Two opposing effects of sulfate reduction on carbonate precipitation in normal marine, hypersaline, and alkaline environments

Sulfate reduction has been suggested as a mechanism to induce precipitation of calcium and magnesium carbonates in marine sediments and microbial mats through most of Earth's history. However, sulfate reduction also causes a drop in pH that favors dissolution rather than precipitation of carbonates. Model results obtained in this study show that in modern seawater, modern hypersaline water, and assumed Precambrian alkaline seawater, sulfate reduction initially lowers the saturation of carbonates due to a rapid decrease in pH. With continuing sulfate reduction, the pH stabilizes between 6.5 and 7, and carbonate saturation slowly increases as a result of increasing dissolved inorganic carbon concentration. However, sulfate reduction in surface microbial mats is not sufficient to cause such an increase in saturation. With increasing salinity, sulfate reduction becomes even less efficient to induce carbonate precipitation. In an alkaline Precambrian ocean, where large amounts of carbonate were formed, induction through sulfate reduction was entirely ineffective. Other metabolic pathways or abiotic factors must be responsible for inducing carbonate formation in microbial mats through Earth's history.