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Melting of submarine methane hydrates as climate warms, Nature Communications

last modified Mar 03, 2017 02:33 PM
Dr Silvana Cardoso

A recent study by a member of this Department, in collaboration with a colleague in Spain, could help understand recent observations of methane bubbling on the seafloor.

The study is published in the journal Nature Communications and is Increased methane emissions from deep osmotic and buoyant convection beneath submarine seeps as climate warms by Silvana S.S. Cardoso and Julyan H.E. Cartwright.

DOI: 10.1038/ncomms13266

Methane-rich seawater is expelled at seep and mud-volcano sites on the seabed. These flows are associated with the presence of methane-hydrate deposits beneath the seafloor. Using fundamental fluid mechanics, we demonstrate that in addition to buoyancy, osmotic effects associated with the presence of methane in the sediments can create large overpressures, capable of recirculating seawater from the seafloor to depth in the sediment layer, then expelling it upwards at rates of up to a few hundreds of metres per year. In the presence of global warming, such deep recirculation of seawater can accelerate the melting of methane-hydrate deposits at depth, drastically increasing the rate of release of methane into the hydrosphere and perhaps the atmosphere.  We estimate  that about 3.5% of the global hydrate inventory, that is ~60 Gt C, might be susceptible to warming by the mechanism proposed here within a timescale of a few decades.

Dr Cardoso is a Reader in Fluid Mechanics and the Environment in this Department. Dr Julyan Cartwright works at the Consejo Superior de Investigaciones Científicas (CSIC), Spain.

Figure 1 | Flow driven by buoyancy and osmotic sources in saturated porous sediment under the seafloor. (a) An extended buoyancy source in a homogeneous sediment, (b) a two-dimensional buoyancy source at a continental margin, (c) a buoyant or osmotic pumping mechanism associated with a developed seep, (d) buoyant or osmotic pumping in a fully developed mud volcano.

Figure 1.  Flow driven by buoyancy and osmotic sources in saturated porous sediment under the seafloor. (a) An extended buoyancy source in a homogeneous sediment, (b) a two-dimensional buoyancy source at a continental margin, (c) a buoyant or osmotic pumping mechanism associated with a developed seep, (d) buoyant or osmotic pumping in a fully developed mud volcano.


Figure 2 | Dissolved methane flux plotted against liquid flux

Figure 2. Dissolved methane flux plotted against liquid flux. An osmotic mechanism can flow more methane and more liquid than competing mechanisms. Comparison of field measurements at seeps and mud volcanoes  with our theoretical predictions for a uniform source of solute and a margin heat plume, and a buoyant or osmotic plume in a developed seep. The predictions are for a sediment permeability of 10-12m2 and an exit methane concentration of 8 μM; the green and orange shaded ellipses represent the range of permeabilities 10-13–10-11m2 (along the major axis) and methane concentrations 0.6–126 μM (along the minor axis) for a buoyant and osmotic seep flow, respectively. An estimate of efflux from sediment compression is shown as a baseline.

 

Top figures: Submarine mud volcano emitting methane and seawater (National Oceanic and Atmospheric Administration (NOAA), USA, in the public domain. Obtained by ROV, credited to the Sea Research Foundation (SRF) and the Ocean Exploration Trust (OET)).

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