The assessment of climate change factors includes a constraint of methane sources and sinks. Although marine geological sources are recognized as significant, unfortunately, most submarine sources remain poorly quantified. Beside cold vents and coastal anoxic sediments, the large number of submarine mud volcanoes (SMV) may contribute significantly to the oceanic methane pool. Recent research suggests that methane primarily released diffusively from deep-sea SMVs is immediately oxidized and, thus, has little climatic impact. New hydro-acoustic, visual, and geochemical observations performed at the deep-sea mud volcano Håkon Mosby reveal the discharge of gas hydrate-coated methane bubbles and gas hydrate flakes forming huge methane plumes extending from the seabed in 1250 m depth up to 750 m high into the water column. This depth coincides with the upper limit of the temperature-pressure field of gas hydrate stability. Hydrographic evidence suggests bubble-induced upwelling within the plume and extending above the hydrate stability zone. Thus, we propose that a significant portion of the methane from discharged methane bubbles can reach the upper water column, which may be explained due to the formation of hydrate skins. As the water mass of the plume rises to shallow water depths, methane dissolved from hydrated bubbles may be transported towards the surface and released to the atmosphere. Repeated acoustic surveys performed in 2002 and 2003 suggest continuous methane emission to the ocean. From seafloor visual observations we estimated a gas flux of 0.2 (0.08-0.36) mol s−1 which translates to several hundred tons yr−1 under the assumption of a steady discharge. Besides, methane was observed to be released by diffusion from sediments as well as by focused outflow of methane-rich water. In contrast to the bubble discharge, emission rates of these two pathways are estimated to be in the range of several tons yr−1 and, thus, to be of minor importance. Very low water column methane oxidation rates derived from incubation experiments with tritiated methane suggest that methane is distributed by currents rather than oxidized rapidly.

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Bubbles in a subsurface ocean layer play a significant role in underwater acoustics and oceanography. Many works have been devoted to different methods of bubble density measurement. Among them, the nonlinear methods of bubble sizing based on combination acoustic scattering have a very high selectivity. These methods were utilized in several types of bubble counters developed in the Institute of Applied Physics for the last decade. In the devices the difference and the sum frequency waves of the incident ones generated by a bubble in the working volume are detected. Another approach to the problem based on the parametric generation of subharmonic of the incident wave frequency was also tested. The results of the measurements of bubble size distribution and density in the subsurface ocean layer and in the wakes of ships are given in the paper. Simulteneous measurements by different linear and nonlinear methods were done to make a comparison between them. The results of linear acoustic echosounding of a subsurface bubble cloud from an underwater research platform are also presented. Possibilities and advantages of linear and nonlinear methods of spectroscopy of structure and movements in a subsurface layer are discussed. [Work supported by RFBR (Grants 94-05-16755, 95-05-15632).]