Abstract
ABSTRACTSulfate 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 his-tory. 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 seawa-ter, modern hypersaline water, and assumed Precambrian alkaline seawater, sulfate reduction initially lowers the saturation of carbonates due to a rapid decrease in pH. With continu-ing 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 suffi cient to cause such an increase in saturation. With increasing salinity, sulfate reduction becomes even less effi cient to induce carbonate pre-cipitation. In an alkaline Precambrian ocean, where large amounts of carbonate were formed, induction through sulfate reduction was entirely ineffective. Other metabolic pathways or abi-otic factors must be responsible for inducing carbonate formation in microbial mats through Earth’s history.
Published Version
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