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The research cruise to the Eastern Mediterranean (GPF-18-2-40) originally planned on RV METEOR was relocated to RV SONNE (Fig. 1.2) due to the reduced number of scientists as part of the corona pandemic. The main objective of the Bremen Ocean Cluster expedition (DFG, EXC2077) was to investigate the interactions between the seabed and ocean water in Greek waters, whereby the plate tectonic constellation of a broad collision zone represents a special tectonic drive. A secondary goal was the sampling of the Sartori mud volcano, which is being processed in Italian waters as part of a separate DFG project and for which the GPF granted an additional permit for ship time (GPF 20-1_054). The expedition began on 12 October in Emden/Germany and ended on 01 December 2020, in Emden. Investigations on mud volcanoes were carried out divided into 3 working areas (Fig. 1.1, the Sartori mud volcano in the Calabrian arc, the so-called Cobblestone Area, the Olimpi mud volcano field including the United Nation Ridge). With the MARUM AUV SEAL (Fig. 1.3) 11 dives were successfully carried out to create high-resolution detailed maps of certain seafloor structures. A total of 38 gravity cores (Fig. 1.4), 30 multicorers (Fig. 1.5) and 4 minicorers were used for sampling sediments and 6 CTD stations for sampling methane in the water column. Furthermore, 10 profiles were carried out with the heat flow lance and 5 observation profiles with the on-board OFOS. In four different provinces, 16 mud volcanoes were examined, 10 of which are characterized by pore waters that show a distinct freshening, while three mud volcanoes, Napoli, Heraklion and Gelendzhik, are characterized by very high salt concentrations. The salt accumulations in these structures are derived from the Messinian salt deposits in the subbed, from which salty brines arise through subrosion, which interact in various ways with the mud volcanoes. The study areas were selected based on preliminary surveys and morphological structures and increased backscatter patterns from multibeam mapping carried out over 3580 nautical miles in Italian and Greek waters. 326

We studied seafloor characteristics, water column anomalies, and sediment methane geochemistry in the German sector of the central North Sea during a research cruise with the German research vessel Heincke in summer 2019. An extensive hydroacoustic mapping campaign revealed the presence and distribution of flares in the water column, indicative for gas bubble releases as well as for geophysical subsurface indications of elevated gas concentrations. We analyzed the spatial distances of detected flares to subsurface salt diapir locations, seismically identified gas accumulations, and abandoned well sites. Continuous and discrete measurements of dissolved methane concentrations in the water column support the identification of seepage from the seafloor. Our data demonstrate that dissolved methane concentrations in the upper water column were not enriched above the studied well sites. At one area, characterized by the presence of shallow gas pockets, we observed methane concentrations ten times enriched compared to background values close to the seafloor. Our results indicate an active natural seep system in the northwestern part of the German North Sea, which is related to updoming salt structures rather than leaking wells, and further underlines that natural seeps may challenge the identification of potentially leaking wells. Due to the shallow water depths of 30 to 50 m in the study area, at least part of the released methane is probably contributing to the atmospheric inventory. This conclusion is based upon our observations of flares reaching close to the sea surface and a slight oversaturation of surface waters in the flare-rich area.

Hydrocarbon seepage is widespread distributed at the southern Gulf of Mexico. During several research cruises in 2003, 2006, and 2015 (SO174, M67/, and M114) we used multidisciplinary approaches, including multi-beam mapping and visual seafloor observations with different underwater vehicles to study the extent and character of complex hydrocarbon seepage in the Bay of Campeche, southern Gulf of Mexico. Our observations showed that seafloor asphalt deposits occur at numerous knolls and ridges in water depths from 1230 to 3150 m. These striking seafloor elevations are formed by diapirs of Jurassic salt deposit associated by hydrocarbon accumulations. The deeper sites like Chapopopte and Mictlan knolls were characterized by asphalt deposits accompanied by extrusion of liquid, and very heavy oil in form of whips or sheets, and in most places by gas emissions, and the presence of gas hydrates (Tsanyao Yang, Mictlan, and Chapopote knolls). Molecular and stable carbon isotopic compositions of gaseous hydrocarbons suggest their primarily thermogenic origin. Relatively fresh solidified asphalt structures were settled by bacterial mats and vestimentiferan tube worms growing through cracks and from under the edges of pavement. The gas hydrates at Tsanyao Yang and Mictlan knolls were covered by a 5-to-10 cm-thick reaction zone composed of authigenic carbonates, detritus, and microbial mats, and were densely colonized by 1–2 m long tube worms, bivalves, snails, and shrimps. The extent of all discovered seepage structure areas indicates that emission of complex hydrocarbons is a widespread, thus important feature of the southern Gulf of Mexico.

