Permafrost slumps spew carbon into streams, study shows

Streams affected by permafrost slumps carry substantially more carbon than typically found in other streams flowing through permafrost landscapes, according to a new study.

The research highlights freshwater’s growing role in the carbon balance of permafrost landscapes, like much of the Northwest Territories, as the world warms.

The work may help researchers refine their estimates of how much carbon is absorbed versus released in northern regions.

Carbon continuously moves through the various places it is stored, such as rocks, plants, the ocean and the atmosphere. The movement of carbon between these reservoirs is known as the carbon cycle, and it plays an important role in keeping carbon levels in different reservoirs balanced.

Human activities that add carbon to the atmosphere upset this balance, leading to climate change. Rising temperatures have knock-on effects of their own on the carbon cycle, too.

The world’s permafrost contains about 1,600 billion tonnes of carbon, roughly twice the amount in the atmosphere, according to Nature. As the North swiftly warms and permafrost thaws, scientists worry that this huge store of carbon will be released as greenhouse gases.

Studies that examine climate change’s effects on carbon in permafrost regions tend to focus on either land or water exclusively, said Scott Zolkos, a postdoctoral fellow at Harvard University and Woodwell Climate Research Center. Zolkos conducted the recent work during his graduate studies at the University of Alberta.

But Zolkos says researchers should consider land and water together to better understand how thawing permafrost is altering the northern carbon balance.

In the new work, Zolkos and colleagues took samples from 33 streams along the Dempster Highway, which stretches between the Yukon and the Northwest Territories. They then compared the amount of carbon carried by freshwater to a modelled estimate of the carbon exchanged on land. The work was described in August in the journal Global Biogeochemical Cycles.

In most of the streams sampled, carbon carried by freshwater was equivalent to six to 16 percent of terrestrial carbon uptake on average. That’s roughly on par with other northern regions, the researchers reported.

However, in watersheds affected by permafrost slumps – caused when thawing permafrost allows rock and soil to move downslope under the influence of gravity – carbon carried by water accounted for nearly 60 percent of terrestrial carbon uptake.

The amount of carbon moving through freshwater matters, Zolkos said, because as permafrost thaws, the link between land and water will be important for understanding future carbon cycling and potential impacts on climate.

Permafrost comes in many forms

The big question is what happens to that carbon carried by freshwater, said Zolkos. How much settles in streams, and how much is consumed by microbes and released into the atmosphere?

The answer may depend where you look.

Permafrost regions are not all the same. Scientists suspect a variety of factors in the landscape, such as glacial history, topography, geology and hydrology, alter carbon cycling.

To get a better handle on how these differences affect carbon carried by freshwater, the team compared stream measurements from four distinct permafrost ecosystems: the Peel Plateau, Mackenzie Lowlands and Travaillant Uplands in the NWT’s Beaufort Delta region, and the Yukon’s Richardson Mountains.

In low-lying areas like the Mackenzie Lowlands, carbon dioxide and methane are the dominant forms of carbon in streams. In the Peel Plateau, which is a hotspot for permafrost thaw and slumping, the researchers found streams carried large amounts of particulate carbon instead.

These different types of carbon may result in different levels of emissions, according to Zolkos. Particulate carbon is generally broken down and released more slowly than dissolved or gaseous forms.

“Given the importance of the changes occurring in permafrost regions, papers like this one are important,” said Ryan Hutchins, a postdoctoral fellow in environmental geochemistry at the University of Waterloo, who was not involved in the work. The research might help scientists create better estimates of carbon emissions from permafrost ecosystems, Hutchins said.

Although there are many popular science stories about permafrost thaw and the large amounts of greenhouse gases released, most research to date has been conducted in Alaska and Siberia, said Hutchins. Those regions were previously unglaciated and carbon could accumulate for a long time.

The new work provides a snapshot of carbon cycling in regions that were previously glaciated, like parts of Canada’s North, and which might end up emitting less carbon to the atmosphere.

One limitation of the study, according to Hutchins, is that all of the water samples came from streams that are close to the highway, which may not be representative of the entire region. He noted that accessing remote areas in the North is a common research challenge.

Integrating water measurements with actual, field-based assessments of terrestrial carbon uptake – rather than the estimates used here – is also an important next step, Zolkos said.

Researchers are beginning to recognize the importance of looking at land and water together when studying carbon cycling in permafrost, he said. He sees the new study as a first step in that direction.

This article is produced under a Creative Commons CC BY-ND 4.0 licence through the Wilfrid Laurier University Climate Change Journalism Fellowship.