Methane discharge from a deep-sea submarine mud volcano into the upper water column by gas hydrate-coated methane bubbles

Abstract

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.

Modeling of methane bubbles released from large sea-floor area: Condition required for methane emission to the atmosphere

“…We conducted the control experiment under the following conditions: a bubble radius of 0.3 cm which is consistent with observations Sauter et al, 2006;Egrov et al, 2003), sea-floor depth of 1000 m, the HSZ deeper than 560 m, and a methane flux of 2.5×10 −2 mol/s·m 2 (equivalent to forty bubbles with a radius of 0.3 cm released per second into the water column from 1 m 2 of the sea-floor) which is the roughly the same order of magnitude to one order of magnitude more than the flux observed in a methane seep recently under investigation (Heeschen et al, 2003;Sauter et al, 2006). We conducted the case studies with different values by changing four parameters: bubble radius, seawater temperature, water depth, and methane oxidation.…”

“…Almost all bubbles released from deep water can rise up hundreds of meters, but not reach the atmosphere Heeschen et al, 2003;Sauter et al, 2006;Matveeva et al, 2003). Models targeted at understanding the behavior of a single bubble ascending the seawater column utilizing observed methane concentration profiles (and other constituent boundary conditions, such as T, DO, salinity, etc) have been developed (Leifer and Patro, 2002;McGinnis et al, 2006;Zheng and Yapa, 2002).…”

The influence of vent systems on pelagic eukaryotic micro-organism composition in the Nordic Seas

“…4). Sea water and plume chemistry have been investigated thoroughly in other studies (Pedersen et al 2005;Sauter et al 2006;Pedersen et al 2010a;Schander et al 2010;Baumberger 2011;Tandberg et al 2011;Jørgensen et al 2012;Kongsrud and Rapp 2012;Stensland 2013), and further discussion related to chemistry will therefore follow below. Five groups dominated the three different water masses (Alveolata, Rhizaria, Protozoa, Metazoa and Heterokonta), but the internal proportion within a given sample varied.…”

Sonar gas flux estimation by bubble insonification: application to methane bubble flux from seep areas in the outer Laptev Sea

“…Seep (r e ) have been measured by video (Leifer, 2010;Römer et al, 2012;Sahling et al, 2009;Sauter et al, 2006) and passive acoustics. Passive acoustic (r e ) measurement has only been demonstrated for low-flow bubble plumes where the individual bubble acoustic signatures can be identified (Leifer and Tang, 2006;Maksimov et al, 2016).…”