Ancient dissolved methane in inland waters revealed by a new collection method at low field concentrations for radiocarbon (14C) analysis.

Methane (CH4) 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 (14C) analysis can indicate both the age and source of CH4 in natural environments. In contrast to CH4 present in bubbles released from aquatic sediments (ebullition), dissolved CH4 in lakes and streams can be present in low concentrations compared to carbon dioxide (CO2), and therefore obtaining sufficient aquatic CH4 for radiocarbon (14C) analysis remains a major technical challenge. Previous studies have shown that freshwater CH4, 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 CH4 for 14C analysis in freshwater environments by circulating water across a hydrophobic, gas-permeable membrane and collecting the CH4 in a large headspace volume. The results of laboratory and field tests show that reliable dissolved δ13CH4 and 14CH4 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 CH4 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 14CH4 signatures from point sources into waterways, e.g. coal seam gas and landfill gas.