As Earth's climate warms, experts predict the rate of greenhouse gas emissions from thawing Arctic permafrost and peat will rise, which will further boost climate warming. Because the rate of permafrost thaw varies widely across the Arctic and data from remote areas is limited, it's been challenging for scientists to monitor actual changes on the ground.
A study published this week in the Proceedings of the National Academy of Sciences by a team of researchers led by Stockholm University, and including James McClelland at the University of Texas Marine Science Institute, demonstrates a novel and practical way to monitor the release of carbon from thawing permafrost.
The team used radiocarbon dating of carbon in four large Siberian-Arctic rivers to pinpoint the patterns in which "old carbon"—preserved in deep permafrost or peat deposits for thousands of years—is released across northern Eurasia.
"Our study quantifies amounts and forms of ancient organic carbon that are currently being transported by some of the largest Arctic rivers," McClelland said, "and provides a benchmark for tracking increasing mobilization of ancient organic carbon as the Arctic continues to warm."
McClelland is a founding member of the Arctic Great Rivers Observatory (ArcticGRO), a research group which provided long-term data central to this study. McClelland also contributed expertise on organic matter sources, transport and processing in major Arctic rivers.
Arctic permafrost and peatlands constitute frozen giants of the global carbon cycle. In the top few meters, the Arctic permafrost stores almost twice as much carbon as in our atmosphere in the form of carbon dioxide (CO2) and more than 200 times as much as atmospheric methane. The large amounts of currently dormant permafrost carbon may be increasingly thawed and mobilized as climate warming progresses – and may then feed additional CO2 and methane into the atmosphere to accelerate climate warming.
"Rivers transport carbon from different sources in their catchments, including carbon mobilized from thawing permafrost or collapsing peat, as well as carbon from the soil surface," says Birgit Wild, researcher at Stockholm University and lead author of the study. "Radiocarbon dating helps us distinguish carbon from these different sources."
Old carbon contains less radiocarbon than modern carbon at the soil surface. The team of researchers calculated the flux of permafrost and peat carbon in the large Siberian rivers Ob, Yenisey, Lena and Kolyma by combining a database of radiocarbon in different deposits with monitoring of radiocarbon in the rivers over ten years and all seasons.
The study shows that permafrost and peat carbon contributed to only 12 percent of the dissolved organic carbon in these rivers, but to more than half of the particulate organic carbon. Seasonal differences suggest that gradual thaw of the seasonally frozen active layer and of surface permafrost is the main source of permafrost- and peat-derived carbon in dissolved form, whereas that in particulate form stems to a large extent from the collapse of deeper deposits that formed thousands of years ago. Differences in the relative amounts of dissolved versus particulate permafrost and peat carbon between rivers were consistent with differences in the type and extent of permafrost in their basins.
"The radiocarbon signal of especially particulate organic carbon in rivers might be a sensitive tool to monitor carbon release from thawing permafrost over the coming decades," adds Örjan Gustafsson, professor at Stockholm University and leader of the team.
This study was funded by the U.S. National Science Foundation, the Swedish Research Council Vetenskapsrådet and the European Research Council.
This post was based in part on a release by Stockholm University.
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