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Role of Microbes in Dolomite Formation

Judith A. McKenzie, Crisogono Vasconcelos, Tomaso Bontognali, and Rolf Warthmann
Geological Institute, ETH-Zentrum, 8092 Zurich, Switzerland

The formation of dolomite has been a long-standing enigma in carbonate diagenesis because it is found widespread in the sedimentary rock record and rarely observed forming in modern carbonate-precipitating environments. Dolomite models have generally focused on thermodynamically driven, hydrologic processes to interpret the occurrence of massive dolomite structures in the rock record, but these models could not provide adequate mechanisms to overcome the kinetic factors inhibiting dolomite precipitation. The recent recognition that microbes can mediate the precipitation of dolomite in natural environments and culture experiments has opened a new direction in dolomite research. The introduction of a microbial factor allows us to study dolomite formation in the laboratory, where experimental conditions and microbial communities can be varied to determine the range of possible environments where dolomite may form. Our research concentrates on saline-tolerant anaerobic microorganisms and the associated geochemical gradients that they establish through their metabolic activity. Microbial dolomite formation appears to occur at major geochemical boundaries where there is elevated microbial activity. In these diagenetic zones, we propose the existence of a symbiotic link within the microbial community between sulfate reducers and sulfide oxidizers, which maintains a high level of sulfate ions. This phenomenon may be particularly important in extremely arid environments, such as the coastal sabkhas of Abu Dhabi, U.A.E., where gypsum and anhydrite precipitate directly beneath the surface in association with early diagenetic dolomite. Thus, the commonly observed association of dolomite and evaporates in many ancient carbonate sequences may also be fundamentally related to a microbial-driven process.