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

Abstract

Abstract Freshwater and marine environments constitute the largest global reservoirs of the greenhouse gas methane (CH 4 ) and natural abundance radiocarbon measurements ( 14 C‐CH 4 ) can allow for high confidence interpretations about CH 4 dynamics operating in these environments. Collecting sufficient amounts of CH 4 sample for a standard, high precision 14 C‐accelerator mass spectrometry (AMS) analysis (∼ 200 μg carbon (C)) was previously unfeasible when sampling from low CH 4 concentration waters, such as much of the surface ocean (∼ 2 nM), which would require collecting the CH 4 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 CH 4 ‐derived carbon dioxide aliquot for 14 C‐AMS analysis is carried out in 3 h on a flow‐through vacuum line that simultaneously prepares aliquots for stable isotope analyses (δ 13 C‐CH 4 and δ 2 H‐CH 4 ). The total process blank of the method is small (5.0 μg CH 4 ‐C) and composes 1.2% of the average collected and prepared sample (424 ± 163 μ g, from a recent campaign; n = 16). The 14 C‐CH 4 blanks prepared on the vacuum line have acceptably low 14 C content (0.23 ± 0.07 percent Modern Carbon (pMC); n = 7) relative to the 14 C‐dead (0 pMC) CH 4 from which they are prepared.