oa Microbial dolomite formation in the Dohat Faishakh Sabkha, Qatar

Dolomite is a common sedimentary mineral in the geological record and common in hydrocarbon reservoirs. However, its origins have remained a controversial problem in sedimentology for more than 200 years. In the 1960's, the hypersaline coastal sabkha of Dohat Faishakh (Qatar) was one of the first settings recognized as a rare modern geological environment where dolomite formation occurs. Studies conducted in the Arabian Gulf led to the development of the first models for low temperature dolomite formation. However, numerous laboratory experiments, which utilized the physico-chemical parameters recorded in modern hypersaline environments to replicate conditions for dolomite formation at Earth surface temperatures and pressures, proved unsuccessful. Since the mid-1990's, the discovery of specific microbes that can mediate dolomite precipitation added a new "biological dimension" to the study of this enigmatic mineral. Based on this biological approach, we returned to the Dohat Faishakh sabkha to test the hypothesis that microbial activity plays an important role for minerals formation in this apparently inhospitable environment. The study of sediments collected in short cores taken along a transect from intertidal to supratidal zones revealed a close association between microbial mats and dolomite. Authigenic dolomite occurs within surface and buried microbial mats, which are composed of exopolymeric substances (EPS). The cation-binding effect of the EPS molecules apparently plays a crucial role for the biomineralization process, influencing the composition of the precipitate. Mineral formation within EPS appears to be enhanced by evaporation with consequent supersaturation of the pore waters with respect to dolomite. Partial EPS degradation during diagenesis may also provide an additional source of cations. These results suggest that the main factor controlling the occurrence of dolomite within the sediments of the Dohat Faishakh sabkha is the presence of an organic matrix (i.e., the buried microbial mats constituted of EPS) and not, as proposed in previous studies, a replacement process transforming primary aragonite into dolomite. Aragonite and dolomite likely precipitate nearly simultaneously from evaporated marine waters. The presence vs. absence of EPS determines whether the carbonate minerals will have a dolomitic or an aragonitic composition, respectively. The next step of our research will be to characterize the chemical composition of the EPS molecules present in the microbial mats and possibly identify the specific molecule involved in dolomite formation. The study of this poorly understood biomineralization process can provide new insights for interpreting micritic dolomite, which is common in oil and gas reservoirs. This research is also relevant for the search of life on other planets, such as Mars, where recent observations have identified the presence of vast evaporitic sedimentary deposits.