Morphological change and activity of submarine mud volcanoes (MVs) are still poorly understood. In this study, we integrated acoustic subbottom profiles, Autonomous Underwater Vehicle (AUV) high-resolution bathymetric and backscatter data, Remotely Operated Vehicle (ROV) seafloor observations, Bottom Water Temperatures (BWT), and compositions of gas samples collected from three research expeditions, to study the morphology and activity of the Helgoland mud volcano (Helgoland MV) in the Sorokin Trough of the Black Sea. Based on the micro bathymetric and backscatter maps, we conducted a geomorphological analysis which differentiates the Helgoland MV into five regions, which are: the active center, the inner crest, the inner moat, the outer crest, and an outer moat. Special seafloor features, such as gas bubble emissions, mud breccia and mud pools, were observed in different regions by ROV observations. They indicate a heterogeneous distribution of materials and activities at the Helgoland MV. Active mud extrusion from the center was inferred from BWT elevations (up to 1.652 °C) that showed the same orientation as a major fault (NW-SE) at the location. Methane stable carbon isotopes (−58.1∼−63.6‰) and molecular compositions of light hydrocarbons (C1/C2+ = 2836, CH4 = 99.545%, C2H6 = 0.033%, C3H8 = 0.002%, CO2 = 0.417%) suggest a predominance of biogenic hydrocarbons with an admixture of thermogenic gas. Circular faults surrounding the Helgoland MV, as observed in the bathymetric and sub-bottom profiles, are speculated to be formed due to subsidence of the MV caldera and gravity driven sediment movement. A conceptual model is proposed to explain the interrelationship between the seafloor morphology and activity of the Helgoland MV. Combining multidisciplinary data, especially from the application of ROVs and AUVs, broaden the scope in which the activity and morphology of submarine mud volcanoes can be investigated.

Long-term monitoring over one year revealed high temporal variability of gas emissions at a cold seep in 1250 m water depth offshore Vancouver Island, British Columbia. Data from the North East Pacific Time series Underwater Networked Experiment observatory operated by Ocean Networks Canada were used. The site is equipped with a 260 kHz Imagenex sonar collecting hourly data, conductivity-temperature-depth sensors, bottom pressure recorders, current meter, and an ocean bottom seismograph. This enables correlation of the data and analyzing trigger mechanisms and regulating criteria of gas discharge activity. Three periods of gas emission activity were observed: (a) short activity phases of few hours lasting several months, (b) alternating activity and inactivity of up to several day-long phases each, and (c) a period of several weeks of permanent activity. These periods can neither be explained by oceanographic conditions nor initiated by earthquakes. However, we found a clear correlation of gas emission with bottom pressure changes controlled by tides. Gas bubbles start emanating during decreasing tidal pressure. Tidally induced pressure changes also influence the subbottom fluid system by shifting the methane solubility resulting in exsolution of gas during falling tides. These pressure changes affect the equilibrium of forces allowing free gas in sediments to emanate into the water column at decreased hydrostatic load. We propose a model for the fluid system at the seep, fueled by a constant sub-surface methane flux and a frequent tidally controlled discharge of gas bubbles into the ocean, transferable to other gas emission sites in the world's oceans.