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Worldwide Gas Hydrate Occurrences and Regional Case Studies |
Geology Department, UNC-CH, Chapel Hill, NC 27599-3315, USA
Deep ODP holes on the Blake Ridge (sites 994, 995 and 997; Leg 164) show that sediments within the interval
200–450 metres below sea floor (mbsf) contain 1–4% gas hydrate based on interstitial geochemistry, pressure core samples, well logs and borehole seismic data. In addition, a considerable methane gas reservoir also exists below the gas hydrate stability zone in the form of free and dissolved methane. Addition of methane from these deep reservoirs to overlying ocean water (and ultimately to the atmosphere) could have profound geological, geochemical and climatic effects. Pervasive flux of methane to the sea floor via diffusion is unlikely because methane is almost completely consumed by reaction with sulphate at the sulphate-methane interface. Further, sulphate and 87Sr/86Sr profiles suggest that upward advection of potential methane-containing fluids is also unlikely. However, point sources for methane advection to the sea floor do occur. Active methane transport along faults occurs at Site 996, and the methane 13C isotopic signature indicates derivation from the gas hydrate-bearing and free gas zones below. 14C data from sediments indicate that the frequency of sediment slumping on the Blake Ridge is higher during the sea-level lowstand associated with the last ice age. Whereas there is no direct evidence that methane exchange is associated with slump events, nor that gas hydrates are necessarily related to the increased slumping frequency, the association between slumps and faults suggest that the amounts of gas escape should be greater during sea-level lowstands.
