Radiocarbon dating of methane and carbon dioxide evaded from a temperate peatland stream

Streams draining peatlands export large quantities of carbon in different chemical forms and are an important part of the carbon cycle. Radiocarbon (¹⁴C) analysis/dating provides unique information on the source and rate that carbon is cycled through ecosystems, as has recently been demonstrated at the air–water interface through analysis of carbon dioxide (CO₂) lost from peatland streams by evasion (degassing). Peatland streams also have the potential to release large amounts of methane (CH₄) and, though ¹⁴C analysis of CH₄ emitted by ebullition (bubbling) has been previously reported, diffusive emissions have not. We describe methods that enable the ¹⁴C analysis of CH₄ evaded from peatland streams. Using these methods, we investigated the ¹⁴C age and stable carbon isotope composition of both CH₄ and CO₂ evaded from a small peatland stream draining a temperate raised mire. Methane was aged between 1617 and 1987 years BP, and was much older than CO₂ which had an age range of 303–521 years BP. Isotope mass balance modelling of the results indicated that the CO₂ and CH₄ evaded from the stream were derived from different source areas, with most evaded CO₂ originating from younger layers located nearer the peat surface compared to CH₄. The study demonstrates the insight that can be gained into peatland carbon cycling from a methodological development which enables dual isotope (¹⁴C and ¹³C) analysis of both CH₄ and CO₂ collected at the same time and in the same way.