Efficient collection and preparation of methane from low concentration waters for natural abundance radiocarbon analysis

Abstract

Freshwater and marine environments constitute the largest global reservoirs of the greenhouse gas methane (CH4) and natural abundance radiocarbon measurements (14C‐CH4) can allow for high confidence interpretations about CH4 dynamics operating in these environments. Collecting sufficient amounts of CH4 sample for a standard, high precision 14C‐accelerator mass spectrometry (AMS) analysis (∼ 200 μg carbon (C)) was previously unfeasible when sampling from low CH4 concentration waters, such as much of the surface ocean (∼ 2 nM), which would require collecting the CH4 from 8500 L of seawater. The method described here involves pumping 20,000–40,000 L of seawater up from depth through a dissolved gas extraction system, which enables the collection of a sample composed of 100s of L of gas in less than 4 h on station. The large volume extracted gas sample is compressed into a 1.7 L cylinder for transport from the ship to the home laboratory. The home laboratory preparation of each sample to a CH4‐derived carbon dioxide aliquot for 14C‐AMS analysis is carried out in 3 h on a flow‐through vacuum line that simultaneously prepares aliquots for stable isotope analyses (δ13C‐CH4 and δ2H‐CH4). The total process blank of the method is small (5.0 μg CH4‐C) and composes 1.2% of the average collected and prepared sample (424 ± 163 μg, from a recent campaign; n = 16). The 14C‐CH4 blanks prepared on the vacuum line have acceptably low 14C content (0.23 ± 0.07 percent Modern Carbon (pMC); n = 7) relative to the 14C‐dead (0 pMC) CH4 from which they are prepared.

Limited contribution of ancient methane to surface waters of the U.S. Beaufort Sea shelf

“…We collected dissolved 14 C-CH 4 samples to test the hypotheses that (i) ancient C sources contribute CH 4 to Arctic Ocean continental shelf waters and (ii) the contribution of ancient C sources to surface water and atmospheric CH 4 in this environment diminishes as proximity to these sources decreases (that is, as water depth and distance from shore increase). Without newly developed techniques (Materials and Methods) ( 20 ), testing these hypotheses would not have been possible due to the challenge of collecting sufficient quantities of CH 4 for natural abundance 14 C-CH 4 analysis in surface waters ( 1 , 3 , 4 , 12 , 13 , 15 , 16 ).…”

Radiocarbon Analysis of Methane at the NERC Radiocarbon Facility (East Kilbride)

“…Similar 14 C concentrations have been reported by previous researchers for 14 C-dead CH4 samples. For example, Sparrow and Kessler (2017) reported a 14 C blank of 0.23±0.07 %modern (0.24 μg C) for their CH4 preparation system, and Elder et al (2018) quantified modern 14 C blank in their CH4 processing apparatus to be ~1.5-2.5 μg C (which compares to 0.5-3.8 μg C for our samples). Palonen et al (2017) measured a 14 C content of 0.06±0.07 %modern for natural gas (after applying a background correction of 0.4±0.1 %modern).…”

Comment on “The origin of methane in the East Siberian Arctic Shelf unraveled with triple isotope analysis” by Sapart et al. (2017)

“…In a recently published study, we demonstrate how useful natural abundance 14 C-CH 4 measurements can be towards understanding the role of ancient sources of methane in arctic shelf seas (Sparrow et al, 2018). Importantly, in this study, we find that the stable isotope (δ 13 C-CH 4 ) and dissolved CH 4 concentration data, together, would suggest an entirely different (and, we argue, incorrect) interpretation of this system, which attests to the importance of 14 C-CH 4 measurements for investigations into the origins of methane.…”

“…In the Supplement, the authors write that "none of the reference and blank measurements were abnormal," without presenting any descriptions of or data stemming from these tests. Refereed techniques for collecting and preparing 14 C-CH 4 samples from natural waters (Dean et al, 2017;Elder et al, 2018;Kessler and Reeburgh, 2005;Pack et al, 2015;Pohlman et al, 2000;Sparrow and Kessler, 2017) include detailed qualitative and quantitative descriptions of the measures taken to validate their methodologies. These measures include processing blank (methane-free) waters and treating methane-free gas and methane of known 14 C-CH 4 content in the same way as samples.…”