Habitat mapping in extreme deep sea environment: geosphere–biosphere interaction in deep anoxic basins

The recent exploration of the Mediterranean Ridge area has resulted in the discovery of some of the most extreme deep-sea environments, characterized by peculiar geochemical and physical condition. Tyro Basin was the first deep sea, hypersaline brinefilled, anoxic depression found. This and subsequent structures were named “hypersaline anoxic basins”, having total dissolved salts as high as 500 g/L and sulphide content upto 20 mM/L. The origin of these structures is morphologically connected to the compressive regime of the Mediterranean Ridge, due to the African plate subducting beneath Eurasia. The brine composition and physical characteristics are mainly due to the dissolution of underlying Messinian evaporites. The brines are characterized by the absence of oxygen, the presence of high concentrations of H2S and CH4, and by an extremely sharp seawater/brine interface, thus with a strong density contrast. This interface is a hotspot for a large variety of mostly redox-controled strong bacterial activity. In the last years, different projects have dealt with with the study of those basins. The work has focussed on the reconstruction of drastic composition changes of the water column, the evaluation of the anoxia trends and of the chemical elements concentration, and more recently, thanks to the technical innovations on seafloor mapping, to a progressively more accurate characterization of the shape and structure of the seabed. In the last three years, the Milan–Bicocca University has been responsible for geophysical data acquisition and analysis in the EU Moccha Project (Multidisciplinary study of continental/ocean climate dynamics using high-resolution records from the eastern Mediterranean), whose cruises (Doppio 2008, Macchiato 2009, Ristretto&Lungo 2010–11) allowed the collection of information (Multibeam Echosounder Simrad EM302) on several target areas at the Mediterranean Ridge. The processing and elaboration of the geophysical data allowed a highly detailed and extended morpho-bathymetric characterization of Urania, Atalante, Discovery, Bannock, Medee, Kryos, and Thetis anoxic basins. The coupling of new geophysical information with the biotechnological results of EU BIODEEP Project (Biotechnologies from the Deep: EU Community 2001–2004), also coordinated by Milan–Bicocca University, will be presented in this work as a first approach to a “Habitat Mapping” definition on the extreme anoxic basin environment, suggesting a methodology for subsequent studies on the same area.