Increased methane emissions from deep osmotic and buoyant convection beneath submarine seeps as climate warms

Methane is of great environmental importance as a greenhouse gas, and marine seeps are estimated to contribute some 37% of the geological sources; likewise mud volcanism provides another 11–17% (ref. 1). Methane hydrates are often found in the vicinity of submarine seeps and mud volcanoes2, and the project of commercializing this energy source is under way. Concomitantly, there is concern that anthropogenic climate change could destabilize hydrates, thereby potentially releasing large quantities of methane into the ocean and atmosphere3,4,5. Thus there are both environmental and economic reasons to be interested in methane associated with seeps and mud volcanism. A mud volcano is a geological structure on land or in the oceans in which water plus fine particulates—‘mud’—issues from a conduit typically topped by a conical hill with a crater. The emerging mud is generally accompanied by methane, both dissolved and, if the concentration exceeds the saturation concentration, as bubbles6. A submarine seep, on the other hand, has similar fluid flow through the porous sediment constituting the seabed without the conical structure or open conduit. What physical forces drive such fluid flows? We find that both buoyancy and osmotic effects are present in cold seeps and mud volcanism in which, rather than being a passive element, methane is its driving force. Some researchers have suggested the importance of considering osmosis in seeps and mud volcanism7,8. Clays and shales are known to possess the semipermeability necessary for osmosis, associated with charge and pore-size effects9,10. Moreover, methane hydrates frequently exist in the sediments around a cold seep or mud volcano2,6. As hydrate forms, the sediment plus hydrate becomes progressively less permeable11,12. However, other mechanisms involving adsorption and chemical reaction can also produce significant osmotic pressures13,14,15,16,17,18,19,20. Methane is shown to adsorb onto sediments21,22,23,24. Hydrates form in pores under a wider range of conditions than in the bulk25 and methane molecules adsorb onto the cages of methane hydrate during the hydrate growth process26,27. Thus, we propose below that given a supply of methane, a submarine cold seep or mud volcano can function as a geological instance of an osmotic pump9,28,29. We find cause for concern that this convective pump mechanism facilitates methane hydrate destabilization under anthropogenic climate change.The financial support of the UK Leverhulme Trust project RPG-2015-002. J.H.E.C. acknowledges the financial support of the Spanish MINCINN project FIS2013-48444-C2-2-Peer reviewe