Methane and Helium Investigations in the Water Column of the Hydrothermal Vent Field Logatchev - Cruise No. M81/2c - April 23 - May 14, 2010 - Bridgetown (Barbados) - Bremerhaven (Germany) (English)

The main goal of M81/2c GASLOG was to investigate the water column around the 'Logatchev'hydrothermal vent field (LHF) on the mid-Atlantic ridge at 14°45’N. This site is known to permanently release hot vent fluids into the water column. These fluids are highly enriched with metal and gases namely hydrogen, methane, and helium. The latter two gases were sampled and analyzed for concentration measurements and later isotope analysis. Moreover, turbidity sensors during CTD casting were used to gather online information about the hydrothermal plume in the water column. Overall a total of 17 CTDs could be conducted around LHF within the three days of station work. One background CTD was conducted one day before reaching LHF approximately 380nm west of the working area. The sampling strategy was to first complete a section south of LHF, then to sample directly above Logatchev I/II, and finally to conduct a N-S section. All stations around LHF (max. distance 6.5nm) show elevated methane concentration in form of a gaussian plume peaking around 2900 m water depth with a vertical plume extension of 400 m. In the nearfield of Logatchev I methane concentration values reach values up to 119nM around 2900 m. Correspondingly, the optical backscatter data plot here with higher values and show three spatially separated plumes at various depths. The data indicate a correlation between optical backscatter and methane concentration. Moreover, the LHF plume clearly plots in TS diagrams. Further analysis of the plume will be possible after interpretation of the 3He and δ 13CCH4 hydrographic sections. Two lowered ADCP were attached during each CTD cast to constrain the currents and a predominant direction towards the north was present close to the seafloor and plume area, respectively. Later processing will allow to derive an integrated water mass flux and the corresponding dissolved gas flux and source strength of the hydrothermal vents. By means of comparing the conservative tracer concentration 3He and CH4 under consideration of the δ 13CCH4 isotopic signature, the hitherto unknown microbial methane oxidation rate can be constrained.