Mediation of Dolomite Precipitation by Methanogenic Microorganisms
Many earlier studies have focused on the thermodynamic and geochemical conditions in which low-temperature dolomite formation is kinetically favorable. More recent studies have begun to consider the role that microorganisms play in overcoming the kinetic barriers to dolomite precipitation at low temperature. Laboratory studies of dolomite formation under many controlled conditions have helped to elucidate the specific roles of different types of microbial metabolism, including sulfate reduction and methanogenesis. In addition to settings with sulfate reduction, there is evidence from both modern and ancient environments that suggests that dolomite forms under methanogenic conditions. The nucleation and precipitation of well-ordered, stoichiometric dolomite phases by mixed methanogenic populations has been demonstrated in the laboratory at low temperature. Dolomite forms in fluids of relatively low Mg:Ca ratio, making this mechanism of dolomite formation broadly applicable for rocks from various settings and ages. There are two distinct metabolic pathways that methanogens utilize, which impact dolomite saturation state and may drive dolomite formation. Autotrophic methanogenesis consumes CO2 in combination with H2 to produce CH4, driving supersaturation with respect to carbonate mineralogy. Acetoclastic methanogenesis drives dolomite supersaturation by producing HCO3- in addition to CH4. These pathways produce distinct isotopic signatures that record these modes of metabolic activity in the rock record. These distinctive isotopic signatures provide methodologies for identifying such dolomite in ancient systems. More complete understanding of dolomite formation in methanogenic systems should serve as models for predicting dolomite distribution in the subsurface.