Nanoscale Investigation of Microbial Role in Promoting the Smectite to Illite Transformation

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Smectite-illite interstratified clay minerals are ubiquitous in sedimentary basins. The smectite to illite transformation is linked to the maturation, migration and trapping of hydrocarbons, the development of pore pressures, rock cementation and porosity reduction, and pore water chemistry. Despite the importance of the reaction, considerable ambiguity exists as to how smectite is converted to illite as a function of geological variables. It is well known that the smectite to illite transformation is approximately concomitant with maturation of petroleum during sediment diagenesis, but the relationship between organic matter and clay mineral reactions has yet to be rigorously established. We hypothesize that the linkage is in the role that microorganisms play, in regards to organic matter oxidation coupled to metal reduction, in the smectite to illite transformation. Sufficient evidence has accumulated that abundant (~106 cells/g) and active microbes and clay minerals co-exist in argillaceous rocks. Metal-reducing microbes have been discovered at burial depths of ~3000 meters, and at up to 100oC. These conditions bracket those commonly found during clay diagenesis. Viable microbes (104-105 cells/g) also exist in organic-rich shales, and their abundance may be higher at the onset of clay diagenesis or in those shales under the .overpressure. condition. Both field and laboratory evidence suggest the important role of microbes in clay mineral reactions, but the functional linkage between microbial activity and clay mineral transformations is poorly understood. Our recent investigations have shown that microbes can transform nontronite (a smectite variety) to illite at room temperature in one month. This reaction typically requires much higher temperatures over extended time periods in absence of microbial activity. We propose to investigate the effects of microbes on the smectite to illite transformation, using unique and powerful analytical tools including environmental cell transmission electron microscopy (EC-TEM) and electron energy loss spectroscopy (EELS). Specifically, the goals of this three-year proposal are to 1) investigate the effects of subsurface bacteria on the extent and rate of the smectite to illite transformation under diagenetic conditions; 2) determine mechanisms of the transformation process; 3) explore how microbes affect kinetics of the transformation. We hypothesize that microorganisms promote the smectite to illite transformation via 1) reduction of structural Fe(III) in smectite; 2) production of organic acids. Fe(III)-containing smectite (0.5-4.2 mmol Fe3+/g) is common in natural environments. To simulate smectite-illite diagenesis, the effects of bacterial Fe(III)-reduction will be studied at three different temperatures (30oC, 61oC, and 90oC) using mesophilic, thermophilic and hyperthermophilic, Fe(III) reducing bacteria (S. putrefaciens, G. metallireducens, Bacillus infernus, Geothermobacterium ferrireducens). A variety of natural smectite samples will be used. Reduction experiments will be designed under well-controlled conditions to mimic diagenesis. Reduction products will be analyzed by a variety of techniques, including chemical extraction, XRD, EC-TEM, EELS, Mossbauer spectroscopy and oxygen isotopes. Attempts will be made to integrate microbial effects into smectite to illite transformation models, which will have major implications for organic matter maturation and petroleum migration. By the end of this project, we will have answered the following questions: 1) do microbes accelerate the smectite to illite transformation under diagenetic conditions? 2) how do they achieve this acceleration? 3) what are mechanisms for the smectite to illite reaction in presence of bacteria? Broader impacts. The proposed work will be carried out with collaborations between national labs and university, and will involve three PI.s and their 3 students for three years. Students will work in both academic institution and governmental labs to gain unique research experiences. Workshops, web pages, course materials, newsletters and museum exhibits will be developed to efficiently and broadly disseminate research results. Major efforts will be made to deliver lectures to those institutions without PhD program and underrepresented, as well as K-12 schools.