Alteration of basaltic glass from the North Pond area, Mid-Atlantic Ridge 22°46'N, 46°05'W, Expedition 336

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Funds are requested for mineralogical and geochemical studies of basalt alteration and the involvement of microorganisms, using drill core samples and incubation slides. Rock samples were recovered during IODP Expedition 336 from the North Pond area on the western flank of the Mid-Atlantic Ridge. North Pond is representative of the globally most abundant, cold and wellventilated ridge flanks. These systems play a pivotal role in setting the chemistry of the oceans, yet, the hydrogeological-geochemical-microbiological couplings controling the rates and magnitudes of chemical exchange are very poorly understood. Expedition 336 installed seafloor observatories that will allow sampling formation fluids, facilitate incubation experiments, and provide insights into subbasement microbiological activities in the forthcoming years. The supplemental analyses of drill core material proposed here include geochemical and x-ray microtomography studies as well as electron- and raman-microcopy investigations. Collaborating scientists in the US will conduct x-ray absorption spectroscopy and molecular microbiological studies using splits of the same materials. The project's primary goal is a detailed and comprehensive description of basalt glass alteration in young ocean crust. One focus is on identifying alteration features believed to be related to microbial activity and collect further evidence for their biogenicity. Geochemical investigations of altered materials and their primary precursor phases will provide information on the direction and magnitude of chemical changes during seawater circulation and the associated alteration of basaltic crust. Trace element and stable isotopic compositions of mineral precipitates are used as a recorder of fluid composition and conditions of fluid-rock interaction. These results will be compared with direct observations on fluid composition and temperature in the borehole. Merging observations and data from the proposed study with those to be obtained by our cooperators and our joint efforts during the next years of observatory studies at North Pond will allow us to obtain fundamentally novel insights into ridge flank processes and their role in exchange budgets between lithosphere, hydrophere, and biosphere in the oceanic realm.