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Submitted by Gretchen Früh-Green, October 2015
This project is a comparative geochemical and stable isotope study of modern serpentinite-carbonate systems, focused on understanding geochemical and microbial processes associated with the formation of high alkaline fluids during serpentinization. Our study builds on an immense set of data from the peridotite-hosted Lost City hydrothermal system (MAR, 30°N), produced in collaboration with an international team of scientists over more than ten years. Lost City is unlike all other hydrothermal known to date and is characterized by metal- and CO2-poor, high pH fluids (9-11) with elevated hydrogen and methane contents resulting from serpentinization processes at depth.We specifically address open questions about the links between the inorganic reactions in the ultramafic basement rocks (i.e. serpentinizing reactions), cycling of carbon and sulfur, and microbial activity in these high pH systems. The overall goal of our project is to quantify C and S pools in active serpentinite-carbonate systems and to constrain their changes over time. To achieve this goal our project involved a comparative organic geochemical and C-and S-isotope study of Lost City with modern high alkaline Ca-OH springs and carbonate deposits associated with present-day serpentinization processes in Liguria (Italy) and resulted in two PhD theses (A. Delacour and E. Schwarzenbach). Our work has been supported by the Swiss National Science Foundation (SNSF) and has involved developing a number of novel analytical techniques to measure stable and radiogenic carbon species in natural samples. Field and laboratory studies are specifically aimed to (1) characterize the organic matter at a molecular and isotopic level; (2) constrain the origin and cycling of C and S in modern marine and meteoric systems; and (3) make quantitative volume estimates of the amount of abiotic and biotic organic carbon and CO2 that is sequestered in these environments. Multidiscipline studies of Lost City and the Atlantis Massif provide the basis for IODP drilling during Expedition 357 in 2015. A number of aspects of this project will continue to be investigated with new funding from SNSF. See also the project “Hydration and carbonation of mantle peridotite: IODP and ICDP drilling projects”.