Numerous articles have recently reported on gas seepage offshore Svalbard, because the gas emission from these Arctic sediments was thought to result from gas hydrate dissociation, possibly triggered by anthropogenic ocean warming. We report on findings of a much broader seepage area, extending from 74° to 79°, where more than a thousand gas discharge sites were imaged as acoustic flares. The gas discharge occurs in water depths at and shallower than the upper edge of the gas hydrate stability zone and generates a dissolved methane plume that is hundreds of kilometer in length. Data collected in the summer of 2015 revealed that 0.02-7.7% of the dissolved methane was aerobically oxidized by microbes and a minor fraction (0.07%) was transferred to the atmosphere during periods of low wind speeds. Most flares were detected in the vicinity of the Hornsund Fracture Zone, leading us to postulate that the gas ascends along this fracture zone. The methane discharges on bathymetric highs characterized by sonic hard grounds, whereas glaciomarine and Holocene sediments in the troughs apparently limit seepage. The large scale seepage reported here is not caused by anthropogenic warming.Methane is, after water vapor and CO 2 , the most abundant greenhouse gas on Earth. When averaged over a 100 yr timescale, the warming effect of methane per unit mass is 28 times higher than that of CO 2 1 . Methane is produced in oceanic sediments either by methanogens at temperatures typically below ~80 °C, or through the breakdown of organic molecules at higher temperatures 2,3 . Buoyancy and pressure gradients can drive gas advection to shallower sediments where methane can be consumed via anaerobic oxidation of methane (AOM) 4 at the sulfate-methane transition zone and aerobic methane oxidation at the sediment surface 5 . Methane can also be sequestered within a cage of water molecules, in a gas hydrate structure, stable under the low temperature and high pressure conditions that define the gas hydrate stability zone 6 . If the upward methane flux is not fully exhausted by these processes, methane is emitted to the ocean either dissolved in the venting fluids or, in case of over-saturation, as gas bubbles 7 . As the bubbles ascend through the water column, a fraction of the methane gas dissolves 8 , generating patches of high methane concentration 9 . When the gas discharge is persistent and vigorous, it leads to the formation of large dissolved methane plumes. The dissolved methane is diluted by mixing with the surrounding ocean water and it is further oxidized by aerobic methanotrophs. Only in cases where dissolved methane reaches the surface-mixed layer in concentrations above saturation, can it be transferred to the atmosphere via sea-air gas exchange 10 . At present, the oceanic methane source to the atmosphere is very small (2-10%) 11 , as it is limited to emissions from vigorous and shallow seeps (<100 m) 1,7,8 . There is, however, an ongoing controversy regarding the methane discharge from sediments during warming events througho...

Abstract. Hydrocarbon seepage is a widespread process at the continental margins of the Gulf of Mexico. We used a multidisciplinary approach, including multibeam 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 previously only known from the Chapopote Knoll also occur at numerous other knolls and ridges in water depths from 1230 to 3150 m. In particular the deeper sites (Chapopopte and Mictlan knolls) were characterized by asphalt deposits accompanied by extrusion of liquid oil in form of whips or sheets, and in some places (Tsanyao Yang, Mictlan, and Chapopote knolls) by gas emission and the presence of gas hydrates in addition. Molecular and stable carbon isotopic compositions of gaseous hydrocarbons suggest their primarily thermogenic origin. Relatively fresh asphalt structures were settled by chemosynthetic communities including bacterial mats and vestimentiferan tube worms, whereas older flows appeared largely inert and devoid of corals and anemones at the deep sites. 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. This study increased knowledge on the occurrences and dimensions of asphalt fields and associated gas hydrates at the Campeche Knolls. 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.

We report on the geochemistry of hydrocarbons and pore waters down to 62.5 mbsf, collected by drilling with the MARUM-MeBo70 and by gravity coring at the Lunde pockmark in the Vestnesa Ridge. Our data document the origin and transformations of volatiles feeding gas emissions previously documented in this region. Gas hydrates are present where a fracture network beneath the pockmark focusses migration of thermogenic hydrocarbons characterized by their C 1 /C 2+ and stable isotopic compositions (δ 2 H-CH 4 , δ 13 C-CH 4 ). Measured geothermal gradients (~80°C/km) and known formation temperatures (>70°C) suggest that those hydrocarbons are formed at depths >800 mbsf. A combined analytical/modeling approach, including concentration and isotopic mass balances, reveals that pockmark sediments experience diffuse migration of thermogenic hydrocarbons. However, at sites without channeled flow this appears to be limited to depths >~50 mbsf. At all sites we document a contribution of microbial methanogenesis to the overall carbon cycle that includes a component of secondary carbonate reduction-that is, reduction of dissolved inorganic carbon generated by anaerobic oxidation of methane in the uppermost methanogenic zone. Anaerobic oxidation of methane and carbonate reduction rates are spatially variable within the pockmark and are highest at high-flux sites. These reactions are revealed by 13 C depletions of dissolved inorganic carbon at the sulfate-methane interface at all sites. However, 13 C depletions of CH 4 are only observed at the low methane flux sites because changes in the isotopic composition of the overall methane pool are masked at high-flux sites. 13 C depletions of total organic carbon suggest that at seeps sites, methane-derived carbon is incorporated into de novo synthesized biomass.

This multidisciplinary study of the hydrocarbon seepage system at Tsanyao Yang Knoll (TYK) in the southern Gulf of Mexico illustrates the amount and fate of hydrocarbons (mainly oil and methane) emanating from the seafloor structure and rising through a 3400 m water column. TYK forms part of the Campeche Knolls and was found to be one of the most active seepage structures at such an exceptional depth. Combining ship-based and AUV-based hydroacoustic mapping with direct seafloor observations and investigations, which used a TV-sled and a remotely operated vehicle with gas and water sampling devices provided an integrated view for the various transport pathways of hydrocarbons from the seafloor to the sea surface. In total, 32 acoustic 'flares,' indicative of gas bubble emission sites, were detected emanating from depressions on top of the knoll. Most of the emission sites were concentrated in two depressions that comprised a main seep field. An estimated volume of 550-4650 L of hydrocarbons per hour (or 8300-70,600 mol CH 4 per hour) are released in the form of gas bubbles, which dissolve almost entirely during their rise in the water column. However, echograms showed gas anomalies to about 500 m below sea surface and some bubbles were seen to burst at the sea surface. Concentrations of dissolved methane were highly elevated (∼30,000 nmol/L) directly above the seafloor emission site, but decreased to background concentrations (3-5 nmol/L) within the lowermost 100 m. Smaller volume flow rates of oil also escaped from the seafloor, rose to the sea surface and generated natural oil slicks visible from the ship and in satellite images. This study shows that hydrocarbon seepage at ∼3400 m water depth can be followed to the sea surface. However, most of the methane dissolves in deeper waters, whereas oil reaches the sea surface.

Abstract:Random variable simulation has been applied to many applications in hydrological modelling, flood risk analysis, environmental impact assessment, etc. However, computer codes for simulation of distributions commonly used in hydrological frequency analysis are not available in most software libraries. This paper presents a frequency-factor-based method for random number generation of five distributions (normal, log-normal, extreme-value type I, Pearson type III and log-Pearson type III) commonly used in hydrological frequency analysis. The proposed method is shown to produce random numbers of desired distributions through three means of validation: (1) graphical comparison of cumulative distribution functions (CDFs) and empirical CDFs derived from generated data; (2) properties of estimated parameters; (3) type I error of goodness-of-fit test. An advantage of the method is that it does not require CDF inversion, and frequency factors of the five commonly used distributions involves only the standard normal deviate.