Methane
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; 25 times as effective as carbon dioxide. The picture to the right shows that methane gas or methane trapped as hydrate (solid block on picture) is able to burn.
Methanogenesis
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 (SO42-)-zone, where sulphate has been exhausted or occurs at very low concentration.
Migrating methane
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.
The figure shows carbon cycling with organic matter degradation and concomitant production of CO2 or CH4. Methane is subsequently oxidized to CO2 and thus origin to (re)production of organic matter.
Methane fluxes in the seabed
The methane flux can be calculated by Fick’s 1st Law of diffusion: Jx= − D (dC/dx), where Jx = the flux of the diffusing species, dC/dx = the concentration gradient, and D = diffusivity or diffusion coefficient (the proportionality constant).
The flux varies strongly depending on the methane production rate (methanogenesis) and on the depth of sulphate penetration into the seabed.
In sediments where methane is completely consumed in the sulphate-methane transition it is possible to calculate the net methane flux and thereby estimate the deep net methane production under the assumption of steady state conditions (i.e. production = consumption). This implies that the upward methane flux (i.e. consumption) equals the methane production. Also sulphate is totally consumed in the sulphate-methane transition and, consequently, the sulfate flux down into this zone should be a measure of the entire methane flux from the deep sub-surface. This sulfate flux can readily be modelled and the process can be experimentally measured, thus providing important information on overall methane fluxes in the seabed.
The figure shows the methane concentration in marine sediments. The concentration gradient, i.e. dC/dx (red line) is used to calculate the methane flux towards the sea floor.