CONTINENTAL MARGIN GAS HYDRATE: SLOPE STABILITY, GEO-HAZARD, AND CLIMATE?

Gas hydrate decomposition is hypothesized to be a factor in generating weakness in continental margin sediments that may help explain some of the observed patterns of continental margin sediment instability. The processes associated with formation and decomposition of gas hydrate can cause the strengthening of sediments in which gas hydrate grow and the weakening of sediments in which gas hydrate decomposes. The weakened sediments may form horizons along which the potential for sediment failure is increased. While a causal relationship between slope failures and gas hydrate decomposition has not been proven, a number of empirical observations support their potential connection. If correct, gas hydrate in near seafloor sediments may also constitute a significant hazard if they are disturbed by seafloor development activities. Unfortunately, standard acoustic techniques are not reliable at detecting the presence of sub-seafloor gas hydrate. The potential connection between gas hydrate and climate is based on the belief that: (1) the inventory of methane gas hydrate deposits is locally enormous, (2) the stability of marine gas hydrate deposits can easily be perturbed by climate induced temperature and pressure changes, (3) enough methane can be released from these deposits to contribute adequate volumes of this isotopically distinct greenhouse gas to alter oceanic or atmospheric reservoirs, and (4) the transfer of methane from deeper geologic reservoirs to the oceanic and/or atmospheric systems is effective. The fate of the gas-hydrate-bound methane contained in the sediments that are disturbed by slumping is one mechanism that needs to be considered in this debate. Our analysis and ROV-conducted field experiments indicate that large slumps could release volumetrically significant volumes of solid gas hydrates that would float upwards in the water column. Large crystals of gas hydrate will reach the upper layers of the ocean before decomposing and some will make it to the ocean’s surface and release methane directly into the atmosphere.