The geophysical data acquired on Leg 164 have contributed substantially to our understanding of methane hydrate distribution, formation, and dissociation on the Blake Ridge with implications for other areas. The data clearly show that a significant portion of the total methane exists as free gas below the base of gas hydrate stability and that large deposits of gas hydrate may be most closely associated with fluid flux along faults. Thus, even in this setting close to a passive margin, high rates of localized advective flux appear to be critical to the development of massive gas hydrates, a result confirmed by recent quantitative modeling. The possible localization of gas hydrate on fault planes within the Blake Ridge may have important implications for seafloor stability and for the volatility of the gas hydrate reservoir during even minor climate change or sedimentological events.
Preliminary analyses of wide-angle seismic data have revealed P-wave anisotropy and a significant shear modulus contrast within the GHSZ on the Blake Ridge. The Leg 164 results have also pointed to a more sophisticated potential cause of seismic blanking than cementation by hydrate, challenging the use of blanking as a means of estimating hydrate concentrations in marine sediments. Wide-angle seismic data and the analysis of P-to-S conversions seem to hold more promise for remotely quantifying rock physical properties and the small amounts of gas hydrate present.
Thermal data acquired on the Blake Ridge imply that temperatures at the BSR are lower than the predicted temperature of dissociation at Sites 995 and 997. At present, this observation can only be explained by the inhibition of gas hydrate stability due to a physical or chemical process or by failure of the accepted phase equilibria to properly represent in situ conditions for the primarily methane system at 32.2 MPa pressure. The ODP Leg 164 temperature data loosely confirm the interpretation that Site 997 may be characterized by enhanced advection along buried faults.
Recent theoretical modeling offers one explanation for the different relationships between the free-gas zone and the base of the GHZ at Sites 994 and 995/997. The occurrence of the free-gas zone tens of meters deeper than the GHZ and the theoretical base of GHSZ at Site 994 may imply that the methane flux at this site is less than a critical value. At Sites 995 and 997, the apparent coincidence between the base of the GHZ, the base of the GHSZ, and the top of the free-gas zone produces a strong BSR and may imply that the critical methane supply rate has been exceeded.