19th January 2012.
When increases in the mixing ratio of methane in the atmosphere are observed, it is useful to know which methane source is contributing to that increase. A recent paper in Geophysical Research Letters (Fisher et al., 2011) has highlighted how isotope data can help determine the source of atmospheric methane in the Arctic.
The starting point for this type of work is knowledge of the carbon isotopic signatures of the major relevant methane sources. If these d13C values are sufficiently distinct, then inverse modelling of data from atmospheric samples can yield insights into the provenance of the methane. In this study, in addition to using published figures for d13C signatures, Fisher and co-authors gathered new data, for example showing that methane from Canadian boreal pine forest fires is relatively enriched in the heavier isotope (d13C -28 ± 1 ‰). They also showed that the isotopic signature of methane from W. Spitzbergen marine clathrates (d13C -50 ± 5 ‰) is variable (d13C -50 ± 5 ‰), and that at present very little of this methane reaches the atmosphere.
Having established the source signatures, measurements of d13C-CH4 from air samples taken at the Zeppelin station in Spitzbergen in late summer/autumn 2008 and 2009, and in spring 2009, were then used to study seasonal variations in the source of that methane. At d13C close to -68 ‰, the late summer/autumn methane samples bore a striking similarity to the highly 13C depleted methane typical of wetland emissions. By contrast samples taken in spring, when the wetlands are frozen, were richer in the heavier isotope (d13C -52.6 ± 6.4 ‰). This is shown by modelling to be consistent with leakage from W Siberian gas infrastructure being the dominant source.
Rebecca Fisher, first author of the study, comments, “Isotopes are a powerful tool for constraining sources, but there are very few stations in the Arctic from which ambient air samples are collected for isotope studies. High frequency, ideally continuous, monitoring of δ13C in methane from a number of Arctic sites, onshore and offshore, will be important if future changes in Arctic sources are to be quantified.”
The conclusion that summer emissions in this part of the Arctic are in the main derived from wetlands meshes well with other studies investigating the importance of wetlands as a high latitude methane source (Tarasova et al., 2009). Given the potential for global warming to create more extensive areas of boreal wetland, and the fact that this methane source could be supplemented by methane from hydrates destabilised under increasing temperatures, the potential for dramatic rises in input of methane to the Arctic atmosphere is a cause for continuing concern.
Fisher, R.E., Sriskantharajah, S., Lowry, D., Lanoisellé, M., Fowler, C.M.R., James, R.H., Hermansen, O., Lund Myhre, C., Stohl, A., Greinert, J., Nisbet-Jones, P.B.R., Mienert, J., and Nisbet, E.G. (2011). Arctic methane sources: Isotopic evidence for atmospheric inputs. Geophysical Research Letters, 38, L21803.
Tarasova, O.A., Houweling, S., Elansky, N. and Brenninkmeijer, C.A.M. (2009). Application of stable isotope analysis for improved understanding of the methane budget: Comparison of TROICA measurements with TM3 model simulations. Journal of Atmospheric Chemistry, 63(1), 49-71.
Photo: air inlet on Zeppelin station roof, taken by Dave Lowry.