Ancient dissolved methane in inland waters revealed by a new collection method at low field concentrations for radiocarbon (C) analysis.

Methane (CH) is a powerful greenhouse gas that plays a prominent role in the terrestrial carbon (C) cycle, and is released to the atmosphere from freshwater systems in numerous biomes globally. Radiocarbon (C) analysis can indicate both the age and source of CH in natural environments. In contrast to CH present in bubbles released from aquatic sediments (ebullition), dissolved CH in lakes and streams can be present in low concentrations compared to carbon dioxide (CO), and therefore obtaining sufficient aquatic CH for radiocarbon (C) analysis remains a major technical challenge. Previous studies have shown that freshwater CH, in both dissolved and ebullitive form, can be significantly older than other forms of aquatic C, and it is therefore important to characterise this part of the terrestrial C balance. This study presents a novel method to capture sufficient amounts of dissolved CH for C analysis in freshwater environments by circulating water across a hydrophobic, gas-permeable membrane and collecting the CH in a large headspace volume. The results of laboratory and field tests show that reliable dissolved δCH and CH samples can be readily collected over short time periods (∼4-24 h), at relatively low cost and from a variety of surface water types. The initial results further support previous findings that dissolved CH may be significantly older than other forms of aquatic C, and is currently unaccounted for in many terrestrial C balances and models. This method is suitable for use in remote locations, and could potentially be used to detect the leakage of unique CH signatures from point sources into waterways, e.g. coal seam gas and landfill gas.