Multidisciplinary investigation on cold seeps with vigorous gas emissions in the Sea of Marmara (MarsiteCruise): Strategy for site detection and sampling and first scientific outcome

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

Clathrites: Archives of near-seafloor pore-fluid evolution (δ44/40Ca, δ13C, δ18O) in gas hydrate environments

Aragonitic clathrites are methane-derived precipitates that are found at sites of massive near-seafloor gas hydrate (clathrate) accumulations at the summit of southern Hydrate Ridge, Cascadia margin. These platy carbonate precipitates form inside or in proximity to gas hydrate, which in our study site currently coexists with a fluid that is highly enriched in dissolved ions as salts are excluded during gas hydrate formation. The clathrites record the preferential incorporation of 18O into the hydrate structure and hence the enrichment of 16O in the surrounding brine. We measured δ18O values as high as 2.27‰ relative to Peedee belemnite that correspond to a fluid composition of −1.18‰ relative to standard mean ocean water. The same trend can be observed in Ca isotopes. Ongoing clathrite precipitation causes enrichment of the 44Ca in the fluid and hence in the carbonates. Carbon isotopes confirm a methane source for the carbonates. Our triple stable isotope approach that uses the three main components of carbonates (Ca, C, O) provides insight into multiple parameters influencing the isotopic composition of the pore water and hence the isotopic composition of the clathrites. This approach provides a tool to monitor the geochemical processes during clathrate and clathrite formation, thus recording the evolution of the geochemical environment of gas hydrate systems

Multiple sulfur isotopic evidence for the origin of elemental sulfur in an iron-dominated gas hydrate-bearing sedimentary environment

Elemental sulfur is commonly regarded as the product of oxidative sulfur cycling in the sediment. However, reports on the occurrence of elemental sulfur in seepage areas are few and thus its origin and mechanisms controlling its distribution are insufficiently understood. Here, we analyzed the multiple sulfur isotopic compositions for elemental sulfur and pyrite from an iron-dominated gas hydrate-bearing sedimentary environment of the South China Sea to unravel the impact of sulfate-driven anaerobic oxidation of methane (SO4-AOM) on the formation of elemental sulfur. The multiple sulfur isotopes reveal variable ranges for both elemental sulfur and pyrite (δ34S: between −15.7 and +23.3‰ for elemental sulfur and between −35.3 and +34.4‰ for pyrite; Δ33S: between −0.08 and +0.06‰ for elemental sulfur and between −0.03 and +0.15‰ for pyrite). The enrichment of 34S in pyrite throughout the sediment core suggests pronounced SO4-AOM in paleo-sulfate-methane transition zones (SMTZ). In addition, the occurrence of seep carbonates with very negative δ13C values (as low as −57‰, V-PDB) coincides with the inferred paleo-SMTZs and agrees with formerly locally pronounced SO4-AOM. Interestingly, the multiple sulfur isotopic composition of elemental sulfur reveals a different pattern from that of pyrite derived from organoclastic sulfate reduction (i.e., with low δ34S and high Δ33S values for the latter). In comparison to coexisting pyrite, most of the elemental sulfur reveals higher δ34S values (as much as +28.9‰), which is best explained by an enrichment of 34S in the residual pool of dissolved sulfide generated by SO4-AOM. As an intermediate sulfur phase, elemental sulfur can form via sulfide oxidation coupled to iron reduction, but it can only persist in the absence of free sulfide. Therefore, the occurrence of 34S enriched elemental sulfur is likely to represent an oxidative product after hydrogen sulfide had vanished due to vertical displacement of the SMTZ. Our observations suggest that elemental sulfur may serve as a useful recorder for reconstructing the dynamics of sulfur cycling in modern and possibly ancient seepage areas