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As temperatures rise, some of the carbon dioxide stored in Arctic permafrost meets an unexpected fate—burial at sea. As many as 2.2 million metric tons of carbon dioxide (CO₂) per year are swept along by a single river system into Arctic Ocean sediment, according to a new study led by Woods Hole Oceanographic Institution (WHOI) researchers and published today in Nature. This process locks away the greenhouse gas and helps stabilize the earth’s CO₂ levels over time, and it may help scientists better predict how natural carbon cycles will interplay with the surge of CO₂ emissions due to human activities.

“The erosion of permafrost carbon is very significant,” says WHOI Associate Scientist Valier Galy, a co-author of the study. “Over thousands of years, this process is sequestering CO₂ away from the atmosphere in a way that amounts to fairly large carbon stocks. If we can understand how this process works, we can predict how it will respond as the climate changes.”

Permafrost—the permanently frozen ground found in the Arctic and Antarctic and in some alpine regions—is known to hold billions of tons of organic material, including vast stores of CO₂. Amid concerns about rising Arctic temperatures and their impact on permafrost, many researchers have directed their efforts to studying the permafrost carbon cycle—the processes through which the carbon circulates between the atmosphere, the soil and surface (the biosphere), and the sea. Yet how this cycle works and how it responds to the warming, changing climate remains poorly understood.

Galy and his colleagues from Durham University, the Institut de Physique du Globe de Paris, the NERC Radiocarbon Facility, Stockholm University, and the Universite Paris-Sud set out to characterize the carbon cycle in one particular piece of the Arctic landscape—northern Canada’s Mackenzie River, the largest river flowing into the Arctic Ocean from North America and that ocean’s greatest source of sediment. The researchers hypothesized that the Mackenzie’s muddy water might erode thawing permafrost along its path and wash that biosphere-derived material and the CO₂ within it into the ocean, preventing the release of that CO₂ into the atmosphere.

In a new study published April 27 in the Proceedings of the National Academy of Sciences, scientists at Woods Hole Oceanographic Institution (WHOI) and their colleague from Rutgers University discovered a surprising new short-circuit to the biological pump. They found that sinking particles of stressed and dying phytoplankton release chemicals that have a jolting, steroid-like effect on marine bacteria feeding on the particles. The chemicals juice up the bacteria’s metabolism causing them to more rapidly convert organic carbon in the particles back into CO₂ before they can sink to the deep ocean.