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Methane release through gas hydrate zones

2005-2009

Funder: Royal Society of New Zealand’s Marsden Fund
GNS Science internal funding

The release of methane from a gas hydrate-bearing seafloor is not yet well understood. Methane is a potent greenhouse gas and thus, it is feared methane release from hydrates following ocean warming may provide a positive feedback to future warming and climate change, a scenario that has been suggested for the past (Dickens et al. 1997; Dickens 1999). More recent research demonstrates that much of the methane from hydrates may be oxidized in the upper layers of sediments or immediately after being released into the ocean (e.g., McGinnis et al. 2006). Nevertheless, because of the vast amount of carbon stored in gas hydrates, hydrate destabilisation is bound to have a major effect on the global carbon cycle (Dickens 2001).

Numerous vent sites that release methane gas into the ocean have been discovered along the Hikurangi margin on the east coast of New Zealand (Lewis & Marshall 1996; Faure et al. 2006; Faure et al. submitted). Sampling in the vicinity of these vents indicate that methane is actively being expelled into the water column (Fig. 1), but no evidence was found that CH4 was reaching the sea surface (Faure et al., submitted).

Figure 1. Contour plot of CH4 associated with vent sites in the water column in the Wairarapa region (Faure et al., submitted).

Figure 1. Contour plot of CH4 associated with vent sites in the water column in the Wairarapa region (Faure et al., submitted).

The seafloor at many of these vent sites is in the gas hydrates stability zone. It is therefore intuitively difficult to understand how gas can travel through sediments within the gas hydrate stability field without being “trapped” as hydrates. Recently, models have been developed that predict how gas may move through highly permeable layers and fractures within the hydrate zone (Haeckel et al. 2004; Liu & Flemings 2007). Due to hydrate formation being slowed down by a lack of water or by an increase of pore-water salinity from expulsion of chloride from molecular hydrate cages.

Figure 2. Location map and seismic line across LM3 Sites, at Rock Garden.  High amplitudes indicate gas that appears to migrate upwards along strata to the base of gas hydrate stability and through faults through the gas hydrate zone (Crutchley et al. submitted).  BSR: Bottom simulating reflection at the base of gas hydrate stability.

Figure 2. Location map and seismic line across LM3 Sites, at Rock Garden. High amplitudes indicate gas that appears to migrate upwards along strata to the base of gas hydrate stability and through faults through the gas hydrate zone (Crutchley et al. submitted). BSR: Bottom simulating reflection at the base of gas hydrate stability.

Figure 1. (above) Contour plot of CH4 associated with vent sites in the water column in the Wairarapa region (Faure et al., 2010).
Figure 2. (above) Location map and seismic line across LM3 Sites, at Rock Garden. High amplitudes indicate gas that appears to migrate upwards along strata to the base of gas hydrate stability and through faults through the gas hydrate zone (Crutchley et al. 2010). BSR: Bottom simulating reflection at the base of gas hydrate stability.

Crustal seismic data (North Island GeopHyiscal Transect, NIGHT) suggested that shallow high amplitude reflections may mark gas conduits that “feed” the LM3 vent site (Lewis & Marshall 1996) a few kilometres to the north of the profile (Pecher et al. 2004). The seismic images from here and across several vent sites on Rock Garden reveal the “plumbing systems” through which gas is provided to seafloor vents. Gas appears to follow highly permeable, dipping layers to outcrops, near-vertical fractures to the seafloor, or the base of gas hydrate stability towards BSR pinchouts (Crutchley et al. submitted).

References

  • Crutchley GJ, Pecher IA, Gorman AR, Henrys S, Greinert J submitted. Seismic imaging of gas conduits beneath seafloor vent sites in a shallow marine gas hydrate province, Hikurangi Margin, New Zealand. Mar. Geol.
    Dickens GR 1999. The blast in the past. Nature 401: 752-753.
  • Dickens GR 2001. The potential volume of oceanic methane hydrates with variable external conditions. Org. Geochem. 32: 1179-1193.
  • Dickens GR, Castillo MM, Walker JCG 1997. A blast of gas in the latest Paleocene: Simulating first-order effects of massive dissociation of oceanic methane hydrates. Geology 25: 259-262.
  • Faure K, Greinert J, von Deimling JS, McGinnes DF, Kipfer R, Linke P submitted. Methane seepage along the Hikurangi margin of New Zealand: geochemical and physical properties of the water column. Mar. Geol.
  • Faure K, Greinert J, Pecher IA, Graham IJ, Massoth GJ, de Ronde CEJ, Wright IC, Baker ET, Olson EJ 2006. Methane seepage and its relation to slumping and gas hydrate at the Hikurangi margin, New Zealand. N. Z. J. Geol. Geophys. 49: 503-516.
  • Haeckel M, Suess E, Wallmann K, Rickert D 2004. Rising methane gas bubbles form massive hydrate layers at the seafloor. Geochim. Cosmochim. Acta 68(21): 4333-4345.
  • seismic section echosounder image through Faure field. The green to red “flares” in the echosounder images are caused by gas bubbles.  Gas appears to migrate along layering to Faure A and along the base of gas hydrate stability towards a BSR pinchout at Faure B (Crutchley et al. submitted).

    seismic section echosounder image through Faure field. The green to red “flares” in the echosounder images are caused by gas bubbles. Gas appears to migrate along layering to Faure A and along the base of gas hydrate stability towards a BSR pinchout at Faure B (Crutchley et al. submitted)

    Lewis KB, Marshall BA 1996. Seep faunas and other indicators of methane-rich dewatering on New Zealand convergent margins. N. Z. J. Geol. Geophys. 39: 181-200.
  • Liu X, Flemings PB 2007. Dynamic multiphase flow model of hydrate formation in marine sediments. J. Geophys. Res. 112: B03101.
  • McGinnis DF, Greinert J, Artemov Y, Beaubien SE, Wuest A 2006. Fate of rising methane bubbles in stratified waters: How much methane reaches the atmosphere? J. Geophys. Res. 111: C09007.
  • 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.
Figure 3a: Location map of the Faure field.
Figure 3b: Seismic section echosounder image through Faure field. The green to red “flares” in the echosounder images are caused by gas bubbles. Gas appears to migrate along layering to Faure A and along the base of gas hydrate stability towards a BSR pinchout at Faure B (Crutchley et al. submitted)