Amongst other factors, the two greenhouse gases carbon dioxide (CO₂) and methane (CH₄) are responsible for the global warming on our planet. They keep excess heat from being released into space. Doing so, a methane molecule is 20 - 60 times more efficient than CO₂. Fortunately, the atmosphere contains less methane than CO₂. Although, methane contributes to about 15% to global warming.

In order to better comprehend and predict the climate development, it is essential to know sources of methane. Whereas the terrestrial methane emitters are fairly well known, oceanic methane sources are still poorly constrained. Often times the source is identified, however the amount of methane released is hardly known.

• sources of methane
• sinks of methane
• how can methane be detected?
• How to get information on the origin of methane?
• How can the fate of methane be tracked in the water column?
• Which portion of methane from marine sources reaches the atmosphere?
• Our target areas
• selected publications

Opposed to the mentioned sources, there are several mechanisms to degrade methane in the ocean. Microbial degradation processes are the most important ones:

A considerable portion of the methane released from submarine sources to the ocean is oxidised aerobically and, thus, is transformed into CO₂ (or dissolved bicarbonate). For this process methane-oxidising bacteria are responsible.

A large amount of methane does not reach the ocean but is degraded already within the sediments. One of the most efficient bio filters of this kind is a consortium of methanotrophic achaea and sulphate reducing bacteria. In closest cooperation these microbes metabolise methane and sea water sulphate into (bi)carbonate and H₂S. In turn, this process can lead to the formation of massive carbonate precipitation, whereas the sulphur of the released H₂S is incorporated as elementary sulphur by sulphur bacteria, typically at the sediment surface. The white mats built by these organisms at the sediment surface can be used as a visual indicator for the occurrence of anaerobic methane oxidation.

In the euphotic zone, methane can be degraded by photo oxidation.

Methane is a very volatile burnable gas and is only little soluble in water. One can use these properties to analyse methane by gas chromatography (GC):

A gaseous sample is injected into the column of a gas chromatograph. Due to its highly volatile behaviour, methane passes the column as one of the first components and can subsequently be detected by means of an attached flame ionization detector.

Methane dissolved in sea water has to be separated from the water prior to the analysis. In the case of high concentrations this can be done by introducing a head space volume above the water sample (headspace method). A subsample is taken from this headspace and injected into the GC as described above. Due to the establishment of a temperature-related equilibrium between methane dissolved in water and the gas phase above, one can calculate back the original methane concentration of the water sample. At low sea water methane concentrations it is recommended to apply a more sophisticated purge and trap procedure where the gas is stripped from the aqueous sample under vacuum and is extracted then in a cool trap.

Methane can be analyzed from sediment samples using cut-off syringes to sample a sediment core from the side (e.g. through taped drill-holes in the core liner). Gas loss can be avoided by using appropriate measures and quick sampling immediately after core retrieval. The sediment is transferred from the syringe into a vial with alkaline solution (in order to stop microbial activity) and is superimposed by an inert gas (e.g. N2 or Ar) prior to immediate closure by a septum lid. Samples are shaken to establish an equilibrium and analysed as described above.

First of all it is important to determine the amount of methane in a particular sample. Besides, it is strongly desired to get information about the origin of the gas. To decipher such information, scientists use the "isotope effect": In nature, atoms of the same element occur with a different number of neutrons causing different atomic masses for one element. Carbon atoms can have the mass 12 and 13, referred to as ¹²C and ¹³C, respectively (¹⁴C with mass 14 has such low natural abundances, that it does not play any role in this context). Due to mass inertia, ¹²C compounds react faster than those with ¹³C. Consequently, ¹³C is enriched or depleted in reactants (isotope fractionation). Studying the involved biogeochemical processes and comparing ¹²C and ¹³C abundances in the compounds involved (isotope signature), scientists can assess the origin of carbon compounds such as methane.

 Scientific example from the Spitsbergen area

Methane from submarine sources can be released via different pathways into the ocean: 

Diffusion: Due to the molecular motion, dissolved methane migrates from high to low concentrations on order to generate equilibrium. Likewise, it diffuses from methane-rich sediments into the methane-poor bottom water. By means of special equipment such as a bottom water sampler, the resulting gradients can be determined. While diffusing through surface sediments, a considerable portion of the venting methane is often degraded by microbial communities.

Advection: Transported along fault structures in the sea floor (e.g. bottom openings and cracks), methane-rich water can be directly released to the bottom water. Methane discharge rates can be quantified by measuring flow velocities and methane concentration in the venting fluid.

