Introduction
Methane (CH4) is produced mostly in ocean margin sediments through the microbial degradation of organic matter buried below the sulphate zone. The methane is formed primarily from acetate or from the reduction of carbon dioxide (CO2) with hydrogen (H2), processes called methanogenesis.
Gas bubbles
As methane builds up in the sediment it migrates upwards towards the sediment surface either by molecular diffusion or as free gas bubbles. The methane occasionally escapes into the bottom water, thereby generating escape structures such as pockmarks.
Although methane is continuously formed in ocean margin sediments, the accumulation of free gas (i.e. bubble formation) is an exception. Gas bubbles develop at sediment depths where the methane concentration exceeds saturation at the ambient hydrostatic pressure. This pressure, and thus the saturating methane concentration, depends on the water depth.
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On a global scale it is estimated that less than 10% of the methane produced in the sediment is released from the sea floor. This implies that by far most of the methane is effectively scavenged before it reaches the sediment surface. In the sub-surface sediment, where there is no oxygen, sulphate is the oxidant for methane which is converted to carbon dioxide. Most methane is oxidized at the depth to which sulphate penetrates down into the sediment, at the sulphate-methane transition.
Archaea are responsible for the direct enzymatic attack on the methane, but it is not clear how the process is coupled to the respiratory reduction of sulphate. It is assumed that reduced compounds, or perhaps electrons, are transferred from the Archaea to the sulphate reducing bacteria. The two types of organisms often grow together and form distinct consortia at sites where the rate of anaerobic oxidation of methane (AOM) is high. The bacteria reduce the sulphate to hydrogen sulphide (H2S).
Enhanced eutrophication by nutrients such as nitrate or phosphate leads to increased production of organic matter in marginal seas like the Baltic Sea. Predicted climate change may increase the water temperature and reduce the deep water ventilation, factors which will stimulate anaerobic degradation of organic matter buried in the sediment. As a consequence, more free methane gas (i.e. gas bubbles) will accumulate in the sediment. This may lead to an enhanced emission of methane to the water column and atmosphere where it acts as a very potent greenhouse gas. Accumulation of shallow gas in the seabed may pose hazards to seabed structures such as wind farms, pipelines, power or communications cables, and off-shore drilling operations by destabilizing the sea floor. Enhanced ebullition from hot-spots of shallow gas will also enhance the emission of hydrogen sulphide. Hydrogen sulphide is toxic to fish and other marine life and is highly corrosive.