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Decreased stability of methane hydrates in marine sediments owing to phase-boundary roughness

Abstract

Below water depths of about 300 metres, pressure and temperature conditions cause methane to form ice-like crystals of methane hydrate1. Marine deposits of methane hydrate are estimated to be large, amassing about 10,000 gigatonnes of carbon2, and are thought to be important to global change3,4 and seafloor stability5,6, as well as representing a potentially exploitable energy resource7. The extent of these deposits can usually be inferred from seismic imaging, in which the base of the methane hydrate stability zone is frequently identifiable as a smooth reflector that runs parallel to the sea floor. Here, using high-resolution seismic sections of seafloor sediments in the Cascadia margin off the coast of Vancouver Island, Canada, we observe lateral variations in the base of the hydrate stability zone, including gas-rich vertical intrusions into the hydrate stability zone. We suggest that these vertical intrusions are associated with upward flow of warmer fluids. Therefore, where seafloor fluid expulsion and methane hydrate deposits coincide, the base of the hydrate stability zone might exhibit significant roughness and increased surface area. Increased area implies that significantly more methane hydrate lies close to being unstable and hence closer to dissociation in the event of a lowering of pressure due to sea-level fall.

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Figure 1: High-resolution seismic images of the hydrate stability zone (upper 220 m of sediment) on the Cascadia margin about halfway between Ocean Drilling Program sites 889 and 890 (ref. 10).
Figure 2: Two views of the same portion of the methane hydrate stability zone.
Figure 3: Finite element modelling results for heat and fluid flux of the wipeout at 2.3 km in Fig. 1.
Figure 4: A comparison of BSRs.

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Acknowledgements

We thank C. Voss for advice on finite element modelling.

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Correspondence to R. D. Hyndman.

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Wood, W., Gettrust, J., Chapman, N. et al. Decreased stability of methane hydrates in marine sediments owing to phase-boundary roughness. Nature 420, 656–660 (2002). https://doi.org/10.1038/nature01263

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