Frozen soils in the Arctic contain vast stores of carbon from decomposed plants that were buried here for millennia. With accelerating global warming, permafrost is thawing, coastlines are eroding, and this carbon is being contributed to the ocean. The question mark has been big: how much of this carbon makes its way into the atmosphere as greenhouse gases, and how much is locked up safely on the seabed?
Researchers from the Alfred Wegener Institute (AWI) and MARUM – Center for Marine Environmental Sciences, University of Bremen have now provided the clearest answer yet, publishing their findings in Nature Geoscience.
By analyzing sediment cores off Qikiqtaruk (Herschel Island) in Canada, the team reconstructed 50 years of deposits. They discovered that while huge amounts of terrestrial carbon wash into the sea, only about 10% is converted by microorganisms into gases that can escape into the atmosphere. The majority is locked into the seabed.
“Although the sea here carries away huge quantities of organic carbon from the coast, surprisingly little of it ends up in the ocean’s active carbon cycle,” explained Dr Manuel Ruben, lead author.
The researchers found that microorganisms in the sediments prefer fresh marine carbon (like algal remains) over the “old” carbon from thawing permafrost.
“The sediment is home to ‘gourmet’ bacteria that apparently prefer fresh carbon… over the old carbon from permafrost deposits,” said Prof. Gesine Mollenhauer, AWI geochemist.
This preference means that permafrost carbon contributes less to greenhouse gas emissions than feared, though some may still break down before reaching the seabed.
Carbon storage: Lots of buried permafrost carbon in the sea bed means little immediate climate effect.
Light productivity: Sediments suspend and discolor coastal waters, which reduces the amount of sunlight reaching algae that rely on it for oxygen- and biomass production. In turn, this indirectly affects fish, crustaceans and seals, which form the mainstay for local communities.
Future risks: With Arctic temperatures rising faster than anywhere else, annual carbon outflow to the ocean could increase by 70–150% by 2100.
The team will expand their research during the Arctic Pulse campaign in 2027, using the Polarstern icebreaker, aircraft, and land-based studies to investigate how rapid environmental change is reshaping Arctic ecosystems.
“Our study shows, more precisely than ever before, how much carbon is safely stored in the seabed – and just how much of the decomposed material actually originates from the old permafrost,” Ruben said. “This provides an important foundation for climate models that can predict the consequences of permafrost thawing for the global climate.”
This work globalizes thawing permafrost away from being a potential future climate villain and into a complex system of microbes, sediments, and sunlight to raise the question of how much carbon is released or not buried below the waves.
