Serpentinization, the hydrothermal alteration of ultramafic rocks, has an important impact on many global biogeochemical cycles and is a unique mineralogical process that can support biological activity through formation of hydrogen and methane. Evidence of microbial activity in the form of sulfur isotope signatures in sulfide minerals has been reported in many peridotite-hosted hydrothermal systems along mid-ocean ridges on the ocean floor and in analogous ancient systems (e.g., outcropping as ophiolite sequences on continents). Recent studies further suggest that biological activity plays a significant role in the long-term storage of sulfur and carbon within serpentinites with a potentially far-reaching impact on the global sulfur and carbon cycles. That said, controls on biological activity in peridotite-hosted hydrothermal systems, whether on land or under the sea, have not yet been conclusively proven and hence the impact of microbial activity on seafloor geochemical cycles remains unknown. This research will characterize the factors that control microbial activity within serpentinites. Determination of the coupling between biological activity and both temperature and fluid chemistry during serpentinization will be assessed via a novel combination of geochemical techniques that will indicate how these key parameters evolve through time. Specifically, multiple sulfur isotope (32S, 33S, 34S) analyses of bulk sulfides occurring in the serpentinites and additional, targeted, in-situ, sulfide/sulfate isotope measurements will be used to detect the presence of microbial activity. These data will be combined with detailed petrological and mineralogical examination and in-situ isotope analyses of host mineral phases. Serpentinites from three different tectonic settings and basic mineralogies will be examined: (1) the continental Iberian Margin, (2) the 15°20" N fracture zone along the Mid-Atlantic Ridge, and (3) serpentinites from an ophiolite sequence in the Northern Apennine, Italy. the first two were drilled as part of the Integrated Ocean Drilling Program (IODP). Research goals will be to determinee how variable fluid compositions and temperatures affect the occurrence of biological activity. This knowledge will provide new insights and constraints on the limits of life on Earth and how sulfur is cycled through altered oceanic crust, as well as how global geochemical cycles and the chemical exchange occur between different Earth reservoirs. Broader impacts of the project involve the support of two early career scientists, both of whom will be trained in the use of new cutting-edge techniques, foster collaboration between Virginia Tech and Harvard University, and engage two undergraduate students in the science.
