Sunlight May be Key to Carbon Dioxide Release from Arctic Permafrost

The permafrost in the Arctic locks away a vast reservoir of carbon. Yet this carbon is gradually converting to carbon dioxide after entering the freshwater system in a process thought to be largely controlled by microbial activity. Now, scientists have discovered that it's not bacteria triggering this process--but sunlight.

"Arctic permafrost contains about half of all the organic carbon trapped in soil on the entire Earth-and equals an amount twice of that in the atmosphere," said Byron Crump, co-author of the new study, in a news release. "This represents a major change in thinking about how the carbon cycle works in the Arctic."

In order to understand what might be triggering the release of carbon in permafrost, the researchers measured the speed at which both bacteria and sunlight converted dissolved organic carbon into carbon dioxide in all types of rivers and lakes in the Alaskan Arctic. They found that in nearly all of the freshwater systems that they measured, sunlight was almost always faster than bacteria at coverting the organic carbon into CO2.

"This is because most of the fresh water in the Arctic is shallow, meaning sunlight can reach the bottom of any river-and most lakes-so that no dissolved organic carbon is kept in the dark," said Crump. "Also, there is little shading of rivers and lakes in the Arctic because there are no trees."

This isn't the only factor that makes sunlight more efficient than bacteria. Cold, nutrient-rich waters actually cause the bacteria to grow more slowly, which limits their contribution.

The findings have implications for future carbon release. While the level of permafrost thawing only gets to be a foot deep or so, even in the summer, climate change could alter that. If the thaw begins earlier due to warmer temperatures and exposes the organic carbon from permafrost to more sunlight, it could potentially trigger the release of more CO2.

The findings are published in the journal Science.