MICRITIC DOLOMITE: A GEOBIOLOGICAL SIGNATURE OF ANAEROBIC MICROBIAL ACTIVITY?

The study of modern and ancient sedimentary dolomite has traditionally been approached as an inorganic geochemical problem. This approach has led to an enigma; although dolomite undoubtedly forms under Earth surface conditions, it has proven impossible to produce it in the laboratory using experimental conditions that replicate the natural environments where modern dolomite precipitates. Recently, however, anaerobic experiments duplicating hypersaline conditions measured in Lagoa Vermelha, Brazil, have resulted in the precipitation of dolomite at low temperatures, but only in the presence of specific sulfate reducing bacteria. Based on these results, we propose that primary dolomite can be considered as a biomineral, a metabolic product of a biologic process, i.e., bacterial sulfate reduction. Thus, in a manner similar to stromatolitic structures being a sedimentary record of photosynthetic microbial (cyanobacterial) activity, micritic dolomite may represent a geobiological signature of microbial (sulfate reducing bacterial) activity, probably associated with high alkalinity, hypersalinity and anoxia. Geochemical signals incorporated into microbial dolomite, such as carbon-isotope ratios and specific intercrystalline organic molecules, can be used to document or confirm a microbial origin.

Micritic dolomite is found widespread in the sedimentary rock record, occurring in some of the oldest sedimentary rock with an age of at least 3.45 Ga. Furthermore, the relative amount of dolomite proportional to limestone and its distribution with respect to geologic time show distinct variations. For example, dolomite was particularly common in the Precambrian, varied cyclically during the Phanerozoic and is distinctly rare in the Holocene. Applying the microbial dolomite model to this record, we interpret times of increased dolomite deposition to be associated with elevated levels of global anoxia and a change in the dominant metabolism of the ocean. Based on this microbial interpretation of the geologic dolomite record, we propose that micritic dolomite can be regarded as a fossil mineral recording specific environmental conditions during the Earth’s evolution.