Thawing Permafrost Carbon Mostly Stays Trapped in the Seabed, Study Finds

Arctic permafrost holds a vast reservoir of organic carbon, and as it thaws and coastlines erode, some of that carbon washes into the ocean, where microorganisms can break it down into greenhouse gases. Researchers from the Alfred Wegener Institute (AWI) and MARUM, the Centre for Marine Environmental Sciences at the University of Bremen, studied what happens to that carbon along the permafrost coast of Qikiqtaruk (Herschel Island) in Canada, and found that a substantial share ends up buried in seabed sediments rather than escaping into the air.

Permafrost ecosystems across the Arctic contain roughly 1,300 gigatonnes of organic carbon, mostly from ancient plant remains, with another 400 gigatonnes stored in ocean and river-delta sediments. Up to 0.02 gigatonnes of that carbon enters the sea each year, and that outflow could rise 70% to 150% by 2100. "How much of this is released back into the atmosphere as a greenhouse gas and how much is stored in the seabed has, until now, been largely unknown," said Manuel Ruben, the study's lead author at AWI.

By analyzing sediment cores collected at different distances from the Herschel Island coast, preserving about 50 years of deposits, the researchers found that while large amounts of organic carbon are swept from the coast, surprisingly little of it enters the ocean's active carbon cycle. "Microorganisms convert around ten percent of the organic carbon from the sediments into gases, which rise into the water and can then enter our atmosphere," Ruben said, with most of the remaining carbon staying buried in the seafloor.

Using isotopes to trace the carbon's source, the team found the microorganisms in the sediment prefer fresh carbon from recent algae over older carbon released from permafrost. "The sediment is home to 'gourmet' bacteria that apparently prefer fresh carbon stemming from, for example, more recent algal remains over the 'old' carbon from permafrost deposits," said AWI geochemist Gesine Mollenhauer. The researchers say more research is needed, since some permafrost carbon may already break down before reaching the seabed. The findings were published in the journal Nature Geoscience.