Hair trigger for gas hydrates

The curious mix of water ice and methane, known as gas hydrate or clathrate, which is stable at ocean depths greater than 300 m, is one of the largest potential components of the active carbon cycle (~1013 t).  Its methane content stems from bacterial breakdown of organic matter buried in anaerobic sea-floor sediments.  As well as being pressure sensitive, gas hydrate also has a narrow stability “window” as regards temperature.  Geothermal heat therefore limits the depth of gas-hydrate accumulations to a few tens to hundreds of metres below the seabed.  Its vast methane content is clearly something on which energy transnationals have an eye.  However, methane is almost four times more powerful as a “greenhouse gas” than CO2 emissions.  Carbon-isotope studies from sedimentary rocks show signs that several times in the distant past methane was released catastrophically to the atmosphere, the timing coinciding with signs of rapid global warming.  The last major event of this kind was around 55 Ma ago, when the end of the Palaeocene Epoch witnessed an 8°C global temperature rise in a matter of a few thousand years (Thomas, D. et al. 2002. Warming the fuel for the fire: Evidence for the thermal dissociation of methane hydrate during the Paleocene-Eocene thermal maximum.  Geology, v. 30, p.1067-1070).  The warming “spike” eases because methane is quickly oxidised to water and CO2 in the atmosphere, but that still allows abnormally warm conditions to linger.

Sonar surveys of the seabed, including that of the North Sea, reveal pits and funnels that probably mark sites of past methane releases from destabilised gas hydrates.  In theory, two general processes lead to their instability: falling global sea level that reduces the pressure on gas hydrates formed at shallow water depths; a rise in the temperature of ocean-bottom water.  The second could produce more widespread methane release than the first.  Refining these crude prognoses needs detail about the structure of gas-hydrate zones beneath the seabed.  Conventional seismic surveys conducted at the sea surface show the clathrate-rich zones just beneath the sea floor, but no detail.  Towing sources and receivers just above the seabed reveals intricate structures (Wood, W.T. et al. 2002.  Decreased stability of methane hydrates in marine sediments owing to phase-boundary roughness.  Nature, v. 420, p. 656-660).  Wood and co-workers from the US Naval Research Laboratory, the University of Victoria and the Pacific Geoscience Centre in British Columbia, Canada surveyed the Pacific floor off Vancouver Island.  Their most striking observation is of many vertical, chimney-like structures that puncture the gas-hydrate zone in the upper sediment layer.  They reckon that these structures are where methane and warm fluids find their way to the seabed; they are probably the expression in cross section of the surface pitting formed by past degassing.  They also may supply gas to the zone where it becomes locked in metastable water ice.  The sheer number of the “chimneys” indicates that the surface area of gas-hydrate stability is many times larger than previously supposed, as a result of their “roughening” effect.  Since the base of the gas-hydrate stability zone is most prone to the effect of warming of sea-bottom water, which shifts the geotherm slightly, an increase in its surface area, together with its closer approach to the seabed around the “chimneys”,  could further increase its sensitivity to small changes.  Up to now, many specialists have suggested that major methane releases resulted from sudden collapses of sea-floor sediments in tectonically unstable areas, such as the Storegga Slide off western Norway.  They may instead have been due to more widespread instability resulting from environmental change.  Since the largest pressure decreases due to sea-level falls accompanied glacial epochs, some clues to whether the “chimney” effect has had an influence may come from a fresh look at methane contents of trapped air bubbles in Antarctic and Greenlandic ice cores.  The extent to which methane releases might effect climate depends on how much is oxidised to CO2 in sea water, before it can enter the atmosphere to enhance the “greenhouse” effect.  Little is know about such processes.

See also:  Pecher, I.A. 2002.  Gas hydrates on the brink.  Nature, v. 420, p, 622-623.

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