Ancient dissolved methane in inland waters revealed by a new collection method at low field concentrations for...

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.