Huge amounts of methane are locked up in deposits that lie deep beneath the sea floor. New seismic images reveal that these deposits possess unexpected features that might affect their stability.
Gas hydrates are an ice-like form of water that has cavities containing gas — usually methane. They exist in vast quantities beneath the ocean floor in certain areas, especially continental margins, where the methane is generated mostly from the bacterial breakdown of organic matter1, and they remain stable under seafloor conditions that are typical of water depths of more than 300 m or so2. There is considerable interest in these hydrates, largely because of their potential as an energy resource and because of the effects that released methane may have, or have had, on climate. Methane release will depend on gas-hydrate stability. The zone of stability extends downwards into the sea-floor sediments until, because of increasing temperature, the 'base of gas hydrate stability' (BGHS) is reached some tens to hundreds of metres beneath the sea floor.
In their report on page 656 of this issue, Wood et al.3 present new data that dramatically change our perception of the BGHS. From conventional seismic data, acquired using sources and receivers towed close to the surface, the BGHS appears to be a continuous boundary that runs more or less parallel to the sea floor. But the images presented by Wood et al., from survey work at locations off the east and west coasts of North America, show that the BGHS exhibits considerable 'roughness'. These features, which may well be a general characteristic of the BGHS elsewhere in the world, could greatly affect the stability of gas-hydrate reservoirs.
The new data were acquired with a seismic source and an array of receivers towed close to the sea floor. This configuration provides an increase in resolution of almost one order of magnitude over previous methods. The images reveal small but densely packed pockets of gas at the BGHS which, in conventional data, appear to be part of continuous gas layers. More intriguingly, narrow, near-vertical 'wipeout zones' (opaque zones with little reflected seismic energy) puncture the gas-hydrate-bearing sediments. Wood et al. interpret these wipeout zones as gas chimneys (Fig. 1a), through which methane can migrate upwards, probably along faults, well into the regional zone of gas-hydrate stability.
Wood et al.3 support this interpretation by modelling fluid and heat flow through the faults. The results suggest that warm fluids moving relatively quickly through highly permeable faults can keep the faults and surrounding sediments warm enough to prevent gas-hydrate formation. That is, although on a regional scale the faults lie well within the zone of gas-hydrate stability, locally, on a scale of several metres, conditions are such that methane will not be transformed to hydrate. Based on evidence of small lenses with high gas-hydrate concentrations4, it has been proposed that such gas chimneys existed in the geological past. These lenses may have formed from gas that was injected along chimneys into the regional gas-hydrate zone.
The chimneys increase the amount of hydrates located close to the BGHS by enlarging the area of the boundary by a factor of 0.5–3. So the gas-hydrate reservoir may be more susceptible to changes in environmental conditions, such as variations in sea level or water temperature, than previously thought. Near the chimneys, high gas-hydrate concentrations would be expected to occur close to the sea floor because of a high methane supply. A sudden change of bottom-water temperature, which is likely to be the most significant environmental cause of gas-hydrate dissociation5, would affect hydrates there more directly and faster than it would deeply buried deposits.
The effects of the chimneys on methane release might go further. Gas-hydrate dissociation in response to environmental change is predicted to take place primarily near the BGHS. As long as gas hydrates remain stable at the sea floor, upward-migrating gas would be expected to become 'trapped' by hydrates before reaching the ocean. So most explanations6 of large-scale methane release into the ocean invoke catastrophic seafloor failure, in which pressure build-up from gas generated at the BGHS produces an underwater eruption. It has long been realized that gas may migrate upwards along faults in a gas-hydrate zone7. But it was not clear why the gas would not be transformed to hydrate within the faults.
From their modelling study, Wood et al.3 predict that in the vicinity of faults the BGHS will almost reach the sea floor. So, to speculate, if the fluids within the faults remain warm up to the sea floor, methane gas could migrate the last few metres through the sediments to the ocean. The chimneys may therefore provide methane migration pathways from the BGHS through the gas-hydrate zone, and (for example, following bottom-water warming) may act as valves by steadily or episodically releasing methane into the ocean without seafloor collapse (Fig. 1b). A similar mechanism has been proposed8 to be occurring at the Blake Ridge Depression, off South Carolina, which has been interpreted as a large hydrate-bearing sediment 'wavefield' (the underwater analogy of sand dunes). Fluid- and gas-migration paths cutting through the gas-hydrate stability zone have been detected where individual sediment 'dunes' meet, and gas may be released episodically through these paths into the ocean.
The study by Wood et al. adds considerably to our knowledge of methane transfer from gas hydrates into the ocean. But what about the possible effects on climate? Methane has a much higher greenhouse potential than CO2 — molecule for molecule, it is estimated to be 3.7 times more powerful in its warming effect9. Carbon-isotope anomalies in oceanic sediments have been linked to a release of methane from gas hydrates10, one implication being that such releases might be a cause of warming episodes in Earth's history. It is unclear, however, how much of this methane reaches the atmosphere — most of it may be oxidized to CO2 in the ocean10. So before we can quantify any connection between gas hydrates and climate change, we need a better understanding of what happens to methane in the ocean.
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