The mighty Mackenzie River, the largest northward-flowing river in North America and one of the main sources of freshwater flowing into the Arctic Ocean, is carrying huge loads of ancient carbon from thawing Canadian permafrost and dumping it into the sea.
Most of that newly freed carbon carried by the river -- about 2.2 million metric tons per year -- is being sequestered at sea, says a new study published in the journal Nature.
Having the carbon buried in marine sediments is much better than the alternative -- its return to the atmosphere in the form of carbon dioxide, said study co-author Valier Galy, an associate scientist with the Woods Hole Oceanographic Institute.
"If you bury the biosphere-based carbon, you have an actual carbon sink -- you're taking carbon that could be out in the atmosphere and locking it away," Galy said in a statement released by Woods Hole.
It is an example of how nature can buffer itself against the impacts of climate change, another co-author said.
"This is a natural process of carbon dioxide draw down, helping to counteract a warming climate," Robert Hilton, the study's lead author and scientist at Durham University in Britain, said in an email.
But the process is no panacea to carbon-induced warming, Hilton and Galy said. It does not come close to offsetting the production of carbon from human activities like fossil-fuel burning, or even the release of other carbon from thawing permafrost sources in the far North, they said.
"Unfortunately, we find that this natural carbon draw down process is too slow to keep up with what we think could be released from permafrost as it thaws over the next 100 years," Hilton said. "It's probably at least 20 times too slow -- it is clear it will not be able to save the day. This is because erosion and transfer by rivers only happens in parts of the landscape. In other parts of the landscape, a lot of frozen soils which thaw will remain exposed and the carbon can be released to the atmosphere."
Though a big amount, 2.2 million metric tons per year is dwarfed by the carbon produced by fossil-fuel burning, a load that is in the billions of tons, he said.
To track the source and fate of the river-swept permafrost carbon, the researchers took samples over three years, working at different locations and depths along the river. After that, they analyzed the samples, using a type of carbon isotope that decays over time to determine age and whether the carbon came from the biosphere -- from plants and other things that grow -- or from rock.
Rock-derived carbon is stable, but the younger biosphere-derived carbon can more easily react with its environments and send carbon dioxide into the environment. So having that biosphere-derived carbon sequestered in the Beaufort Sea prevents such releases, the authors said.
Does the deposition of the carbon-bearing sediments contribute to ocean acidification in the Beaufort, where chemistry is already changing significantly?
No, the authors say.
The "vast majority of the permafrost carbon is actually preserved and buried in sediments," not remineralized in the water, Galy said in an email.
Jessica Cross, an expert in Alaska ocean acidification expert and a scientist not involved in the Nature study, said the authors present evidence that the Mackenzie River's geological record shows that the rate of remineralization is slow compared to the burial rate. In other words, it's being buried in the sediment the Mackenzie is depositing more quickly than it can dissolve in Arctic seawater.
That indicates that "not much of that permafrost carbon actually gets respired, and thereby would not contribute to air-sea flux of CO2 or acidification events," Cross, a research scientist with the National Oceanic and Atmospheric Administration, said in an email. "This could mean that permafrost carbon may not be as big a player in acidification events as it could be."
The efficiency of that burial process is a subject for further study, as is the question of impacts to ocean chemistry, Cross said.
The Mackenzie is the second-largest river in North America, draining one-fifth of Canada's land mass. It pours significant amounts of freshwater into the Arctic Ocean -- enough to warm vast sections and melt large amounts of sea ice -- and is also a dominant source of sediment flowing into the Beaufort Sea.
Climate warming in the territory drained by the river has dramatically increased its discharges of freshwater and sediments into the Beaufort Sea, according to a recent study published in the journal Biogeosciences.
Since 2003, the discharge of freshwater has increased by 25 percent and the discharge of "terrestrial particles" -- suspended solids and organic carbon -- has increased by 50 percent, said the study, by scientists from the Villefranche Oceanographic Laboratory in France and the Takuvik Joint International Laboratory of Quebec's Laval University.
Timing of the freshwater flow has also changed, advancing in the spring by several days over the past four decades as snow melts earlier, according to a study published last year in the journal Quaternary International.