Baltic Gas

An important energy source and a strong greenhouse gas

Methane (CH₄) is one of the most important energy sources in the industrialized world, but it is also a strong greenhouse gas when emitted into the atmosphere; 23 times as effective as carbon dioxide (CO₂).

Most of the methane in marine environments is of microbiological origin. It is produced in great quantities in the sediments by methane-producing microorganisms during organic matter decay through a process named methanogenesis. In marine sediments, bacterial sulphate-reduction dominates in the upper sub-surface layers because sulphate reducing bacteria are energetically more effective in the degradation of organic matter than methane-producing microorganisms. Methanogenesis takes over deeper in the sediment, below the sulphate (SO₄^2-)-zone, where sulphate has been exhausted or occurs at very low concentration.

The methane concentration builds up below the sulphate zone and methane migrates up towards the sediment surface by diffusion or as gas bubbles. The migrating methane penetrates up into the sulphate zone where a distinct depth interval of overlapping methane and sulphate occurs, and thus named the sulphate-methane transition (SMT) zone. In this zone, methane is oxidized to carbon dioxide, probably by archaea in syntrophic association with sulphate reducing bacteria. The carbon dioxide diffuses up into the water column and carbon dioxide may subsequently be taken up by plankton algae and incorporated into organic biomass through photosynthesis.

On a global scale, more than 90% of all methane produced in the seabed is trapped in the sulphate-methane transition and oxidised to carbon dioxide. Methane which ascends with deep fluids up through cold seeps or hot vents is mostly exported into the bottom water, however, and so are gas bubbles which rise by focused or diffuse ebullition from the sea floor.