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

Silvana S S Cardoso,Julyan H E Cartwright

doi:10.1038/ncomms13266

Abstract

High speeds have been measured at seep and mud-volcano sites expelling methane-rich fluids from the seabed. Thermal or solute-driven convection alone cannot explain such high velocities in low-permeability sediments. Here we demonstrate that in addition to buoyancy, osmotic effects generated by the adsorption of methane onto 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 hydrates at depth from timescales of millennia to just decades, and can drastically increase the rate of release of methane into the hydrosphere and perhaps the atmosphere.

Highlights

High speeds have been measured at seep and mud-volcano sites expelling methane-rich fluids from the seabed
Methane hydrates are often found in the vicinity of submarine seeps and mud volcanoes[2], and the project of commercializing this energy source is under way
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

Summary

Introduction

High speeds have been measured at seep and mud-volcano sites expelling methane-rich fluids from the seabed. There is concern that anthropogenic climate change could destabilize hydrates, thereby potentially releasing large quantities of methane into the ocean and atmosphere[3,4,5]. There are both environmental and economic reasons to be interested in methane associated with seeps and mud volcanism. 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. 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 pump[9,28,29]. We find cause for concern that this convective pump mechanism facilitates methane hydrate destabilization under anthropogenic climate change

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Conclusion