Biogeochemical Investigations of In-Situ Dolomite Formation: Providing New Insights into an Old Enigma

temperature dolomite has remained an enigmatic mineral for over a century. The formation mechanisms have been controversial debated as there are large abundances of low-temperature dolomite in ancient rocks, but a continual decrease as time progressed with little to no low-temperature dolomite forming today. Indeed, the mechanisms controlling ancient abundances of dolomite can still not be explained. Recent laboratory experiments have shown that dolomite can be mediated by functional groups along cell surfaces and excretions referred to as exopolymeric substances (EPS). It has been theorized that biologically formed functional groups of EPS act as nucleation surfaces which reduced the energy required to form dolomite. Although, the formation of low-T dolomite in association with EPS has been shown in the laboratory it does not explain observations in the rock record. It also remains unclear why many modern environments do no host dolomite despite its supersaturation. Thus, this thesis aimed to examine in-situ formation of dolomite from a select modern environment, the hypersaline sabkhas of Qatar, and to provide new insight into an old enigma. Through a series of field and laboratory investigations this thesis shows that specific environmental drivers, such as salinity, facilitated a particular microbial community dynamic that produced EPS specifically suited to nucleating dolomite. The microbial community dynamics were represented by fluctuations in oxygenic and anoxygenic phototrophs which appear to have become sparse through geologic time due to the oxygenation of Earth’s atmosphere. This connection provided a possible explanation for the decreased abundance of low-T dolomite in the rock record. Furthermore, this work has shown through laboratory experiments that natural extracted EPS achieves rates of carbonate/dolomite nucleation at many times the rate of abiotic surfaces, such as clays. Additionally, the presence of carotenoid pigments associated with these dynamics provide potential targets for biomarker analysis, which have implications for astrobiology as well as oil and gas exploration. Overall, these findings help improve established models of dolomite precipitation and highlight the importance of considering microbe communities-mineral interactions with regard to global bio-geochemical cycles.Ph.D