Description: Raw data of pCO2 measurements during Celtic Explorer cruise CE12010 at station 44ROV12 (Gas Release Experiment). The gas discharge of the gas release experiment was observed in situ during a 4 hour dive with GEOMAR's remotely operated vehicle ROV Kiel 6000 equipped with HD camera/video device and a sonar system. The spread of the dissolved CO2 plume was monitored geochemically using the commercial HydroC pCO2 sensor (S/N 0412-006, Kongsberg Maritime Contros GmbH) mounted to the front porch of the ROV. The sensor was calibrated for pCO2 signals up to 3,000 µatm (accuracy ~1% of reading resolution; resolution: 〈30 µatm) and was programmed to measure in 60 s intervals, which is equal to the sensor's response time.

Description: Hydrocarbon seeps are ubiquitous at gas-prone Cenozoic deltas such as the Nile Deep Sea Fan (NDSF) where seepage into the bottom water has been observed at several mud volcanoes (MVs) including North Alex MV (NAMV). Here we investigated the sources of hydrocarbon gases and sedimentary organic matter together with biomarkers of microbial activity at four locations of NAMV to constrain how venting at the seafloor relates to the generation of hydrocarbon gases in deeper sediments. At the centre, high upward flux of hot (70 °C) hydrocarbon-rich fluids is indicated by an absence of biomarkers of Anaerobic Oxidation of Methane (AOM) and nearly constant methane (CH4) concentration depth-profile. The presence of lipids of incompatible thermal maturities points to mixing between early-mature petroleum and immature organic matter, indicating that shallow mud has been mobilized by the influx of deep-sourced hydrocarbon-rich fluids. Methane is enriched in the heavier isotopes, with values of d13C ~-46.6 per mil VPDB and dD ~-228 per mil VSMOW, and is associated with high amounts of heavier homologues (C2+) suggesting a co-genetic origin with the petroleum. On the contrary at the periphery, a lower but sustained CH4 flux is indicated by deeper sulphate-methane transition zones and the presence of 13C-depleted biomarkers of AOM, consistent with predominantly immature organic matter. Values of d13C-CH4 ~-60 per mil VPDB and decreased concentrations of 13C-enriched C2+ are typical of mixed microbial CH4 and biodegraded thermogenic gas from Plio-Pleistocene reservoirs of the region. The maturity of gas condensate migrated from pre-Miocene sources into Miocene reservoirs of the Western NDSF is higher than that of the gas vented at the centre of NAMV, supporting the hypothesis that it is rather released from the degradation of oil in Neogene reservoirs. Combined with the finding of hot pore water and petroleum at the centre, our results suggest that clay mineral dehydration of Neogene sediments, which takes place posterior to reservoir filling, may contribute to intense gas generation at high sedimentation rate deltas.

Description: Gridded multibeam bathymetry from Poseidon cruise 408 and Pelagia cruises 64PE350 and 64PE351 within the Jeddah Transect Project. The raw-data were post-processed and gridded at a resolution of 30 m with QPS Fledermaus Pro. For smaller file size and better handling 11 tiles were created with GlobalMapper (5 columns, 5 lines).

Description: The presence of surface methanogenesis, located within the sulfate-reducing zone (0-30 centimeters below seafloor, cmbsf), was investigated in sediments of the seasonally hypoxic Eckernförde Bay, southwestern Baltic Sea. Water column parameters like oxygen, temperature and salinity together with porewater geochemistry and benthic methanogenesis rates were determined in the sampling area 'Boknis Eck' quarterly from March 2013 to September 2014, to investigate the effect of seasonal environmental changes on the rate and distribution of surface methanogenesis and to estimate its potential contribution to benthic methane emissions. Water column parameters where determined via CTD (temperature, salinity, pressure), as well as gas chromatography (methane) and fluorometric methods (chlorophyll a). For porewater and sediment geochemistry various method were used including photometry (sulfide), ion chromatography (sulfate), N/C Analysis (DIC), Carbo-Elba element analysis (POC, C/N), gas chromatography (methane). Sediment net methanogenesis rates were determined via the methane increase (measured with gas chromatography) over time in sediment slurry batch incubations. Sediment hydrogenotrophic methanogenesis was measured by adding radiotracer (14C-bicarbonate) to sediment samples and measuring the production of 14C-methane (via scintillation counting) after a specific period of time. For further details (sample preparation and analysis) see the related publication (Maltby et al., 2017, Biogeosciences Discussions)

Description: Video showing the gas release experiment, which has been conducted at ~80 m water depth in the vicinity of the Sleipner CO2 storage site in the North Sea. The gas discharge was observed in situ during a 4 hour dive with GEOMAR's remotely operated vehicle ROV Kiel 6000 equipped with HD camera/video device.

Description: We report on newly discovered mud volcanoes located at about 4500 m water depth 90 km west of the deformation front of the accretionary wedge of the Gulf of Cadiz, and thus outside of their typical geotectonic environment. Seismic data suggest that fluid flow is mediated by a 〉400-km-long strike-slip fault marking the transcurrent plate boundary between Africa and Eurasia. Geochemical data (Cl, B, Sr, 87Sr/86Sr, Delta18O, DeltaD) reveal that fluids originate in oceanic crust older than 140 Ma. On their rise to the surface, these fluids receive strong geochemical signals from recrystallization of Upper Jurassic carbonates and clay-mineral dehydration in younger terrigeneous units. At present, reports of mud volcanoes in similar deep-sea settings are rare, but given that the large area of transform-type plate boundaries has been barely investigated, such pathways of fluid discharge may provide an important, yet unappreciated link between the deeply buried oceanic crust and the deep ocean.