mineralogist, 97, 556-567.

Zhang, F., Xu, H., Konishi, H., Kemp, J. M., and Roden, E. E. (2012b): Dissolved sulfide-catalyzed crystallization of Ca-Mg carbonates and implications for the formation mechanism of sedimentary dolomite. Geochimica et Cosmochimica, 97, 148-165.

Zhang, F., Yan, C., Teng, H., Roden, E. E., and Xu, H. (2013): In situ AFM observations of Ca-Mg carbonate crystallization catalyzed by dissolved sulfide: Implications for sedimentary dolomite formation. Geochimica et Cosmochimica, 105, 44-55.