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

摘要

Freshwater and marine environments constitute the largest global reservoirs of the greenhouse gas methane (CH4) and natural abundance radiocarbon measurements (C-14-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 C-14-accelerator mass spectrometry (AMS) analysis (similar to 200 mu g carbon (C)) was previously unfeasible when sampling from low CH4 concentration waters, such as much of the surface ocean (similar to 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 C-14-AMS analysis is carried out in 3 h on a flow-through vacuum line that simultaneously prepares aliquots for stable isotope analyses (delta C-13-CH4 and delta H-2-CH4). The total process blank of the method is small (5.0 mu g CH4-C) and composes 1.2% of the average collected and prepared sample (424 +/- 163 mu g, from a recent campaign; n=16). The C-14-CH4 blanks prepared on the vacuum line have acceptably low C-14 content (0.23 +/- 0.07 percent Modern Carbon (pMC); n=7) relative to the C-14-dead (0 pMC) CH4 from which they are prepared.