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Gas hydrates and seafloor stability

Funder: Royal Society of New Zealand’s Marsden Fund

Programme: How do gas hydrates weaken the seafloor, causing submarine slides and tsunamis? 2005-2009

Gas hydrates may represent a significant natural hazard. Dissociation (“melting”) of hydrate generates large volumes of gas that may weaken the seafloor, causing submarine landslides and tsunamis. East of the North Island, in an area with abundant gas hydrates, two undersea ridges, Rock Garden and parts of Ritchie Banks, appear to be "chopped off" 600 m below sea level suggesting erosion of the seafloor at this depth. The plateau-like crests of the ridges are flanked by pinchouts of bottom simulating reflections (BSRs). BSR pinchouts mark the top of the gas hydrate stability zone in the ocean.

We postulate that the top of gas hydrate stability is linked to seafloor erosion (Pecher et al. 2005). Tectonic modelling supports the conclusion that ridge flattening is not caused by tectonic processes (Ellis et al. submitted). We have tested the feasibility of several mechanisms related to gas hydrates that may lead to seafloor erosion with a multidisciplinary study of the ridge and quantitative modelling (Crutchley submitted; Ellis et al. submitted). Results are of fundamental significance for understanding how gas hydrates may cause submarine landslides and tsunamis.

Location of the Rock Garden and Ritchie Banks on the Hikurangi Margin.  NIGHT: Crustal seismic line in which BSRs were first observed on Rock Garden (Pecher et al. 2004).

Location of the Rock Garden and Ritchie Banks on the Hikurangi Margin. NIGHT: Crustal seismic line in which BSRs were first observed on Rock Garden (Pecher et al. 2004).

Conceptual model: “Freeze-thaw” cycles of gas hydrates leading to seafloor weakening – modelling suggests this process is unlikely to be effective.

Researchers

  • Ingo Pecher, Susan Ellis (GNS Science)
  • Stephen Chiswell, Helen Neil (NIWA)
  • Andrew Gorman (University of Otago)
  • Nina Kukowski (GeoForschungsZentrum Potsdam , Germany)

Repeated formation/dissociation of gas hydrates

Predicted profiles of gas hydrate concentration as a function of water depth during uplift of Rock Garden at a rate of 5 mm/yr. (Ellis et al. submitted). Initial stage is a smooth profile of gas hydrate concentration beneath a seafloor at 680 m water depth.  The gas hydrate zone is thinning and hydrate saturation increases during uplift to 605 m after 15,000 years because gas that dissociates at the base of gas hydrate stability is “recycled” back into the hydrate stability zone to form hydrate.

Predicted profiles of gas hydrate concentration as a function of water depth during uplift of Rock Garden at a rate of 5 mm/yr. (Ellis et al. submitted). Initial stage is a smooth profile of gas hydrate concentration beneath a seafloor at 680 m water depth. The gas hydrate zone is thinning and hydrate saturation increases during uplift to 605 m after 15,000 years because gas that dissociates at the base of gas hydrate stability is “recycled” back into the hydrate stability zone to form hydrate.

Bottom-water temperatures on Rock Garden vary over time and hydrates beneath the seafloor may therefore repeatedly "freeze" and "thaw". We postulated that expansion and contraction during hydrate dissociation and formation may weaken the seafloor by a process similar to frost heave. However, pore pressure estimates from a commonly used model for gas hydrate formation and dissociation (Xu & Ruppel 1999; Xu 2004) suggest for this process to be viable, unusually high methane flux rates are needed over a wide area in order to form significant near-seafloor hydrates. For lower methane fluxes and short cycles of water-temperature fluctuation (1 year), the temperature signal is unlikely to reach gas-hydrate-bearing sediments whereas for long cycles (100 years), changes are slow enough to allow dissipation of any overpressure. However, in conjunction with a permeability reduction during hydrate formation, hydrate “freeze-thaw” cycles may contribute to seafloor weakening from overpressure at the base of gas hydrates stability (Ellis et al. submitted).

Overpressure at the base of gas hydrate stability

Tectonic compaction of the accretionary prism on the Hikurangi Margin leads to significant fluid flow. Modelling of a gas hydrate system during uplift revealed that significant pore pressure may build up beneath shallow gas hydrates easily reaching lithostatic pressure and thus, seafloor fracturing. Key to this process is a decrease of permeability with increasing hydrate saturation combined with an increase of hydrate saturation during ridge uplift from “recycling” of gas into the gas hydrate stability field (Ellis et al. submitted).

Gas pockets beneath the base of gas hydrate stability

Numerous gas pockets beneath a shallow gas hydrate stability zone have been observed beneath Rock Garden (Crutchley et al. submitted-a). Gas can generate pressure by its buoyancy (“gas column”). The effect of gas columns was studied using 2-D two-phase modelling in collaboration with Heriot-Watt University, Edinburgh . It was found that gas buoyancy may contribute significantly to fluid-flow-generated overpressure facilitating fracturing of sediments (Crutchley et al. submitted-b).

Seismic image of a gas pocket beneath a shallow gas hydrate stability zone on Rock Garden, BGHS: Base of gas hydrate stability zone (Crutchley et al. submitted-b).