If the venting fluid is over-saturated in respect to methane, free gas can be formed, which is released as bubbles into the ocean. The bubble plume can be visualised and followed by hydro-acoustic means such as fish echo sounders. If gaseous methane is discharged under conditions where gas hydrate is stable (i.e. within the gas hydrate stability zone (GHSZ), bubbles are normally coated by a thin gas hydrate skin immediately upon release. Protected like this, methane bubbles dissolve much more slowly during their rise in the water column.

The floating methane bubbles cause the ambient water to flow upward as well (bubble-induced up-welling). Above the GHSZ the bubbles lose their hydrate skin and dissolve rapidly. Due to momentum conservation, the water body of the plume ascends further up and carries the dissolved methane into higher water layers.

The estimation of this portion requires further research. Isotope investigations and rate measurements with tritiated methane suggest that the oxidation process of methane can be slow in the upper water column, particularly in cold regions. Especially during the winter season, the surface ocean can be mixed down to several hundred meters depth. This guarantees for an enhanced air-water gas exchange which allows methane even from deep sources to be released to the atmosphere. However, the quantification of sea-air fluxes is to be further investigated. In warm water columns where microbial activities are normally higher in the water column, dissolved methane can be oxidized more efficiently. Correspondingly, a smaller portion of the methane released would make it into the atmosphere.

Håkon Mosby mud volcano (HMMV) situated in the western Barents Sea was intensively studied in recent years. In close cooperation with partner institutions such as IFREMER, MPI for Marine Microbiology, University Bremen and University Nishny Novgorod, the AWI conducted studies on the release and spreading of methane at HMMV. Several missions were supported by the powerful deep-sea remotely operated vehicle (ROV) "Victor 6000" (IFREMER).

Methane discharge from inoxic sediments was studied in Eckernförde Bay (western Baltic Sea). Methane release was found to be strongly linked to submarine groundwater discharge (EU project SubGATE).

On south west Spitzbergen continental shelf methane anomalies were found and investigated in the Hornsundfjord and the van Mijenfjord. Read more …

Methane release is currently being studied in the tidal flats of the Cuxwatt in association with small scale fresh water discharge.

2006

Damm, E., Schauer, U., Rudels, B., Haas, C. (2006). Sea-air flux of submarine methane in an Arctic latent heat polynya, Geophysical research letters.

Sauter, E. J., Muyakshin, S. I., Charlou, J. -L., Schlüter, M., Boetius, A., Jerosch, K., Damm, E., Foucher, J. -P., Klages, M. (2006). Methane discharge from a deep-sea submarine mud volcano into the upper water column by gas hydrate-coated methane bubbles, Earth and planetary science letters, 243(3/4), 354-365. DOI: 10.1016/j.epsl.2006.01.041

Sauter, E. J., Muyakshin, S. I., Charlou, J. -L., Schlüter, M., Boetius, A., Jerosch, K., Damm, E., Foucher, J. -P., Klages, M. (2006). Methane discharge from a deep-sea submarine mud volcano into the upper water column by gas hydrate-coated methane bubbles, Earth and planetary science letters, 243(3/4), 354-365. DOI: 10.1016/j.epsl.2006.01.041

Beer, D. de, Sauter, E., Niemann, H., Kaul, N., Foucher, J.-P., Witte, U., Schlüter, M., Boetius, A. (2006). In situ fluxes and zonation of microbial activity in surface sediments of the Håkon Mosby Mud Volcano, Limnology and oceanography, 51 (3), 1315-1331.

2005

Damm, E., Mackensen, A., Budéus, G., Faber, E., Hanfland, C.(2005). Pathways of methane in seawater: Plume spreading in an Arctic shelf environment (SW-Spitsbergen), Continental shelf research, 25, 1453-1472. DOI: 10.1016/j.csr.2005.03.003

2004

Schlüter, M., Sauter, E. J., Andersen, C. E., Dahlgaard, H., Dando, P. (2004). Spatial Distribution and Budget for Submarine Groundwater Discharge in Eckernförde Bay (W-Baltic Sea), Limnology and Oceanography, 49(1), 157-167.

2003

Damm, E., Budéus, G.(2003). Fate of vent-derived methane in seawater above the Håkon Mosby mud volcano (Norwegian Sea), Marine chemistry, 82, 1-11. DOI: 10.1016/S0304-4203(03)00031-8

For additional publications, please visit the personal homepages of E. Damm and E. Sauter

Publications