Description: Multibeam echosounder (MBES) data recorded during RV METEOR cruise M146 between 17.03.2018 and 16.04.2018. The data covers the transit (cruise started in Receive/Brazil) in international waters crossing the Atlantic Ocean, a single survey line crossing Tropiquito seamount (SW of Tropic seamount) and the main target at Henry seamount, southeast of El Hierro, Canary Islands. The main objective of this cruise was to discover hydrothermal venting sites at Henry Seamount as data acquired during the previous cruise M66/1 showed evidences that there might be active hydrothermal circulation at Henry Seamount. Therefore, the bathymetric surveys conducted with hull-mounted MBES and the MARUM AUV SEAL were accompanied with investigations in the water column, heat flow measurements, TV-sled dives and high-resolution reflection seismic. CI Citation: Paul Wintersteller (seafloor-imaging@marum.de) as responsible party for bathymetry and backscatter post-processing and its products. Description of the data source: During the RV METEOR cruise M146 the Kongsberg EM122 multibeam echosounder with a nominal sounding frequency of 11.5 to 12.5 kHz was utilized. 288 beams (and up to 864 soundings in equidistant and dual swath mode) are formed for each ping with a 1°(Tx)/2°(Rx) footprint while the seafloor is detected using amplitude and phase information for each beam sounding. For further information consult https://www.km.kongsberg.com/. The EM122 was recording constantly within the permitted areas, either designated to bathymetry surveys or flare imaging-surveys. In total, three different subsets of hydroacoustic data were acquired: a transatlantic dataset covering the transit in international waters; a second dataset covering the area of Tropiquito Seamount, which is situated about 100 km southwest of Tropico seamount and the main dataset of the area of Henry Seamount, which is located roughly 40 km southeast of El Hierro (Canary Islands). Responsible person during this cruise / PI: Miriam Römer (mroemer@marum.de) & Paul Wintersteller (pwintersteller@marum.de) Description of data processing: Postprocessing and products were conducted by the Seafloor-Imaging & Mapping group of MARUM/FB5, responsible person: Paul Wintersteller (seafloor-imaging@marum.de). The open source software MB-System suite (Caress, D.W., and D.N. Chayes, MB-System Version 5.5, open source software distributed from the MBARI and L-DEO web sites, 2000-2012.) was utilized for this purpose. A tide correction was applied, based on the Oregon State University (OSU) tidal prediction software (OTPS) that is retrievable through MB-System. During M146, a CTD mounted on the heatflow lance and an autonomous sound velocity profiler were used several times between the MBES surveys. The resulting sound velocity profiles (SVP) were applied during the acquisition of the hydro acoustic data. Since no CTD cast was conducted during the transit from Recife (Brazil) to Tropiquito, the sound velocity of the transatlantic dataset was corrected utilizing the MB-System tool mblevitus. It generates annual mean water SVPs for a specified location using temperature and salinity data from the 1982 Climatological Atlas of the World Ocean (Levitus, S., Climatological Atlas of the World Ocean, NOAA Professional Paper 13, U.S. Government Printing Office, Washington D.C., 173pp, 1982). The SVP values are calculated using the DelGrosso equations (Dusha, B. D., Worcester P. F., Cornuelle B. D. and Howe, B. M., "On equations for the speed of sound in seawater", J. Acoust. Soc. Am., 93, 255-275, 1993). Roll, pitch and heave corrections were not applied for the M146 data. Bathymetric data has been manually cleaned for existing artefacts with mbeditviz. NetCDF (GMT) grids of the product and the statistics were created using mbgrid. No total propagated uncertainty (TPU) has been calculated to gather vertical or horizontal accuracy. The currently published bathymetric grids of the cruise have a resolution of 50 m (Tropiquito dataset) and 70 m (Transatlantic and Henry Seamount dataset). A higher resolution is, at least partly, achievable. The grid extended with _num represents a raster dataset with the statistical number of beams/depths taken into account to create the depth of the cell. The extended _sd -grid contains the standard deviation for each cell. All grids produced are retrievable through the PANGAEA database (www.pangaea.de). Chief Scientist: A. Klügel (akluegel@uni-bremen.de) CR: not yet available CSR: not yet available A special thanks goes to the watchkeeper during M146: Rachel Barrett, Philipp Held, Kai Frederik Lenz, Katja Lindhorst, Anne-Christin Melcher, Laura Kramer, Miriam Römer, Nikolas Stange.

Description: The continental slopes of the Black Sea show abundant manifestations of gas seepage in water depth of 〈720 m, but underlying controls are still not fully understood. Here, we investigate gas seepage along the Bulgarian and Romanian Black Sea margin using acoustic multibeam water column, bathymetry, backscatter, and sub-bottom profiler data to determine linkages between sub-seafloor structures, seafloor gas seeps, and gas discharge into the water column. More than 10,000 seepage sites over an area of ∼3,000 km 2 were identified. The maximum water depth of gas seepage is controlled by the onset of the structure I gas hydrate stability zone in ∼720 m depth. However, gas seepage is not randomly distributed elsewhere. We classify three factors controlling on gas seepage locations into depositional, erosional, and tectonic factors. Depositional factors are associated with regionally occurring sediment waves forming focusing effects and mass-transport deposits (MTDs) with limited sediment drape. Elongated seafloor depressions linked to faulting and gas seepage develop at the base between adjacent sediment waves. The elongated depressions become progressively wider and deeper toward shallow water depths and culminate in some locations into clusters of pockmarks. MTDs cover larger regions and level out paleo-topography. Their surface morphology results in fault-like deformation patterns of the sediment drape on top of the MTDs that is locally utilized for gas migration. Erosional factors are seen along channels and canyons as well as slope failures, where gas discharge occurs along head-scarps and ridges. Sediment that was removed by slope failures cover larger regions down-slope. Those regions are devoid of gas seepage either by forming impermeable barriers to gas migration or by removal of the formerly gas-rich sediments. Deep-rooted tectonic control on gas migration is seen in the eastern study region with wide-spread normal faulting promoting gas migration. Overall, gas seepage is widespread along the margin. Gas migration appears more vigorous in shallow waters below ∼160 m water depth, but the number of flare sites is not necessarily an indicator of the total volume of gas released.