Results from surface geochemical prospecting, seismic exploration and satellite remote sensing have documented oil and gas seeps in marine basins around the world. Seeps are a dynamic component of the carbon cycle and can be important indicators for economically significant hydrocarbon deposits. The northern Gulf of Mexico contains hundreds of active seeps that can be studied experimentally with the use of submarines and Remotely Operated Vehicles (ROV). Hydrocarbon flux through surface sediments profoundly alters benthic ecology and seafloor geology at seeps. In water depths of 500–2000 m, rapid gas flux results in shallow, metastable deposits of gas hydrate, which reduce sediment porosity and affect seepage rates. This paper details the processes that occur during the final, brief transition — as oil and gas escape from the seafloor, rise through the water and dissolve, are consumed by microbial processes, or disperse into the atmosphere. The geology of the upper sediment column determines whether discharge is rapid and episodic, as occurs in mud volcanoes, or more gradual and steady, as occurs where the seep orifice is plugged with gas hydrate. In both cases, seep oil and gas appear to rise through the water in close proximity instead of separating. Chemical alteration of the oil is relatively minor during transit through the water column, but once at the sea surface its more volatile components rapidly evaporate. Gas bubbles rapidly dissolve as they rise, although observations suggest that oil coatings on the bubbles inhibit dissolution. At the sea surface, the floating oil forms slicks, detectable by remote sensing, whose origins are laterally within ∼1000 m of the seafloor vent. This contradicts the much larger distance predicted if oil drops rise through a 500 m water column at an expected rate of ∼0.01 m s−1 while subjected to lateral currents of ∼0.2 m s−1 or greater. It indicates that oil rises with the gas bubbles at speeds of ∼0.15 m s−1 all the way to the surface.

[1] The East Siberian Arctic Shelf (ESAS), which includes the Laptev Sea, the East Siberian Sea, and the Russian part of the Chukchi Sea, has not been considered to be a methane (CH 4 ) source to hydrosphere or atmosphere because subsea permafrost, which underlies most of the ESAS, was believed, first, not to be conducive to methanogenesis and, second, to act as an impermeable lid, preventing CH 4 escape through the seabed. Here recent observational data obtained during summer (2005)(2006) and winter (2007) expeditions indicate the ubiquitous presence of elevated dissolved CH 4 and an elevated atmospheric CH 4 mixing ratio. The CH 4 data were also analyzed together with high resolution seismic (HRS) data obtained by means of a "Sonic M-141" system consisting of a high-resolution profiler and side-scan sonar mounted in a towed fish during the Transdrift-X Expedition (2004) onboard the R/V Yakov Smirnitskiy. Results show anomalously high concentrations of dissolved CH 4 (up to 5 mM) and an episodically (nongradually) increasing atmospheric mixing ratio of CH 4 (up to 8.2 ppm) in some areas of the ESAS. A most likely source is yearround CH 4 release through taliks (columns of thawed sediments within permafrost) from seabed CH 4 reservoirs such as shallow hydrates and geological sources. Such releases occur not only within the areas underlain by fault zones but also outside of them. This points to permafrost's failure to further preserve CH 4 deposits in the ESAS. The total amount of carbon preserved within the ESAS as organic matter and ready to release CH 4 from seabed deposits is predicted to be ∼1400 Gt. Release of only a small fraction of this reservoir, which was sealed with impermeable permafrost for thousands of years, would significantly alter the annual CH 4 budget and have global implications, because the shallowness of the ESAS allows the majority of CH 4 to pass through the water column and escape to the atmosphere.

Laboratory results have demonstrated that bubble plumes are a very efficient air‐water gas transfer mechanism. Because breaking waves generate bubble plumes, it could be possible to correlate the air‐sea gas transport velocity kL with whitecap coverage. This correlation would then allow kL to be predicted from measurements of apparent microwave brightness temperature through the increase in sea surface microwave emissivity associated with breaking waves. In order to develop this remote‐sensing‐based method for predicting air‐sea gas fluxes, a whitecap simulation tank was used to measure evasive and invasive kL values for air‐seawater transfer of carbon dioxide, oxygen, helium, sulfur hexafluoride, and dimethyl sulfide at cleaned and surfactant‐influenced water surfaces. An empirical model has been developed that can predict kL from bubble plume coverage, diffusivity, and solubility. The observed dependence of kL on molecular diffusivity and aqueous‐phase solubility agrees with the predictions of modeling studies of bubble‐driven air‐water gas transfer. It has also been shown that soluble surfactants can decrease kL even in the presence of breaking waves.

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Bubble plumes from hydrocarbon seeps drive upwelling flows in the water column that can disappear if the bubbles dissolve. This may lead to formation of a layer enriched in gases and substances transported by the bubbles, a process we term bubble deposition. A review of observed dissolved methane layers in the North Sea showed their existence in an area of active seeping pockmarks at a height of ∼ 20–30 m above the sea bed, well below the thermocline. To test the bubble deposition hypothesis, rising seep bubbles were simulated numerically. The model predicted a dissolution depth consistent with the observed methane layer for ∼ 2700‐µm‐radius bubbles. The model also predicted that bubbles smaller than 3400 µm dissolved subsurface, decreasing to 2000 µm for a 10‐cm s−1 upwelling flow. We speculate that this layer may be attractive to marine organisms. Although North Sea seeps are not oily, this mechanism also applies to oily bubbles from hydrocarbon seeps or a leaking undersea gas/oil pipeline. Thus bubble deposition can create a subsurface oil layer which rises far slower than either the bubble stream or droplets entrained in the stream.