Radiocarbon and stable carbon analysis of dissolved methane and carbon dioxide from the profile of a raised peat bog

Abstract

We developed and tested a new method to separate CO<sub>2</sub> and CH<sub>4</sub> from bulk gas samples for radiocarbon and stable-carbon analysis that utilizes zeolite molecular sieve. To validate the technique, tests were performed using a suite of standard gases, composed of CO<sub>2</sub> and CH<sub>4</sub> of distinctly different isotopic composition. We employed the method to investigate the carbon isotopic composition of samples of dissolved CO<sub>2</sub> and CH<sub>4</sub> collected <i>in situ</i> from the near surface to deep layers of an ombrotrophic raised peat bog. Results showed that the age of both the CO<sub>2</sub> and CH<sub>4</sub> components of the dissolved gases increased with depth from ~ 0–300 BP at 0.25 m to ~ 4000 BP at 4 m. CH4 was mainly similar or slightly older in age compared to CO<sub>2</sub>, with the greatest difference in ages occurring at 1 m depth where CH<sub>4</sub> was older by 430-615 years. The δ<sup>13</sup>C values of CO<sub>2</sub> increased with depth from -12.4 ‰ and -8.0 ‰ at 0.25 m to +6.9 ‰ and +8.3 ‰ at 4 m, whereas the δ<sup>13</sup>C of CH<sub>4</sub> stayed in the range -58.4 ‰ to -70.6 ‰. The 14<sup>C</sup> results from the deepest layers are consistent with a similar source for both gases. <sup>14</sup>C ages for the CO<sub>2</sub> component were younger compared to CH<sub>4</sub>, within the shallower depths of the peat bog (≤ 1 m) and demonstrate the incorporation of acrotelm-derived respired CO<sub>2</sub> into the catotelm