11. GAS IN COHESIVE SEDIMENTS

Abstract

In the previous chapters we have argued that cohesive sediment contains various materials, such as sand, organic residue of plants, and sometimes contaminants. The organic matter is degraded under aerobic (top ~ 10 cm of the seabed) or anaerobic conditions (below ~10 cm within the seabed). A large number of bacteria species are involved in this degradation, resulting in the production of mainly two gasses: methane and carbon dioxide. After a time scale of weeks, years or centuries, depending on ambient conditions (temperature and pressure), the pore water can become saturated with dissolved gas, and a gas phase develops in the form of small bubbles. A cohesive sediment bed is an almost perfect trap for these gas bubbles, which behave as low density "particles". A large amount of gas can be stored in the sediment as bubbles, but when the sediment's bulk density becomes less than that of water, stability of the sediment bed is at risk. The presence of methane stored around the world in marine cohesive sediments is probably of the order of the presently known amounts of fossil fuel reserves. Releases of methane from marine sediments would form one of the major contributions to the global greenhouse effect and there are indications that such releases have played an important role in the cycles of ice-ages (Henriet et al., 1996). In this chapter, a concise description of the underlying processes is given, illustrated with a few case studies.