An investigation of the combined stable isotopic composition of methane emissions from northern wetlands.
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 C~
concentration measurements alone. Conventional analytical techniques for studying dual stable
isotopic composition of methane (813e and 8D) require prohibitively large quantities of CH4 for
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 ofCH4 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 817M. 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 817M ranging from -34 to -17%0. 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%0 shift in 817M. 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 817M. It was concluded that the seasonal variation in the isotopic composition of
methane emission is linked to the plant growth cycle. Comparison of 817M values determined for
methane emissions in Snowdonia with published 813C and 8D data leads to the conclusion that
methane is produced mainly by C02 reduction.
Contrasting terrains in a paisa mire in the Arctic region of Finland exhibited methane _
emissions with distinct 817M values: lakes, +4.8 ±1.2%0; pools, -3.9 ±O.IO/oo and hummocks, -28.6
±5.8%0 .. 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.