Seismic image of a gas pocket beneath a shallow gas hydrate stability zone on Rock Garden, BGHS: Base of gas hydrate stability zone (Crutchley et al. submitted-b).

Over and underpressure caused by the gas pocket, Pf: fluid pressure, Ph: hydrostatic pressure.  Significant overpressure is generated at the BGHS.  Underpressure (less than hydrostatic, blue) is caused by buoyant gas “sucking up” fluids from below (Crutchley et al. submitted-b)

Over and underpressure caused by the gas pocket, Pf: fluid pressure, Ph: hydrostatic pressure. Significant overpressure is generated at the BGHS. Underpressure (less than hydrostatic, blue) is caused by buoyant gas “sucking up” fluids from below (Crutchley et al. submitted-b)

Tsunami hazard

Weakening of the seafloor may lead to submarine slides, potentially causing tsunamis. First “back-of-the-envelope” calculations suggest sliding on Rock Garden may, in an extreme scenario, be tsunamogenic (Kukowski et al. submitted). More detailed analyses are currently being conducted based on seismic images from Ritchie Banks.

Additional considerations and conclusions

Additional aspects that may affect gas hydrate formation on Rock Garden include (Pecher et al. 2008):

  • A temperature probe deployed for 450 days indicates that both amplitudes and wavelengths of water temperature fluctuations on Rock Garden are larger than previously thought. This would cause temperature variations to penetrate deeper into the seafloor than the 2-3 m originally estimated (Pecher et al. 2005) and thus, make it more likely to reach hydrate-bearing sediments.
  • Dredge samples suggest that the “country rock” on Rock Garden consists of fine-grained, inundated mudstones. Gas hydrate formation in mudstones is not well understood. Also, permeability of mudstones is often dominated by fractures that may be easily “clogged” by hydrates, facilitating the build-up of overpressure beneath gas hydrates.

We summarize findings from our project as follows:

  • A purely tectonic origin of the flat ridge crests is highly unlikely – processes linked to a shallow gas hydrate zone are most likely involved in ridge erosion.
  • “Freeze-thaw cycles” (repeated formation and dissociation) of gas hydrates require unusually high rates of methane supply to be a viable mechanism for seafloor weakening.
  • Overpressure at the base of gas hydrate stability beneath an uplifted ridge on the other hand is predicted to lead to seafloor weakening and thus, seafloor erosion.
  • Buoyancy from gas pockets may significantly contribute to overpressure.
  • A reduction of permeability caused by precipitation of gas hydrates in sediment pores or fractures appears to be the key parameter controlling overpressure.
  • The tsunami risk from possible slides linked to gas hydrates on Rock Garden and Ritchie Banks is still being investigated.

References

  • Crutchley G submitted. Gas hydrates on New Zealand's Hikurangi Margin: the importance of focused fluid flow for highly-concentrated deposits, methane seepage, and seafloor erosion. PhD thesis, University of Otago, Dunedin. 172 p.
  • Crutchley GJ, Pecher IA, Gorman AR, Henrys S, Greinert J submitted-a. Seismic imaging of gas conduits beneath seafloor vent sites in a shallow marine gas hydrate province, Hikurangi Margin, New Zealand. Mar. Geol.
  • Crutchley GJ, Geiger S, Pecher IA, Gorman AR, Zhu H, Henrys SA submitted-b. The potential influence of shallow gas and gas hydrates on seafloor erosion of Rock Garden, an uplifted ridge offshore of New Zealand. Geo Mar. Lett.
  • Ellis S, Pecher IA, Kukowski N, Xu W, Greinert J, Henrys S submitted. Testing proposed mechanisms for seafloor weakening at the top of gas hydrate stability, Rock Garden, New Zealand Mar. Geol.
  • Kukowski N, Greinert J, Henrys S submitted. Morphometric and critical taper analysis of the Rock Garden region, Hikurangi Margin, New Zealand: implications for slope stability and potential tsunami generation. Mar. Geol.
  • Pecher IA, Henrys SA, Zhu H 2004. Seismic images of gas conduits beneath vents and gas hydrates on Ritchie Ridge, Hikurangi margin, New Zealand. N. Z. J. Geol. Geophys. 47: 275-279.
  • Pecher IA, Henrys SA, Ellis S, Chiswell SM, Kukowski N 2005. Erosion of the seafloor at the top of the gas hydrate stability zone on the Hikurangi Margin, New Zealand. Geophys. Res. Lett. 32: L24603.
  • Pecher IA, Henrys SA, Ellis S, Crutchley G, Fohrmann M, Gorman AR, Greinert J, Chiswell SM, TAN0607 Scientific Party, SO191 Scientific Party 2008. Erosion of seafloor ridges at the top of the gas hydrate stability zone, Hikurangi margin, New Zealand – new insights from research cruises between 2005 and 2007. Proc. 6th International Conference on Gas Hydrates. Vancouver. Pp. 10 pp.
  • Xu W 2004. Modeling dynamic marine gas hydrate systems. Amer. Miner. 89: 1271-1279.
  • Xu W, Ruppel CD 1999. Predicting the occurrence, distribution, and evolution of methane gas hydrate in porous marine sediments. J. Geophys. Res. 104: 5081-5095.