Methane is the ecologically most beneficial hydrocarbon energy source. Nevertheless, it is also one of the important atmospheric greenhouse gases. Methane is formed at anaerobic conditions in deep and shallow sediments. It is accumulated in clathrates and reservoirs, dissolved in ocean, lake and river waters, and occurs in the atmosphere as one of the radiative trace gases. Global warming is partly related to the global increase of atmospheric methane concentration. Tracing the fate of methane from sediments through the water column into the atmosphere contributes to global atmospheric methane flux budgeting, which contributes to global temperature modelling.
In order to trace methane fluxes, concentrations and stable carbon isotope ratios of methane in sediments and in the water column have been determined. Marine environments like different areas in the Pacific, the Indian Ocean and the North Sea have been investigated as well as fresh water systems of Alpine lakes in Germany and Austria. Some general statements on the results of current investigations can be drawn:
-Shallow marine and fresh water sediments are often rich in methane, especially of bacterial origin.
-Methane diffusion from sediments into the water has not been detected so far, thus it is inferred to be negligible compared to other processes.
-Large amounts of methane enter the sea water only at specific sites, e.g. through vent systems.
-Vent-methane spreads out in ocean waters only locally: Horizontally some tens of kilometres, vertically some hundreds of meters only.
-Vent-methane has significantly different isotopic signatures compared to that of deep water "background" methane.
-Vent-methane entering deep ocean water does not reach surface water layers or the atmosphere.
-Near surface water (ca. 0 to 150 m) is often enriched in methane compared to atmospheric equilibrium. This methane is isotopically characterised as bacterial (d13C1 < -55 ). Although rich in oxygen, formation of methane in the surface water of ocean, lake and river waters has to be inferred, as lateral transport from lithoral areas is unlikely to cause the high concentrations in ocean and lake waters.
-Bacterial methane escapes from surface waters, hence certain oceanic and lake areas have to be considered as sources for atmospheric methane.
On the basis of the measured data, calculated methane flux rates range e.g. between 133 to 1700 kg km-2yr-1 for Lake Constance and ca. 30 kg km-2yr-1 for the Ganges Delta/Arabian Sea. These rates are higher than oceanic fluxes reported so far and allow to improve the data on mean oceanic methane flux to the atmosphere. Although these rates may be higher than considered before, methane emission rates from fresh and marine waters are low compared to the anthropogenic sources. Nevertheless, the new isotope data are used to improve considerably the up to now inconclusive global atmospheric methane budget based on carbon isotopes.