A rapid method to collect methane from peatland streams for radiocarbon analysis

Abstract

Peatland streams typically contain high methane concentrations and act as conduits for the release of this greenhouse gas to the atmosphere. Radiocarbon analysis provides a unique tracer that can be used to identify the methane source, and quantify the time elapsed between carbon fixation and return to the atmosphere as CH 4 . Few studies -those that have focus largely on sites with bubble (ebullition) emissions -have investigated the 14 C age of methane in surface waters because of the difficulty in collecting sufficient CH 4 for analysis. Here, we describe new sampling methods for the collection of CH 4 samples from CH 4 -oversaturated peatland streams for radiocarbon analysis. We report the results of a suite of tests, including using methane 14 C standards and replicated field measurements, to verify the methods. The methods are not restricted to ebullition sites, and can be applied to peatland streams with lower methane concentrations. We report the 14 C age of methane extracted from surface water samples (~4-13 l) at two contrasting locations in a temperate raised peat bog. Results indicate substantial spatial variation with ages ranging from~400 (ditch in afforested peatland) to~3000 years BP (bog perimeter stream). These contrasting ages suggest that methane in stream water can be derived from a wide range of peat depths. This new method provides a rapid (10-15 min per sample) and convenient approach, which should make 14 CH 4 dating of surface water more accessible and lead to an increased understanding of carbon cycling within the soil-water-atmosphere system.

The Potential Hidden Age of Dissolved Organic Carbon Exported by Peatland Streams

“…Direct in situ measurements of 14 CO 2 and 14 CH 4 signatures are needed to improve our understanding of the contribution of old C to inland water C emissions across a range of environmental settings. Methodological advances now allow for the collection of 14 CO 2 and 14 CH 4 samples relatively quickly and easily in even remote field locations (Billett et al, ; Dean et al, ; Garnett, Billett, et al, ; Garnett, Gulliver, et al, ). For aquatic CO 2 and CH 4 14 C samples, age distribution analysis (e.g., Dean, van der Velde, et al, ) or multisource isotopic mass balance (e.g., Elder et al, ) can help determine relative contributions of different aged C.…”

A new field approach for the collection of samples for aquatic 14CO2 analysis using headspace equilibration and molecular sieve traps: the super headspace method

“…Despite these field results suggesting that the samples were fully equilibrated within 2 min, we chose to standardize our sampling method with a shaking time of 3 min to allow for manual differences in shaking technique between operators. This was also consistent with our headspace equilibration method for the collection of methane samples for 14 C analysis (Garnett et al ., ), thus permitting the collection of samples of both CO 2 and CH 4 for 14 C analysis using the same equilibrated water sample.…”

“…Errors could arise because of either (1) a small contribution of geological carbonate contributing to gas‐stripped samples, and not the headspace‐equilibrated samples [for practical reasons, the water samples were not filtered, but a small particle of as little as ~2 mg 14 C‐dead CaCO 3 in the 485‐ml water sample would be sufficient to account for the discrepancy between headspace‐equilibrated and gas‐stripped results; although the stream flows from a peatland, the underlying geological formation (Clackmannan Group) does contain limestone; British Geological Society; http://mapapps.bgs.ac.uk/geologyofbritain/home.html]; (2) disequilibrium between aquatic CO 2 and DIC due to unfavourable mixing in the stream (suggested by Billett and Garnett, , who also reported older ages for gas‐stripped DIC samples compared with CO 2 collected using a floating chamber); and (3) a sample preservation issue (the DIC sample was stored for ~4 months in a refrigerator without any other means of sample preservation, prior to processing, and therefore, transfer of carbon between different inorganic pools within the sample may have occurred. Although we did not measure methane during this study, it has previously been found to be abundant and much older than CO 2 at this site (up to 3291 years BP; Garnett et al ., ), and so methane oxidation during sample storage could explain a significant proportion of the offset between headspace‐equilibrated and gas‐stripped samples). In this respect, it should be noted that by avoiding the need for the acidification of water samples, and instead relying on rapid equilibration of samples by shaking and the isolation of aquatic CO 2 from the water sample in the field and stable storage on molecular sieve, the new method completely avoids all of the confounding issues associated with the gas‐stripping method.…”

“…In our method, the established headspace equilibration technique (Kling et al, 1991;Hope et al, 1995;Jahangir et al, 2012) is scaled up to provide the necessary larger volume samples using a sampling method originally devised for the collection of methane from water for 14 C analysis (Garnett et al, 2016). Headspace gases are injected using a large syringe into the same molecular sieve traps that have previously been proven to be very reliable for the collection of 14 C samples of aquatic CO 2 either collected passively or from a floating chamber (Billett et al, 2006).…”

“…All field tests were performed at a small (~40-cmwide × 30-cm-deep) peatland stream adjacent to Langlands Moss, a domed ombrotrophic peat bog in central south-west Scotland, UK (55°44′5·5″N, 04°10′ 25·8″W). The site has been used previously for aquatic 14 C measurements of CO 2 and CH 4 (Garnett et al, 2016). Peat depth in the centre of Langlands Moss is at least 8 m (Langdon and Barber, 2005).…”