Collaborative Research: Multiple Sulfur Isotope Tracers of the Subsurface Biosphere in Oceanic Basement

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ABSTRACT 0622949/0622982/0623550 (Alt/Rouxel/Ono)Intellectual Merit: Fluids circulate at low temperatures (100C) through the upper oceanic crust on mid ocean ridge flanks and in ocean basins providing vast potential habitats for a subsurface biosphere. This work tests the hypothesis that microbial sulfate reduction occurs in crystalline oceanic basement rocks (basalts and gabbros), and that it is a widespread and important processes affecting the sulfur budget of the oceanic crust. The research plan entails determining the contents and stable isotope ratios of sulfur (S) and selenium (Se) in altered oceanic basement. Specific analyses include ion microprobe analyses of del 34S of individual sulfide grains, whole rock S and Se contents, Se isotopes and multiple-sulfur isotope ratio analyses (del 33S, del 34S and del 36S). The data enable testing for microbial signatures, provide evidence for fluid evolution, help constrain redox conditions and the sources of S in the rocks (remobilized basaltic sulfide or seawater), and aid in understanding the effects of microbial activity on the budget for S during alteration of oceanic basement. The research focuses on upper oceanic basement from well-studied IODP Site U1301, but also includes gabbros from ODP Hole 735B, and limited analyses of other basaltic basement. Results of the work help improve our understanding of microbial activity during alteration of ocean crust, and provide a basis for establishing links between geochemistry and microbiology. Broader Impacts:This research will support two early career scientists and one senior scientist in Washington DC, Massachusetts, and Michigan, respectively. The research is multi-institutional and will engage undergraduates in research. The work leverages NSF investments by using samples collected by the NSF-Ocean Drilling program.