Scientists have known for a while that warming global temperatures are causing Arctic lakes to release methane, a potent greenhouse gas that leads to even more warming. In a new study published in the journal Nature, a team of researchers including U of M researcher Jacques Finlay, found that Siberian lakes have actually pulled more greenhouse gasses from the atmosphere than they have released into it since the last Ice Age, causing an overall slight cooling effect.
Permafrost, especially that in the Siberian Arctic, contains significant amounts of all organic carbon found on Earth locked away in frozen soils. Warming global temperatures in the 15,000 years since the last Ice Age have begun to thaw the permafrost, leading to the widespread formation of lakes.
"As the lakes form, they thaw sediments that have been frozen for thousands of years," Finlay said. "Previous work by the lead author and others shows that this process accelerates methane production, leading to a large atmospheric flux of this greenhouse gas."
With this continued warming over thousands of years, however, the sediment destabilizes and the lakes fill and subsequently drain. Scientists had never studied the role of thaw, or thermokarst, in lakes in the global carbon cycle past the initial burst of greenhouse gas release.
"[The goal of our study] was to try to have an understanding of the long-term carbon dynamics of these lakes," Finlay said. He and his colleagues' work, published online July 24, shows that deep thermokarst lakes have taken more carbon and greenhouse gasses from the atmosphere than they have released since the last Ice Age.
However, Finlay cautions that the lakes have buried this carbon over a period of thousands of years. He says that scientists currently cannot predict if the "ice box" effect that rapidly freezes plant matter in the permafrost will continue to do so as atmospheric temperatures rise in the short term.
"If permafrost thaws rapidly, there is a large potential to release carbon back to the atmosphere from carbon rich soils and lake sediments," he said.
A key piece of the research was the identification of the source of the carbon collected in soil samples from these thermokarst lakes. One possibility was that all the carbon in lake sediments had been there since before the last Ice Age, indicating that soil carbon was simply moved around by the lake formation processes.
Finlay and colleagues instead found that much of the carbon had been buried there within the past 15,000 years. "The fossil data shows that a lot of the carbon that is there came there because of a burst of productivity that followed lake formation," Finlay said. "There were a lot of happy plants for a little while."
As the lakes drained, the plants and their sequestered carbon essentially were frozen into the permafrost. "The high rates of carbon burial exist because you have this ice box in the landscape that leads to a rapid transition back to a terrestrial environment with high rates of carbon burial," Finlay said. The study suggests that thermokarst lakes eventually compensate for their initial large release of greenhouse gases.
Finlay is an associate professor in the U of M's College of Biological Sciences and a resident fellow in the Institute on the Environment.