GNGTS 2014 - Atti del 33° Convegno Nazionale

Higher resolution acquisitions and seafloor sampling were carried out in 2011 (Rovere et al. , 2014a). The cold seeps in the area are represented by 2 mud volcanoes (RMV and R1MV structures) and 3 main mud diapir structures characterized by the presence of pockmarks (D1, D2, D3). There are other 4 pockmark-related dome structures in the area dissected by normal faults (Fig. 1a). The data lead to important findings and the discovery of active gas venting at the seafloor in the mud volcanoes and ceased seepage activity in the mud diapirs. In the study area, when the fluid flow is vigorous, it produces mud volcanoes, gas plumes in the water column and precipitates oxy-hydroxides, sulfides and siderites in the sub-seafloor. Conversely, when the flux is very low or even absent, authigenic calcite and aragonite, associated with chemosymbiontic fauna, are formed near the seabed on pockmarks-punctuated mud diapirs. The mud volcanoes RMV and R1MV are characterized by high backscatter signature, related to the presence of gas venting at the seabed. Intermediate backscatter typifies areas where free venting is hampered by the presence of mudflows at the seafloor, downslope the mud volcano summits. The mud diapirs are, on the contrary, characterized by low seafloor backscatter, because the authigenic limestones are mostly buried below 1-2 m of sediment and locally exposed only along the normal faults. The carbonates have the typical isotopic signature indicating formation during sulfate-dependent microbially-mediated anaerobic oxidation of methane. They are associated with Lucinoma borealis , the youngest being dated 490 +/- 30 BP and with Acharax sp. the youngest dated 20,010 +/- 90 BP. This is the first documented occurrence of Acharax sp. in the Quaternary of the Mediterranean region and probably shows the replacement of the Acharax sp. with Lucina sp. in the cold seep habitat during the last sea level rise and general warmer conditions (Rovere et al. , 2014b). Siderites precipitated in burrows below the mud volcanoes and mud flows showed enrichment in �δ 13 C and �δ 18 O, which are compatible with their precipitation in the methanogenic zone (Rovere et al. , in press). The heavy-oxygen isotopic compositions of the siderites are possibly related to the dissociation of gas hydrates, which have not been mapped in the seismic data (1 kJ sparker). The same seismic data showed that NW-SE and NNW-SSE-oriented normal faults were the most probable mechanism for the emplacement of the mud mobile structures (Fig. 1). These faults can be regarded as the marine prolongation of the fault zones that dissect the Calabrian Arc from SE to NW (Fig. 1a). According to the seismic data, diapirism is no longer active. The normal faults are exploited as primary conduits for upward fluid migration and control the seepage distribution and activity. In particular, tectonic structures control the shape and the distribution of the pockmarks, and the location where authigenic carbonates, associated with chemosymbiontic fauna, formed hardgrounds at the seafloor (Rovere et al. , 2014a). Data and methods. In June 2014 a multichannel seismic survey was undertaken in the cold seep area during the campaign MarBeep 2014 on board the R/V CNR Urania. The seismics were designed to image the deeper faults which were already suggested to be related to the seeps and that were not perfectly visible in the previously available data. 322 km of multichannel profiles were acquired (Fig. 1a). The seismic source was a single 60 in 3 mini GI-gun set in Harmonic Mode (30 Generator + 30 Injector) with a shot interval of 9.375 m at 2000 psi. The data were collected by a 300 m long, 96 channels digital streamer with a trace distance of 3.125 m. Both the gun and the streamer were towed at 1.5 m below sea level to minimize the ghost effect on the emitted spectrum (centred well above 200 Hz), thus preserving the high frequencies needed to better resolve the shallower targets. During the acquisition four birds leveller were used as a whole to keep the streamer at a constant depth. The choice of such an equipment and parameters drove to an excellent compromise between penetration (700 – 800 m) and resolution (metric along the vertical axes if assuming a λ/4 criterion, with an actual 1.56 m lateral distance between the traces in the stack section). The acquisition parameters are summarized in Tab. 1. With this configuration, resulting in an effective horizontal sampling of 1.56 m in the stacked section, the maximum attainable fold coverage was 16 traces / CDP. GNGTS 2014 S essione 1.2 171