Birot, Dominique

The Black Sea bottom is well known to be earth’s largest anaerobic methane source, hosting a huge amount of cold seeps releasing significant volumes of methane of both thermogenic and biogenic origin. Taking into account the well-known effects of methane concerning global warming, including the warming up of the oceans, an effective monitoring of its output from the Black Sea is nowadays an essential target for interdisciplinary studies. We discuss the results achieved during monitoring campaigns aimed to detect and track methane flares from the seafloor of the Romanian sector of the Black Sea, in order to better constrain the possible mechanisms responsible for its injection from the marine sediments, through the water column, into the atmosphere. In the mainframe of the ENVRI-Plus project, we deployed a multidisciplinary seafloor observatory for short, mid and long time monitoring and collected samples of the water column. The multidisciplinary seafloor observatory was equipped with probes for passive acoustic signals, dissolved CH4 and chemical-physical parameters. The collected data showed a high concentration of dissolved methane up to values of 5.8 micromol/L. Passive acoustics data in the frequencies range 40–2,500 Hz allow us to discriminate different degassing mechanisms and degassing styles. The acoustic energy associated with gas bubbling is interpreted as a consequence of the gas dynamics along the water column while the acoustic range 2–20 Hz reveals vibration mechanisms generated by gas dynamic’s along the cracks and inside the sediments.

MarsiteCruise was undertaken in October/November 2014 in the Sea of Marmara to gain detailed insight into the fate of fluids migrating within the sedimentary column and partially released into the water column. The overall objective of the project was to achieve a more global understanding of cold-seep dynamics in the context of a major active strike-slip fault. Five remotely operated vehicle (ROV) dives were performed at selected areas along the North Anatolian Fault and inherited faults. To efficiently detect, select and sample the gas seeps, we applied an original procedure. It combines sequentially (1) the acquisition of ship-borne multibeam acoustic data from the water column prior to each dive to detect gas emission sites and to design the tracks of the ROV dives, (2) in situ and real-time Raman spectroscopy analysis of the gas stream, and (3) onboard determination of molecular and isotopic compositions of the collected gas bubbles. The in situ Raman spectroscopy was used as a decision-making tool to evaluate the need for continuing with the sampling of gases from the discovered seep, or to move to another one. Push cores were gathered to study buried carbonates and pore waters at the surficial sediment, while CTD-Rosette allowed collecting samples to measure dissolved-methane concentration within the water column followed by a comparison with measurements from samples collected with the submersible Nautile during the Marnaut cruise in 2007. Overall, the visited sites were characterized by a wide diversity of seeps. CO2- and oil-rich seeps were found at the westernmost part of the sea in the Tekirdag Basin, while amphipods, anemones and coral populated the sites visited at the easternmost part in the Cinarcik Basin. Methane-derived authigenic carbonates and bacterial mats were widespread on the seafloor at all sites with variable size and distributions. The measured methane concentrations in the water column were up to 377 μmol, and the dissolved pore-water profiles indicated the occurrence of sulfate depleting processes accompanied with carbonate precipitation. The pore-water profiles display evidence of biogeochemical transformations leading to the fast depletion of seawater sulfate within the first 25-cm depth of the sediment. These results show that the North Anatolian Fault and inherited faults are important migration paths for fluids for which a significant part is discharged into the water column, contributing to the increase of methane concentration at the bottom seawater and favoring the development of specific ecosystems