Quantifying the role of degasification subsidence in wetland loss -- Mississippi delta plain

Modem and ancient delta plains in Louisiana that border the northern Gulf of Mexico are undergoing unprecedented rates of land loss. Subsidence due to compaction and loss of pore water is believed to be one of the primary land loss factors, whereas subsidence due to gas emission is generally believed to play a significant role only in the upper one or two meters of a wetland. Evidence to the contrary is presented here that suggests that degasification subsidence is a more important mechanism contributing to land loss than previously thought. In addition, potential degasification subsidence can be quantified if the depression from normal compressional and shear wave velocities is known. Velocity reductions of 30 to 75% from compressional wave velocity in water and extending to depths of tens of meters have been observed in a saltwater marsh in Louisiana. An order of magnitude reduction in velocity has been observed previously in gas-rich sediments on the adjacent continental shelf, where biogenic gas is observed to depths of 1 km or more. The ratio of normal to depressed compressional wave velocities is used to calculate potential degasification subsidence.