Radiocarbon and Stable Carbon Analysis of Dissolved Methane and Carbon Dioxide from the Profile of a Raised Peat Bog

Abstract

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  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  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.

Radiocarbon dating of methane and carbon dioxide evaded from a temperate peatland stream

“…This therefore suggests that an additional, much younger source is 354 contributing to evasion CO 2 . Evidence for an additional source for evasion CO 2 is also supported by the δ 13 C values; in the peatland adjacent to our stream dissolved CO 2 had a δ 13 C 356 value of -2.6 and -0.6 ‰ at 1 m depth (Garnett et al 2011), whereas CO 2 evasion was much more 13 C-depleted (-25 to -24 ‰). Therefore an additional source contributes to evaded CO 2 358 that is relatively young and with a δ 13 C value of less than ~ -25 to -24 ‰.…”

“…For reasons mentioned above (Steinmann et al 2008), CO 2 374 was slightly younger than CH 4 at this depth (Garnett et al 2011), but still considerably older than the evaded CO 2 ; this fraction therefore represents the 'old' component (aged 1150-1454 376 years BP i.e. on average ~ 85 %modern) and based on measurements at our site we also know that it should have a δ 13 C value of ~ -1.6 ‰ (Garnett et al 2011). We can therefore calculate 378 the isotopic composition of the 'young' component using our measured values for evaded CO 2 (averaged 14 C = 95 %modern and δ 13 C = -24 ‰) and isotope mass balance:…”

“…Given that the 370 concentration of CO 2 in the peat at this depth is likely to have been approximately the same (Garnett et al 2011) or even greater (Clymo and Bryant 2008) than CH 4 , then it is reasonable 372 also to assume that a proportion of the evaded CO 2 in our peatland stream may also have been sourced from around this depth. For reasons mentioned above (Steinmann et al 2008), CO 2 374 was slightly younger than CH 4 at this depth (Garnett et al 2011), but still considerably older than the evaded CO 2 ; this fraction therefore represents the 'old' component (aged 1150-1454 376 years BP i.e. on average ~ 85 %modern) and based on measurements at our site we also know that it should have a δ 13 C value of ~ -1.6 ‰ (Garnett et al 2011).…”

“…First, we assume that CH 4 has a 366 single source, the methanogenic decomposition of substrates in the waterlogged peat. The evaded CH 4 is unlikely to originate from a single peat depth, but if we assume that the CH 4 in 368 our peatland stream has a 14 C age that reflects the average depth in the peat where the CH 4 was acquired, this corresponds to a depth of ~ 1 m (Garnett et al 2011). Given that the 370 concentration of CO 2 in the peat at this depth is likely to have been approximately the same (Garnett et al 2011) or even greater (Clymo and Bryant 2008) than CH 4 , then it is reasonable 372 also to assume that a proportion of the evaded CO 2 in our peatland stream may also have been sourced from around this depth.…”

“…The evaded CH 4 is unlikely to originate from a single peat depth, but if we assume that the CH 4 in 368 our peatland stream has a 14 C age that reflects the average depth in the peat where the CH 4 was acquired, this corresponds to a depth of ~ 1 m (Garnett et al 2011). Given that the 370 concentration of CO 2 in the peat at this depth is likely to have been approximately the same (Garnett et al 2011) or even greater (Clymo and Bryant 2008) than CH 4 , then it is reasonable 372 also to assume that a proportion of the evaded CO 2 in our peatland stream may also have been sourced from around this depth. For reasons mentioned above (Steinmann et al 2008), CO 2 374 was slightly younger than CH 4 at this depth (Garnett et al 2011), but still considerably older than the evaded CO 2 ; this fraction therefore represents the 'old' component (aged 1150-1454 376 years BP i.e.…”