Methane in high latitudes
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. According to current estimates methane contributes to about 15% to global warming.
The ocean may act as a source as well as a sink for atmospheric methane. The marine background concentration in surface waters corresponds to the atmospheric equilibrium level. The ocean acts as a sink for atmospheric methane if atmospheric methane is transported by downward diffusion and convective ventilation into the deeper ocean whereby microbial methane oxidation occurs over the entire water column.
However, a methane surplus relative to the atmospheric equilibrium concentration is a persistent feature of most ocean surface waters and, thus, the ocean may act as a methane source releasing methane to the atmosphere. Although it is evident that microbial in situ methane production within the photic zone generates this supersaturation, the mechanism of methanogenesis is yet only poorly understood.
Additional fossil methane released from gas venting sites at the sea floor can also contribute directly and indirectly as a source. From the geological perspective, there are many natural sources of fossil methane that have been so far neglected in the assessment of the global methane budget. These natural sources include macro and micro seepages as well as mud volcanoes found worldwide along coastal margins.
Apart its greenhouse contribution, methane sources bear a large energy potential, which is of potential interest for future exploitation. Beside the occurrence of gaseous methane in bedrock caverns, large quantities of methane are fixed as methane clathrates (often referred to as “gas hydrates”) below the sea floor of continental margins as well as in deeper strata of high latitude permafrost areas. However, much more technological and ecological research is needed before such energy sources can be exploited in an efficient as well as a environmentally sustainable manner.
For both the assessment of the environmental as well as the energetic potential, sub-seafloor methane sources are to be better constraint. Against this background, the AWI Geochemistry Group contributes to marine methane research e.g. with the following activities
- Methane inventories in marine and coastal sediments
- Quantification of submarine methane release from diffusive, focused outflow and bubble discharge
- Fate of methane in the water column above seep locations
- Pelagic in situ methane production
- Isotope fractionation processes and isotopic signatures of methane from specific sources