RADIOCARBON AND STABLE CARBON ANALYSIS OF DISSOLVED METHANE AND CARBON DIOXIDE FROM THE PROFILE OF A RAISED PEAT BOG

We developed and tested a new method to separate CO2 and CH4 from bulk gas samples for radiocarbon and stable-carbon analysis that utilizes a zeolite molecular sieve. To validate the technique, tests were performed using a suite of standard gases, composed of CO2 and CH4 of distinctly different isotopic composition. We employed the method to investigate the carbon isotopic composition of samples of dissolved CO2 and CH4 collected in situ from the near surface to deep layers of an ombrotrophic raised peat bog. Results showed that the age of both the CO2 and CH4 components of the dissolved gases increased with depth from similar to 0-300 BP at 0.25 m to similar to 4000 BP at 4 m. CH4 was mainly similar or slightly older in age compared to CO2, with the greatest difference in ages occurring at 1 m depth where CH4 was older by 430-615 yr. The delta C-13 values of CO2 increased with depth from -12.4 parts per thousand and -8.0 parts per thousand at 0.25 m to + 6.9 parts per thousand and + 8.3 parts per thousand at 4 m, whereas the delta C-13 of CH4 stayed in the range -58.4 parts per thousand to -70.6 parts per thousand. The C-14 results from the deepest layers are consistent with a similar source for both gases. C-14 ages for the CO2 component were younger compared to CH4, within the shallower depths of the peat bog (<= 1 m) and demonstrate the incorporation of acrotelm-derived respired CO2 into the catotelm.