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Thawing permafrost a significant source of carbon

June 16, 2006

FAIRBANKS, Alaska — Permafrost, permanently frozen soil, isn't staying frozen and a type of soil called loess contained deep within thawing permafrost may be releasing significant, and previously unaccounted for, amounts of carbon into the atmosphere, according to authors of a paper published this week in the journal Science.

Preliminary assessments by scientists from Russia, the University of Florida, and the University of Alaska Fairbanks indicate that loess permafrost, which covers more than a million square kilometers in Siberia and Alaska, is a large carbon reservoir with the potential to be a significant contributor of atmospheric carbon, yet it is seldom incorporated into analyses of changes in global carbon reservoirs.

"The unique aspect of the Siberian loess permafrost is that it is quite deep — 20 to 40 meters—and has a surprisingly high carbon concentration at depth for a mineral soil," said Terry Chapin, co-author from the Institute of Arctic Biology at UAF. "This paper explains the processes that led to the accumulation of large amounts of soil carbon and the processes that could lead to its return to the atmosphere."

The largest carbon reservoir on Earth is the ocean, which scientists estimate holds about 40,000 gigatons; soils contain about 2,500 Gt and vegetation about 650 Gt. According to the authors, about 500 Gt of carbon are contained in the thaw-threatened loess, also called yedoma, of Siberia and Alaska.

"I was surprised, because it is unusual to find major new large carbon stocks," Chapin said. "We have spent more than five years discussing among ourselves all the details of the calculations, because initially I did not believe that the pool could be both so large and so decomposible (once thawed)."

Permafrost has been seldom incorporated into global carbon budgets in part because the "... size of the carbon pool was so poorly quantified ... and in part because global data bases for soils have been standardized to provide data only for the top meter of soil," Chapin said. "People know about carbon in permafrost—it's not a trivial amount," said Ted Schuur, co-author from the University of Florida. "Normally, scientists look for carbon in the upper layers of permafrost where organic matter decomposes."

Laboratory and field experiments by the scientists demonstrate that the organic matter in yedoma decomposes quickly when it is thawed and produces rates of carbon release similar to those of productive northern grassland soils. "If these rates continue as field observations suggest, most carbon in recently thawed yedoma will be released within a century—a striking contrast to the preservation of carbon for tens of thousands of years when frozen in permafrost," state the authors.

The National Science Foundation provided financial support for this research.

Institute of Arctic Biology