title: Ancient dissolved methane in inland waters revealed by a new collection method at low field concentrations for radiocarbon (14C) analysis.
abstract: 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 CH4in natural environments. In contrast to CH4present in bubbles released from aquatic sediments (ebullition), dissolved CH4in lakes and streams can be present in low concentrations compared to carbon dioxide (CO2), and therefore obtaining sufficient aquatic CH4for 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 CH4for14C analysis in freshwater environments by circulating water across a hydrophobic, gas-permeable membrane and collecting the CH4in a large headspace volume. The results of laboratory and field tests show that reliable dissolved δ13CH4and14CH4samples 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 CH4may 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 unique14CH4signatures from point sources into waterways, e.g. coal seam gas and landfill gas.
authors.affiliation: NERC Radiocarbon Facility, East Kilbride, G75 0QF, UK; Department of Earth Sciences, Vrije Universiteit Amsterdam, 1081 HV, Amsterdam, The Netherlands. Electronic address: j.f.dean@vu.nl.
authors.forename: Joshua F
authors.initials: JF
authors.lastname: Dean
authors.affiliation: Biological and Environment Sciences, University of Stirling, Stirling, FK9 4LA, UK.
authors.forename: Michael F
authors.initials: MF
authors.lastname: Billett
authors.affiliation: NERC Radiocarbon Facility, East Kilbride, G75 0QF, UK.
authors.forename: Callum
authors.initials: C
authors.lastname: Murray
authors.affiliation: NERC Radiocarbon Facility, East Kilbride, G75 0QF, UK.
authors.forename: Mark H
authors.initials: MH
authors.lastname: Garnett
mesh_terms: D001272:Atmosphere; D057486:Carbon Cycle; D002245:Carbon Dioxide; D060106:Lakes; D008697:Methane
chemical_list: D002245:Carbon Dioxide; D008697:Methane
keywords: Global carbon cycle; Inland waters; Methane (CH(4)); Method development; Peatlands; Radiocarbon ((14)C)