Organic Carbon in Altered Submarine Basalts: Tracing the Subsurface Biosphere and Role in the Global Carbon Cycle

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Recent analyses show that submarine basalts contain organic carbon at the level of a few hundred parts per million. The distribution and extent of this carbon in the subsurface, however, is presently not known. Because seafloor basalts cover two-thirds of Earth's surface, the carbon that is held within them may be important for the global carbon cycle; and it is possible that this carbon and its characteristics can be utilized as a tracer of the subsurface biosphere. Preliminary work shows that elevated sulfur and organic carbon contents and isotopic signatures in seafloor basalts are consistent with functional genes that indicate the presence of sulfate reducing microbes and methanogens in these rocks, as well as changes in the organic carbon in basement fluids. Although, sulfur isotopes have been measured in many different subsurface environments, the microbial impacts on carbon in these different settings is not known. This research will measure carbon contents and the isotopic signature of carbon in seafloor basalts to trace the distribution and extent of microbial effects in the subsurface. Work will include analyzing total carbon contents and the organic carbon content of bulk rocks and the 13C/12C isotopic ratios of both the total carbon and the organic carbon. These data will be combined with sulfur isotope and microbiological data from related studies by the investigator and his collaborators, with the the new carbon data providing a better understanding of the subsurface biosphere in oceanic basement. Samples will be analyzed from three deep basement sites that represent different thermal and fluid flow regimes: (1) the North Atlantic, (2) the southwest Pacific Ocean, and (3) the northeast Pacific Ocean. The distribution of carbon and biological effects in the samples will be examined from the mineral (centimeter scale) to the rock (tens to hundreds of meters) scale. Goals are to understand how these element and isotopic indicators are related to ocean crust fluid flow pathways (alteration halos, breccias), lithology (massive vs pillow basalt), and inorganic alteration effects. This will enable a better understanding of the controls on microbial activity in ocean basement. The data for carbon contents of different rock types and flow regimes will be used to calculate local and global mass balances, targeting the role and quantification of organic carbon in submarine basalts. Broader impacts of the work inclde undergraduate student support and incorportating them into the full scientific process. Additional impacts include support of an NSF-funded stable isotope facility, incorporation of reserch results into university level course work, and outreach and interactions with middle school teachers and students in Ann Arbor, Michigan.