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Gas
within the GHSZ on Hydrate Ridge, Oregon, and Implications for Rate of Gas Hydrate Formation
1 Oregon State University, Corvallis, USA, [email protected],
[email protected]
2 Geomar, Kiel, Germany, [email protected]
3 University of Bremen, Bremen, Germany, [email protected]
4 Geoteck, Northants, UK, [email protected]
5 Eastern Kentucky University, Richmond, USA, [email protected]
6 University of Tokyo, Tokyo, Japan, [email protected]
Since the onset of studies on Hydrate Ridge in the Cascadia subduction zone, transport of free gas within the gas hydrate stability zone (GHSZ) has been postulated based on observations of bubble discharge over topographic highs of this accretionary margin. Drilling during ODP Leg 204 established the co-existence of large amounts of methane gas hydrate, gaseous methane, and highly saline porewater near the southern summit. Direct evidence for free gas was documented by logging a core recovered at in situ pressure from 14 mbsf. Seismic reflection data suggest that movement of methane gas below the bottom simulating reflector (BSR) is focused towards the ridge crest along a high-permeability horizon (Horizon A), which is imaged as a strong reflector with negative amplitude.
A one dimensional, non-steady state, transport reaction model was developed to simulate the observed chloride enrichment at the Hydrate Ridge summit. Our model shows that in order to reach the observed high chloride values at the ridge summit, methane must be transported in the gas phase from the depth of the BSR to the seafloor. Methane transport exclusively in the dissolved phase is not enough to form methane hydrate at the rates needed to generate the observed chloride anomalies.
In order to reproduce the observed abrupt change in chloride and gas hydrate content at 25
+ 5 mbsf, the model requires an enhanced rate of hydrate formation in near surface sediment. We postulate that the observed change in fabric and concentration of gas hydrate above 25 mbsf is due to changes in geomechanical properties of the sediment. At depths shallower than 25 mbsf the internal pressure of the growing crystals can overcome effective overburden stress, and hydrate growth proceeds by particle displacement, thus minimizing capillary inhibition effects.
Our calculations indicate the hydrates in the upper sediments of the ridge summit are probably younger than 1,500 years. Combining our model results with previous seafloor observations at this site, we can estimate that gas
hydrate formation rates at the ridge crest are in the order of
102 mol m-2yr-1. These rates are several orders of magnitude higher than those estimated for Site 997 on the Blake Ridge, highlighting the dynamic nature of these deposits.