Description: Submarine mud volcanism represents an important. pathway for methane from deeper reservoirs to the surface, where it enters the benthic carbon cycle. To quantify overall methane release from the Captain Arutyunov mud volcano (CAMV) and to assess the contribution of macrobenthic seep organisms to the regulation of the benthic methane flux, we linked water column methane concentrations, seabed methane emission and pore water geochemistry to the spatial distribution of seep biota. Prominent organisms of the CAMV seep biota were 3 different species of frenulate tubeworms. Seabed methane emission ranged from 0.001 to 0.66 mmol m(-2) d(-1). Dense patches of tubeworms were associated with the lowest seabed methane emission. Elevated methane emission was associated with a sporadic distribution of tubeworms and the occurrence of numerous mud clasts. Despite the presence of a large subsurface methane reservoir, the estimated total methane release from CAMV was low (0.006 x 10(6) mol yr(-1)). In addition to direct methane consumption by Siboglinum poseidoni, the tubeworms likely contribute to the retention of methane carbon in the sediment by affecting bacterial communities in the proximity of the tubes. The siboglinids create new meso-scale habitats on the sediment Surface, increasing habitat heterogeneity and introducing niches for bacterial communities.

Description: Cruise SO268 is fully integrated into the second phase of the European collaborative JPI-Oceans project MiningImpact and is designed to assess the environmental impacts of deep-sea mining of polymetallic nodules in the Clarion-Clipperton Fracture Zone (CCZ). In particular, the cruise aimed at conducting an independent scientific monitoring of the first industrial test of a pre-protoype nodule collector by the Belgian company DEME-GSR. The work includes collecting the required baseline data in the designated trial and reference sites in the Belgian and German contract areas, a quantification of the spatial and temporal spread of the produced sediment plume during the trials as well as a first assessment of the generated environmental impacts. However, during SO268 Leg 1 DEME-GSR informed us that the collector trials would not take place as scheduled due to unresolvable technical problems. Thus, we adjusted our work plan accordingly by implementing our backup plan. This involved conducting a small-scale sediment plume experiment with a small chain dredge to quantify the spatial and temporal dispersal of the suspended sediment particles, their concentration in the plume as well as the spatial footprint and thickness of the deposited sediment blanket on the seabed. Leg 1 and 2 acquired detailed environmental baseline data in the designated collector trial and reference sites as well as the site of the small-scale sediment plume experiment. The plume experiment was monitored by an array of acoustic and optical sensors and the impacted area was investigated in order to develop standards and protocols for impact assessments and recommendations for marine policy and international legislation. A more technical aim of the cruise was to test tools, technologies, and a concept for the environmental monitoring of future deep-sea mining operations. This comprised oceanographic, biological, microbiological, biogeochemical, and geologic investigations which required the deployment of a multitude of seagoing equipment, such as ROV Kiel 6000 for sampling of sediments, nodules, and benthic fauna as well as carrying out in situ measurements and experiments, and the deployment of the plume sensor array. AUV ABYSS and ROV Kiel 6000 were used for high-resolution acoustic mapping of the seafloor using mounted multibeam systems and video/photo surveys of the manganese nodule habitat. This work was accompanied by video observations with the OFOS system. Several benthic landers and moorings with acoustic and optical sensors were deployed and recovered for the measurements of physical and chemical oceanographic variables. Coring devices (i.e., box corer, gravity corer, TV-guided multiple corer, ROV-operaten push cores) were used to collect sediment samples for biological, geochemical, and microbiological analyses, and a CTD rosette water sampler, in situ pumps, and a bottom water sampler sampled the water column. In addition, recolonization experiments for nodule-associated fauna were started by deploying artificial hard substrates on the seabed of the working areas.