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 2 and CH 4 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 CO 2 and CH 4 of distinctly different isotopic composition. We employed the method to investigate the carbon isotopic composition of samples of dissolved CO 2 and CH 4 collected in situ from the near surface to deep layers of an ombrotrophic raised peat bog. Results showed that the age of both the CO 2 and CH 4 components of the dissolved gases increased with depth from ≃0–300 BP at 0.25 m to ≃4000 BP at 4 m. CH 4 was mainly similar or slightly older in age compared to CO 2 , with the greatest difference in ages occurring at 1 m depth where CH 4 was older by 430–615 yr. The δ 13 C values of CO 2 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 13 C of CH 4 stayed in the range −58.4‰ to −70.6‰. The 14 C results from the deepest layers are consistent with a similar source for both gases. 14 C ages for the CO 2 component were younger compared to CH 4 , within the shallower depths of the peat bog (≤1 m) and demonstrate the incorporation of acrotelm-derived respired CO 2 into the catotelm.