Dolomite is an important constituent of many economically important gas and oil reservoir rocks. Studies conducted in modern environments combined with microbiological laboratory experiments have shown that microbes and their extracellular polymeric substances (EPS) play an important role for the formation of primary dolomite at Earth's surface conditions. These studies showed that, at low temperature, Mg is incorporated into the carbonate mineral exclusively in the presence of specific organic molecules. However, because the organic molecules involved in the mineralization process rarely survive metamorphism and are usually not preserved in the carbonate mineral, identifying microbial dolomite in ancient rocks represents a challenging task. It remains, as yet, unclear what percentage of sedimentary dolomite is a primary microbially mediated precipitate vs. a secondary replacement product that formed during diagenesis or at high temperatures during metamorphism. A useful approach for evaluating the microbial origin of ancient dolomite is that of searching for microfossils and other microstructures of biological origin associated with the dolomite crystals. Here, we present the results of scanning electron microscopy (SEM) investigations of various ancient sedimentary dolomites. Several investigated samples include abundant filamentous microstructures that we interpret to be mineralized EPS. In some cases, these filaments form a well-structured alveolar pattern, whose architecture appears too complex to be the result of an abiotic process. In order to reinforce our interpretation, we also investigated modern biofilms, which include microstructures of EPS that are morphologically identical to the mineralized and preserved EPS in ancient dolomites. In some rare cases, we also found microfossils (i.e. mineralized cells) in close spatial association with the dolomite crystals. These occurrences are remarkable, considering that it is commonly thought that only chert has the potential of preserving the soft tissues constituting microbial cells. Although we cannot prove conclusively that the fossilized cells and EPS mediated the formation of the adjacent dolomite crystals, our results indicate that dolomite formation took place in environments where microorganisms and biofilms were an important component of the depositional setting. The "microbial factor", which has been proven to be essential for the precipitation of dolomite at low temperature in modern environments, may, therefore, have also been active during the formation of these ancient carbonates.


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