For our second working area after the Karasik seamount, we selected a small, steep and rocky mound of 8 km diameter, which shows evidence of active hydrothermalism. The mount rises more than 1000 m from the rift valley and appears to have recently been volcanically active, as evidenced by the vast occurrence of fresh pillow basalts and glass shards near its summit. We were able to map the area precisely using the ships own Multibeam Echosounder, an ATLAS Hydrosweep system. Common echosounders such as those used for fishing or as a depth sensor use only one beam of sound for imaging. But this one has 960 individual beams to record the shape of the seafloor below the ship across a wide angle. The lower figures show a 3D view as seen from the northwest and a profile view between each of the sites. This bathymetric data is then used for further exploration and sampling of the seafloor (Fig. 2).

Already during the first dive, we discovered hydrothermal precipitates and potential seepage sites at the seafloor near the summit of the volcano (Fig. 4). The type of hydrothermalism present at the seafloor depends primarily on the amount of heat available and the types of rock percolated by the fluids. Currently, we are looking for the main vents of the mound, which produce the characteristic plumes of energy-rich substances and particles above the mound. If we find them, then their specific morphology, the color of the chimneys and the fluids emanating, as well as the types of life associated with the vent structures will tell us more about the underlying geology and chemistry of the ongoing hydrothermalism at Gakkel Ridge.

Molecular hydrogen and methane are major reduced compounds emitted from hydrothermal vents. To quantify these potential microbial energy sources, their concentrations were determined from water samples retrieved in different water sampler casts with on board gas chromatographs. The fresh plume has very high hydrogen and methane concentrations with up to 350 nM hydrogen and 280 nM of methane indicating that there are several mM of these compounds in the source vent fluids. Notably, the ratios of hydrogen and methane are highly variable in different plume samples. This indicates that the individual plumes may derive from different venting sources with substantially different chemistry. Alternatively, this variability may reflect different preferences of the microorganisms for the plume gases.  Previous studies have shown that hydrogen consumption in plumes is more rapid than methane oxidation, thus ‘older’ plumes may have more methane remaining than hydrogen. To determine whether the kinetics of microbial hydrogen and methane oxidation are due to different affinities by microorganisms, incubations with natural plume microbial communities have been started by the microbiologists on board. Reduced compounds of the plume waters may provide an energy source for chemosynthetic bacteria, which can fix CO2 just like plants, but without sunlight. First results show that the microbial activity in the plume waters is much higher than in control experiments conducted with water collected from reference sites, distant from the hydrothermal waters. We additionally have a novel spectrometric device on board that lets us take a first glimpse on the origin of the methane. It is isotopically much heavier than the atmospheric methane, suggesting that it is formed by abiotic processes in the seabed.