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Signs of instability

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The finding that pools of gas hydrates — compounds that trap natural gas emissions — in ocean sediments are deeper than expected implies that the hydrates are destabilizing, and might release gigatonnes of methane. See Letter p.527

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Methane hydrate is an ice-like substance that exists at low temperatures and high pressures, and which fills the pore space of sediments in the seabed and sub-seabed. Increases in sea temperature could destabilize methane hydrate, releasing methane gas into the ocean and possibly into the atmosphere — where, if released in sufficient quantities, it would act as a potent greenhouse gas and thus contribute to global warming. On page 527 of this issue, Phrampus and Hornbach1 report a combination of seismic data and thermal models that suggests that changes in the Gulf Stream are rapidly destabilizing methane hydrate along the North American continental margin — the zone of ocean floor where the deep ocean meets the shallow continental shelves, and which marks the transition between continental and oceanic crust.

Methane hydrates are found within a region of sediments known as the gas-hydrate stability zone (GHSZ). The depth of the GHSZ varies mainly with pressure (which depends on water depth); with the composition of the natural gas trapped in the hydrates (which depends on whether the gas is produced by microbes at shallow depths or by thermal processes at greater depths); with the temperature gradient beneath the seabed; and with the water temperature at the seabed. If the deep gas reservoirs in continental margins are leaking, they may contribute methane to the shallower gas-hydrate reservoir system.

One of the most pressing issues in climate change is the relationship between ocean warming and methane escape from the seabed2,3,4,5 (Fig. 1). The extent to which such methane venting is connected to the thawing of methane-hydrate reservoirs is the crux of this issue, but also of two big, related stories involving methane in the environment: the large-scale collapse of continental slopes (steep inclines at the edge of continental shelves) and the increasing methane levels in the atmosphere. For example, some areas of the ocean margins along the east coast of the United States6 and the Norwegian Arctic7 have cracked, and seem to be susceptible to future slope failures. This, in turn, may enable mechanisms to develop that allow the rapid transfer of methane from sub-seabed gas-hydrate deposits into the upper ocean and the atmosphere, which would add to greenhouse-gas emissions. Such destabilization in the methane-hydrate reservoirs and continental slope may occur at intermediate depths along upper continental margins, down to approximately 1,000 metres below sea level.

Figure 1: Bubbling up.
figure1

Hydrocarbon gas, including methane, seeps from sub-seabed reservoirs and forms plumes of bubbles. The graphic shows two plumes in the Gulf of Mexico as an east–west trace, based on echo-sounding data10. Phrampus and Hornbach1 report that changes in the Gulf Stream could trigger the release of gigatonnes of methane from ocean sediments off the east coast of America. (Figure adapted from ref. 10.)

Phrampus and Hornbach's paper adds to the scientific debate concerning the risk of substantial, rapid methane emissions in the future, and will certainly draw considerable attention to the cracked continental slope of the east coast of North America. The authors' seismic data reveal that the GHSZ in this region is deeper than predicted from models based on current ocean temperatures, indicating that transient processes for methane-hydrate melting are occurring in the sub-seabed. The researchers rule out several factors that could explain this observation, such as changes of sea level, increases in the contribution of thermogenic gas from deep hydrocarbon reservoirs, increased sedimentation rates and lower heat flow in the sub-seabed. Instead, they conclude that the discrepancy was probably caused solely by warming of the ocean at intermediate depths over the past few thousand years, the heat from which has not yet fully penetrated the seabed, and so has yet to affect the depth of the GHSZ.

Phrampus and Hornbach argue that the required warming can be explained by changes in the temperature or path of the Gulf Stream within the past 5,000 years or so. Extrapolating from their data, they estimate that these changes will ultimately warm the western North Atlantic margin by as much as 8 °C, and will trigger the destabilization of 2.5 gigatonnes of methane hydrate. What's more, taking into account changes observed in the Arctic environment4,8 that could also cause methane-hydrate destabilization, the authors suggest that this quantity may represent only a fraction of the methane hydrate that is currently destabilizing around the world. However, the extent of global destabilization is difficult to determine exactly, because estimates of the temperatures of ocean water at intermediate depths on glacial–interglacial timescales of several thousand years suggest that ocean warming may be different between oceans9.

Whether the destabilization of gigatonnes of methane hydrate from the east coast of North America would adversely affect climate in the future depends on how rapid temperature changes associated with the Gulf Stream will affect approximately 10,000 square kilometres of the US eastern continental margin, an area prone to submarine landslides. More broadly, the big unknowns regarding this century's ocean-temperature shifts are to what extent, and how rapidly, such shifts will reduce the stability of methane hydrate in ocean margins. It may (hopefully) turn out to be the case that intermittent increases in ocean temperature along methane-hydrated regions are not enough to increase ocean acidification and — if methane is released from the ocean — the level of carbon in the atmosphere.

Phrampus and Hornbach's GHSZ mapping may provide a timely lesson about how the conditions of the sea floor down to a depth of about 1,000 metres below sea level can change from favouring methane-hydrate formation to favouring methane-hydrate melting. It shows that we must use seismic records as indicators of past conditions affecting ocean methane hydrates, and also suggests that real-time, long-term recording of anticipated sea-floor releases of gigatonnes of methane are needed to provide the ultimate proof that a warming Gulf Stream is causing methane-hydrate melting.

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Correspondence to Juergen Mienert.

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Mienert, J. Signs of instability. Nature 490, 491–492 (2012) doi:10.1038/490491a

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