An investigation of the combined stable isotopic composition of methane emissions from northern wetlands

Jackson, Sarah May (1999). An investigation of the combined stable isotopic composition of methane emissions from northern wetlands. PhD thesis The Open University.



Methane is a radiatively active, naturally occurring atmospheric trace gas which is thought to account for as much as 19% of the enhanced greenhouse effect. Ice core studies have shown that the atmospheric concentration has more than doubled since pre-industrial times. Wetlands are the largest natural source of atmospheric methane, contributing around 21 % of the annual global flux. The magnitude of various sources of methane is still poorly defined. Stable isotope measurements are increasingly being used to constrain global budgets of atmospheric trace gases because isotopic analysis provides a much clearer picture of global atmospheric chemistry than CH4 concentration measurements alone. Conventional analytical techniques for studying dual stable isotopic composition of methane (δ13C and δD) require prohibitively large quantities of CH4 for analysis.

At the Planetary Sciences Research Institute of the Open University, a highly sensitive static mass spectrometer has been developed which uniquely uses CH4 as the analyte. The method requires only 8 ng of CH4 for analysis «10 ml ambient air), making replicated measurements of the isotopic composition of CH4 emissions from wetlands feasible for the first time.

Methane emissions from an ombrotrophic mire in Snowdonia have been measured over 2 years, (1995-1997) and analysed for δ17M. Parallel laboratory studies have also been conducted, to constrain the effects of environmental variables such as peat temperature and water table depth. The presence of vascular plants enhanced methane flux. In the field, methane flux showed seasonal variation. Peat temperature and water table depth could account for 68% of this variation.

The isotopic composition of methane flux from the ombrotrophic mire also exhibited seasonal variation, with δ17M ranging from -34 to -17‰. The lowest values were observed in summer and the highest in winter. Variations in the isotopic composition of peat water are unlikely to account for more than a 2‰ shift in δ17M. Although there was a strong correlation between peat temperature and methane isotopic composition in the field, peat temperature is thought to be an indirect effect, because in laboratory studies this relationship was absent. There was no relationship between water table depth and δ17M. It was concluded that the seasonal variation in the isotopic composition of methane emission is linked to the plant growth cycle. Comparison of δ17M values determined for methane emissions in Snowdonia with published δ13C and δD data leads to the conclusion that methane is produced mainly by CO2 reduction.

Contrasting terrains in a paisa mire in the Arctic region of Finland exhibited methane _ emissions with distinct δ17M values: lakes, +4.8 ±1.2‰; pools, -3.9 ±0.1‰ and hummocks, -28.6 ±5.8‰ .. From these isotope data it was concluded that in pool and lake sediments the methanogenic pathway is acetate fermentation, while in hummocks methane is produced by CO2 reduction.

This study is the first investigation of the stable isotopic composition of methane emissions from wetlands in the UK. The data collected in Snowdonia, and in Finland, show the need for systematic, year round isotopic analysis of methane emissions, if isotope data are to be used in constraining the global methane budget.

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