Award: OCE-1449604

Intellectual Merit: This project resulted in microbiological and geochemical characterrization of seafloor sediments in Guaymas Basin, a hydrothermally active spreading center in the Gulf of California (Mexico), and the adjacent Sonora Margin along the Mexican mainland. The sediments were sampled as 3-5 m long gravity cores, on RV El Puma in October 2014.The sediments showed the geochemical and microbial signatures for off-axis seepage on the Guaymas Basin ridge flanks, where shallow buried basalt sills (lava emplacements into the sediment layers) drive hydrothermal circulation even at a considerable distance off the central spreading center, where hydrothermal activity is usually focused. A mixture of hydrothermally and microbially generated methane, in its stable isotopic composition intermediate between hydrothermal methane at the spreading center and biogenic methane on the Sonora Magin, emerges at these off-axis seeps. Non-hydrothermal control sediments in Guaymas Basin show the standard geochemical and microbial signatures of suboxic, organic-rich seafloor sediments, and seep sediments from the nearby Sonora Margin are permeated with the extremely reduced, fully sulfidic and methane super-saturated fluids that are produced by sediment compaction and fluid expulsion along continental margins. All in all, Guaymas Basin provides contrasting, coexisting geological, geochemical and microbial regimes within the small area of this nascent ocean basin, ca. 50 miles from end to end. Broader Impacts: The sampling locations had been strategically selected to characterize promising locations in preparation for a deep drilling proposal within the International Ocean Discovery program [IODP]. The site survey that we conducted produced essential data in support of the IODP proposal and its rationale: to probe the deep subsurface across the entire width of the Guaymas Basin spreading center and to investigate the geochemical, sedimentological, and microbial characteristics of this model system for an emergent ocean basin surrounded by young continental margins. The IODP proposal, including the new site survey data generated in this project, was submitted in October 2016, and will be evaluated at the next IODP Science Evaluation Panel [SEP] meeting in La Jolla, CA, In January 2017. With the preparation and submission of the IODP proposal, this project has fulfilled its purpose and is finished; major results were already presented at the 2015 Fall AGU meeting, and will be published separately in a more fully fleshed-out version. Now it remains to be seen whether this proposal will have a successful afterlife as a fully fledged IODP drilling cruise. Last Modified: 11/30/2016 Submitted by: Andreas P Teske