Deep-water depositional systems and cold seeps of the Western Mediterranean, Gulf of Cadiz and Norwegian continental margins. Preliminary results of investigations during the TTR-16 cruise of RV Professor Logachev, May-July, 2006
Akhmetzhanov, A.M.; Kenyon, N.H.; Ivanov, M.K.; Westbrook, G.; Mazzini, A. (Ed.) (2008). Deep-water depositional systems and cold seeps of the Western Mediterranean, Gulf of Cadiz and Norwegian continental margins. Preliminary results of investigations during the TTR-16 cruise of RV Professor Logachev, May-July, 2006. Technical Series. Intergovernmental Oceanographic Commission = Série technique, 76. UNESCO: Paris. iii, 58 + annexes pp.
Part of: IOC Technical Series. Intergovernmental Oceanographic Commission = Série technique. UNESCO: Paris. ISSN 0074-1175
Interdisciplinary studies of deep-water environments including depositional systems and cold seeps were conducted by RV Professor Logachev in the Western Mediterranean, Gulf of Cadiz and Norwegian continental margins during the 16th Training-through-Research Cruise of UNESCO-IOC in the summer of 2006.During the first leg TTR was working in the distal setting of the Rhone Neofan looking at the terminal depositional lobes that are developing there. The sampling showed that most of the depositional elements of the distal Rhone Neofan have been draped by hemipelgic sediments since it last active phase. The new data provides new insights into the hemipelagic processes in the area and emphasises the importance of cascading currents, originating in shallow water, for the modern deep-water depositional environment of the distal Rhone Neofan.During the second leg in the Gulf of Cadiz a newly acquired high resolution sidescan sonar survey produced an excellent image of the two largest mud volcanoes in the Gulf - Yuma and Ginsburg. The new data were particularly useful for mapping the most active sites on the volcanoes which, in addition to being in the craters, were also found on their slopes.Another high resolution sidescan sonar survey covered Pen Duick escarpment and imaged two extensive faults bounding the structure to the east and west. Video survey and sampling showed the western fault to be the more active seepage site and large carbonate crusts and chemosynthetic fauna were recovered here by a grab sampler.The Darwin mud volcano, discovered a few months earlier by RRS Charles Darwin and which was found to host the largest population of chemosynthetic mussels Bathymodiolus Sp. observed so far in the Gulf of Cadiz, was mapped with high resolution sidescan sonar and sampled with TV-guided grab. The sidescan data show that the most active part of the volcano is on its top, being only about 100 m in diameter. Authigenic methane-derived carbonate crusts as well as living specimens of Bathymodiolus observed during video runs were successfully retrieved onboard.Traces of gas hydrate presence were noticed on almost all deep-water mud volcanoes of the Portuguese margin. The long awaited acoustic image of the Bonjardim mud volcano allowed the selection of a sampling site from which large, up to 6 cm across, aggregates of gas hydrates were recovered.Two gas chimneys were investigated in the Nyegga region on the Norwegian margin during the third leg with identical seismic experiments using an array of OBSs. The seismic experiment was designed to determine the structure of a chimney, which is typically 300-m in diameter, the presence of gas within it, the hydrate within and around it, and the presence and orientation of fractures through which gas and fluids migrate. The depth of investigation of the experiment was between 200 and 500 m beneath the seabed in a water depth of ~750 m, depending on the seismic source, and the experiment was to measure P-wave and Swave velocities, and seismic anisotropy, through S-wave splitting.For the first time, gas hydrates were sampled here from several sites, supporting the inference of several authors that hydrate was present, but which had never been verified by tangible evidence. As gas hydrate or strong bubbling were generally observed in the bottom part of several cores from different structures, it is inferred that the presence of gas hydrate prevented further penetration of the gravity corer at these sites. Although the presence of gas hydrates does not necessarily provide evidence of actual or very recent flow of free gas at the locations investigated, a near-constant flux of methane-rich fluid through the features containing hydrate is required to maintain the continued existence of hydrate, so close to the seabed.
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