Microbial transformations of organic matter in black shales and implications for global biogeochemical cycles

The various roles that microorganisms play in transformation of organic matter in geologic environments are yet to be fully revealed. Many of these roles influence and perhaps control the composition of earthʹs atmosphere over geologic time by directly impacting global-scale cycling of carbon dioxide, oxygen, and methane. One example is the weathering of black shales. Exposure of organic matter-rich sedimentary rocks to oxygenated earth surface conditions results in significant changes in outcrop-scale rock geochemistry with implications for element cycling on a global scale. This article reviews the progress of ongoing research of a black shale weathering profile exposed near Clay City, Kentucky, USA. Using tools of molecular biology, microbial ecology, isotope geochemistry, and organic geochemistry, this research explores the role of microorganisms in utilization and oxidation of sedimentary organic matter and sulfide mineral oxidation, and examines the communities of microorganisms that may live in this environment. From this and related studies, we are developing greater awareness of the importance of microorganisms in transfer of organic materials among various reservoirs of the geosphere, biosphere, hydrosphere, and atmosphere.