Nonskeletal sedimentary carbonate rocks are an important component of the Precambrian geological record, but consensus on their origin is lacking. Phanerozoic carbonates are almost exclusively biogenic products of shelly fossils, but it has generally been assumed that carbonate rocks deposited before a shelly biota evolved in the marine environment formed by direct precipitation from supersaturated solution in seawater. However, there is no unequivocal empirical evidence that calcium carbonate or dolomite precipitates directly from modern seawater, and it has been suggested that kinetic inhibitors to carbonate precipitation, related to the low concentration and activity of the carbonate ion, cation hydration and ion complexing, are especially effective in saline waters. On the other hand, there is increasing evidence that these inhibitors can be overcome through microbial mediation. Bacteria have b een implicated in calcium carbonate precipitation since the Archaean, and though best known in seas and lakes, microbial carbonates are also important in fluviatile, spring, cave, and soil environments. The mechanisms of microbial mineral precipitation appear diverse, but many bacteria exhibit an ability to change solution chemistry and control pH at the microscale, passively or actively, thereby creating the ambient conditions for both oversaturation of Ca2+ and CO32- ions, and removal of kinetic inhibitors. Bacteria dominated the ecosystems of Precambrian shallow marine environments, enhancing their potential involvement in widespread carbonate formation. Chemical precipitation of evaporite minerals is generally accepted, but the involvement of microbes may be significant and underestimated. This review evaluates current knowledge and attempts to define some of the many questions that await resolution.
Bibliographical noteFunding Information:
This work was supported by the Medical and health research projects in Hainan Province (Grant No. 2001320243A2009). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
- Kinetic inhibitors
- Microbial mediation
- Nonskeletal carbonate