Isotope (14C and 13C) analysis of deep peat CO2 using a passive sampling technique

Abstract

10 11We developed and tested a new method to collect CO 2 from the surface to deep layers of a 12 peatland for radiocarbon analysis. The method comprises two components: i) a probe equipped 13 with a hydrophobic filter that allows entry of peat gases by diffusion, whilst simultaneously 14 excluding water, and, ii) a cartridge containing zeolite molecular sieve that traps CO 2 passively. 15We field tested the method by sampling at depths of between 0.25 and 4 m at duplicate sites 16 within a temperate raised peat bog. CO 2 was trapped at a depth-dependent rate of between ~ 0.2-17 0.8 ml d -1 , enabling sufficient CO 2 for routine 14 C analysis to be collected when left in place for 18 several weeks. The age of peatland CO 2 increased with depth from modern to ~ 170 BP for 19 samples collected from 0.25 m, to ~ 4000 BP at 4 m. The CO 2 was younger, but followed a 20 similar trend to the age profile of bulk peat previously reported for the site (Langdon and Barber, 21 2005). δ 13 C values of recovered CO 2 increased with depth. CO 2 collected from the deepest 22 sampling probes was considerably 13 C-enriched (up to ~ +9 ‰) and agreed well with results 23 reported for other peatlands where this phenomenon has been attributed to fermentation 24 processes. CO 2 collected from plant-free static chambers at the surface of the mire was slightly 25 * Manuscript Click here to view linked References -2 -14 C-enriched compared to the contemporary atmosphere, suggesting that surface CO 2 emissions 26 were predominantly derived from carbon fixed during the post-bomb era. However, consistent 27 trends of enriched 13 C and depleted 14 C in chamber CO 2 between autumn and winter samples 28 were most likely explained by an increased contribution of deep peat CO 2 to the surface efflux in 29 winter. The passive sampling technique is readily portable, easy to install and operate, causes 30 minimal site disturbance, and can be reliably used to collect peatland CO 2 from a wide range of 31 depths. 32 33

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

“…This was particularly evident with the 2 deepest sets (2 and 4 m) where results for both CO 2 and CH 4 components were within measurement uncertainty (<2 ) of each other. 14 C results for these 2 depths also agreed very closely with dissolved CO 2 results reported previously from Langlands Moss using the passive sampling method (Garnett and Hardie 2009 A were 2524 ± 46 and 4019 ± 64 BP, both within 1  of the results for CO 2 collected at the same depths, only ~3 m away, using the passive sampling method (2571 ± 48 and 4006 ± 63 BP, respectively; Garnett and Hardie 2009). Therefore, the similarities in 14 C results between Sites A and B in this study (both CO 2 and CH 4 fractions), and the excellent agreement of the 14 C results for CO 2 between this study and the earlier study, provide strong support that the new sampling and processing methods have performed reliably.…”

“…The MSC used has previously been described and tested several times before for collection of CO 2 from various atmospheres (e.g. Hardie et al 2005;Garnett and Hardie 2009;Garnett and Hartley 2010). Very briefly, it consists of a quartz glass tube containing ~3-4 g of Type 13X zeolite molecular sieve (1/ 16 pellets, BDH, UK) held within a central chamber using quartz wool.…”

“…We collected samples of dissolved gas from a raised peat bog using evacuated glass flasks (volumẽ 215 mL) connected to stainless steel sampling probes previously described and tested by Garnett and Hardie (2009). Briefly, the probes were composed of lengths of steel tubing (6 mm OD) carefully pushed into the surface of the peat to the depth under investigation.…”

“…The sampling probes were inserted to 4 depths (0.25, 1, 2, and 4 m) at 2 locations (Sites A and B) situated ~20 m apart on 13 November 2009. These sampling locations were approximately 3 m away from sites previously used by Garnett and Hardie (2009) to collect dissolved CO 2 . After insertion into the peat profile, each probe was briefly evacuated using a battery-powered air pump (MiDan Co., Chino, California, USA) in order to reduce the small amount of atmospheric CO 2 that would have been present.…”

Technical note: In situ measurement of flux and isotopic composition of CO&lt;sub&gt;2&lt;/sub&gt; released during oxidative weathering of sedimentary rocks

“…CO 2 evading the rock accumulates in the chamber and can be sampled using a zeolite molecular sieve trap Garnett and Hardie, 2009;Hardie et al, 2005 Figure 2. An example of the monitoring of the CO 2 accumulating in a chamber.…”