More old carbon is draining away from water systems in the Arctic than previously thought, according to scientists in the UK and the Netherlands.
The researchers – who measured for the first time the radiocarbon content of aquatic carbon dioxide and dissolved organic carbon in headwaters of the western Canadian Arctic – believe that carbon originating from before 1750 comprises up to 37% of total fluxes.
The results hint at the existence of a permafrost carbon feedback, in which old carbon released into the atmosphere causes the melting of more permafrost and so the release of more old carbon, although the researchers say this would need to be confirmed in future studies.
“The presence of old carbon does not necessarily mean a permafrost feedback is occurring, because old carbon can [still] be released by natural seasonal thaw processes in permafrost systems,” said Joshua Dean of Vrije Universiteit Amsterdam.
Permafrost in the Arctic contains some 1500 billion tonnes of organic carbon – equivalent to roughly half the carbon emitted from fossil fuels since the beginning of the industrial revolution. Some of the permafrost carbon is 50,000 years old, or perhaps even older.
Scientists would like to know how much of this old carbon is draining away from permafrost and finding its way into the atmosphere, having been transformed by microbes into greenhouse gases such as carbon dioxide and methane. To know how old the carbon is, researchers can identify levels of radiocarbon – carbon-14 – which is present in diminishing amounts as samples age.
Over the summer of 2014, Dean and his co-workers collected samples of dissolved organic carbon, carbon dioxide and methane from remote headwaters in the western Canadian Arctic, including streams, ponds and lakes.
“Access to the site was only by helicopter,” said Dean, “so we needed to develop methods to collect our samples in a way that was possible in very difficult conditions.” To analyse the samples, the team had to separate the genuine radiocarbon signal from that imprinted by post-1950s nuclear bomb testing.
“The bomb spike complicates [our] signal because it is no longer a smooth linear trend in carbon-14… This spike is so strong that it can hide older carbon,” Dean added. “Here we use a modelling technique to try and tease these different signals apart, and it appears to work quite well.”
As the thawed layer deepened over the summer, the researchers found, the age of dissolved organic carbon rose by 120-125%, while the age of carbon dioxide rose by 59-63%. What’s more, carbon originating from before 1750 ultimately comprised between 15 and 37% of all carbon fluxes.
“This is the first time this signal has been observed directly in carbon dioxide in Arctic aquatic systems, and mirrors previous observations that considered only soil systems,” said Dean. “Aquatic systems give a more integrated signal of the whole area they drain, meaning they can provide a snapshot of a larger area than soil studies.”
To estimate the level of any permafrost feedback, Dean and co-workers must work out how old they expect the carbon exported by natural seasonal permafrost thaw to be. If the actual age is larger than expected, there may indeed be a permafrost carbon feedback.
The team published the study in Environmental Research Letters (ERL).