GNGTS 2014 - Atti del 33° Convegno Nazionale

Normal faults control ���� ��� ���������� �� ��� ���� �� ��� fl �� ��� �� ��� uid flow structures at the rear of the Calabrian Arc (Paola Ridge, southeastern Tyrrhenian S M. Rovere 1 , F. Gamberi 1 , F. Zgur 2 , L. Facchin 2 , L. Sormani 2 , G. Visnovic 2 , A. Mercorella 1 , E. Leidi 1 1 Istituto di Scienze Marine, Consiglio Nazionale delle Ricerche, Bologna, Italy 2 Osservatorio Geofisico Sperimentale, Sgonico (TS), Italy Introduction. Cold seeps are seafloor expressions of focused and diffuse fluid flow in the marine environment (Judd and Hovland, 2007). Cold seeps can have different seabed morphologies and deeper structures to form their plumping system, they can be distinguished in: oil and gas seeps, gas hydrate pingoes, brine pools, pockmarks, mud volcanoes and diapirs. Several processes have been invocated to explain, for example, the main mechanisms involved in the formation and growth of pockmarks, which are the most distinct feature at gas seeps and escapes, but these have remained, so far, largely unclear (Marcon et al. , 2013), thought physical models have been advanced (Cathles et al. , 2010). Normal fault mechanisms are now often recognized to be associated with the presence of seeps, mud volcanoes and pockmarks (Ostanin et al. , 2013; Sultan et al. , 2014), with faults acting as the deep-rooted plumbing system, often along salt diapir anks and related faults, along which the uid ux is high (Serié et al. , 2012). Cold seeps slowly release hydrogen sulfide, methane and other hydrocarbon-rich fluids in the water column and, for this reason, they are considered possible players in the global greenhouse gas emission budget and the consumption of oxygen at the seafloor (Boetius and Wenzhöfer, 2013). Uncertainties remain regarding the quantity of free methane that is emitted from deep-water seeps into the water column, with several authors showing that, at least in gas hydrates scenarios, most of the methane emitted per year within the gas hydrate stability zone remain trapped in the deep ocean (Römer et al. , 2012). The detection of the gas bubbles can be direct, using sophisticated sampling methodologies, or indirect by means of acoustic echo-sounders that detect gas bubbles in the water column, due to the acoustic impedance contrast between water and the free gas in the bubbles. More and more often swath bathymetry multibeam systems are now used not only to detect the gas flares (Schneider von Deimling et al. , 2007), but also to precisely locate and map them and to assess a gross quantification of gas emission (Nikolovska et al. , 2008). The recognition of seep sites on the sea oor is often favoured by the detection of anomalously high acoustic seafloor backscatter on side-scan sonar or multibeam backscatter (Klaucke et al. , 2008). High seafloor backscatter is caused by the enhanced acoustic impedance contrast between certain regions of the sea oor and their surroundings. At cold seeps, this contrast is caused by: sharp changes in the sea oor morphology (pockmarks, mud volcanoes); precipitation of authigenic carbonates at the seafloor; chemosynthetic organisms (clams, tube worms); bubbles or gas hydrates in the sediment (Naudts et al. , 2008, for a complete review). The cold seeps support biomes whose primary producers do not depend directly on photosynthesis. The alternative function is served by chemotrophic bacteria and archaea, which have symbiotic relationships to heterotrophic organisms which host them, often intracellularly. These bacteria are sul de oxidizers, using the free energy yield from the oxidation of sul de with oxygen to x carbon dioxide. In exchange for providing nutrition for the host, the symbionts are sheltered from grazing and they receive a steady source of sul de and oxygen (Boetius, 2005). The most common heterotrophic organisms found at these sites are: Vestimentiferan tube worms, mytilid mussels, vesicomyid clams and infaunal lucinids. Chemosynthetic bivalves are prominent constituents of the cold seep fauna and are represented by five families: Solemyidae, Lucinidae, Vesicomyidae, Thyasiridae and Mytilidae (Oliver et al. , 2011). Bacterial mats are also frequently present at active sites. Study area. Cold seeps were discovered at 500-1000 m water depth along the Paola Ridge, on the continental slope of the NW Calabrian margin (south-eastern Tyrrhenian Sea, Fig. 1a), GNGTS 2014 S essione 1.2 169