Deep-Sea Investigations on Hydrothermal Site Rainbow (MAR 36°14 N)

Hydrothermal site Rainbow, one of the few known site on an ultramafic basement, is an exceptional target for the multidisciplinary study of hydrothermal phenomena. It is one of the two targets of the MoMAR (Monitoring the Mid Atlantic Ridge) project patronized by InterRidge, and is the focus of an IODP drilling project. What makes this site exceptional is the abundance of natural hydrogen, methane, and iron, an element which plays a major role in active processes, down to the scale of molecules. During Cruise MomarDream (25 Aug. - 15 Sept. 2008), R/V L'Atalante and ROV Victor spent 3 weeks on site Rainbow to carry out detailed investigation of this unique area. The goals of the cruise were, first, to study the role of iron in the geological, hydrological, and biological processes, and second, to identify potential drilling targets. Beyond the requirement of a "zero state" for the repeated observations and in fine the site monitoring in the framework of the MOMAR project, the completion of an exhaustive inventory of the biological populations is needed for the sake of preservation of a fragile environment. Multibeam bathymetry and magnetics have been collected by ROV Victor 50 m above the seafloor on a 4 km × 3 km wide box centered on the site and covering about 25% of the Rainbow Massif. Similarly, multibeam bathymetry, magnetics, and high resolution photographs have been acquired 10 m above the seafloor on a 650 m × 500 m box centered on the site, and on a 300 m × 300 m box centered on a field of dead clams. A nearly full coverage was obtained in these boxes. Direct geological exploration was also carried out and allowed the collection of rock samples, complemented by an intensive dredge program when the ROV was onboard. A large part of the cruise was devoted to biological studies sensu lato, including the collection of fluids dedicated to the study of abiotic organic molecules and metagenomics, the collection of sulfide for microbiological investigations, in situ chemical measurements to characterize the hydrothermal environment, the collection of shrimps and mussels, and the deployment and recollection of colonization modules. As for the initial geological results, the microbathymetry and the rock sampling are presented in two posters (Gente et al.; Ildefonse et al.). A very strong positive magnetic anomaly is clearly associated to the site and likely reflects the strong magnetization of serpentinites impregnated by sulfides. Another strong positive anomaly corresponds to a zone of similar size located 500 m south of Site Rainbow, and may correspond to a fossil site, as suggested by tenuous evidences from rock samples collected in this area. Momardream party : K. Bukas, V. Cueff Gauchard, L. Durand, F. Gaill, C. Konn, F. Lartaud, N. Le Bris, G. Musset, A. Nunes, J. Renard, V. Riou, A. Tasiemski, P. Torres, I. Vojdani, M. Zbinden