Geological Problems with Microbiological Solutions: Deciphering the Authigenesis of Calcite, Dolomite, and Native Sulfur in Salty Environments

Microbial activity is known to impact the formation and alteration of many different rock types. For carbonate caprock (CCR), a lithology found on salt diapirs, it is generally accepted that microbial activity drives the precipitation of carbonate minerals, forming limestone and/or dolomite and native (elemental) sulfur. It appears that there are two types of CCR: 1) limestone associated with native sulfur (S0) and 2) limestone associated with dolomite. The mechanics of CCR formation are poorly understood. For example, it is unclear why native sulfur and dolomite are rarely found in the same CCR assemblage, and why either are formed at all. Filling these gaps in knowledge is important for several reasons. From the perspective of oil and gas exploration, CCR serves as archives of the temperature and fluid history at salt diapirs, can act as reservoirs or conduits for hydrocarbons, can pose drilling hazards or result in dry holes if not identified or misidentified as a different lithology, and finally, may hold critical information on microbial hydrocarbon degradation coupled to the production of sulfide – a process called oil souring that poses major health hazards and is a significant cost to the industry due to corrosion of drilling equipment. From the perspective of basic science, CCR is a unique natural laboratory in which microbial processes took place over long durations in the subsurface with hostile properties. These environments are characterized by elevated pressure and temperature, high salinities, extreme scarceness of oxidants, and are prone to accumulation of sulfide and carbon dioxide. Such conditions are extremely difficult to achieve and maintain under laboratory conditions. As archive of such processes, CCR opens a window into microbial activity in Earth’s subsurface, as well as Earth surface processes in a distant past, and may provide critical clues to the understanding and discovery of life beyond Earth. For my Ph.D., I studied lithological and geochemical signatures of different CCR types to elucidate the geologic setting and dynamics and associated microbial metabolic pathways responsible for the genesis of the different mineral assemblages. I also developed a tool to identify yet undiscovered microbial sulfur transformations in culturing experiments. Central to these activities was the testing of two hypotheses: 1) CCR formation is intimately tied to abiotic and/or microbial sulfur transformations, and 2) native sulfur associated with CCR can be produced by microbial activity in the absence of molecular oxygen (O2), an oxidant that is usually inferred to play a critical role in the genesis of large native sulfur deposits. The dissertation comprises two chapters that tackle the second hypothesis (Chapters 2 and 3), a chapter that investigates an example that may challenge the first hypothesis (Chapter 4), and a chapter that describes the development of an isotope tracer to identify yet undiscovered microbial sulfur transformations in culturing experiments (Chapter 5). Following an outlook on future research (Chapter 6), the appendix includes a manuscript that provides a general overview on the current state of CCR research. As second author I collaboratively developed with my advisor Dr. Brunner, who is lead author, the concept for the paper, contributed to the literature research that went into this review, compiled data, drafted figures, wrote parts of the manuscript, and coordinated the communication with the total of 23 authors in the writing and revision of the manuscript. At the point of the submission of this dissertation, the overview paper and Chapter 2 have been published (Brunner et al., 2019; Labrado et al., 2019), and Chapter 3 is in preparation for submission to Geochimica et Cosmochimica Acta