New study finds Arctic permafrost carbon largely sequestered in seafloor

As Arctic permafrost thaws and coastlines erode, enormous quantities of ancient carbon that has sat frozen in the ground for thousands of years are being carried into the sea. Scientists have long worried that microorganisms in the ocean would break this carbon down and release it as greenhouse gases, adding a dangerous new feedback loop to climate change. A new study led by researchers at the Alfred Wegener Institute and MARUM, the Centre for Marine Environmental Sciences at the University of Bremen, has found a more reassuring picture, at least for now. Examining sediment cores collected off the coast of Qikiqtaruk, also known as Herschel Island, in Canada, the team found that the vast majority of this land-based carbon simply becomes buried in the seafloor, with only around 10 per cent converted into greenhouse gases.

According to the Alfred Wegener Institute, Arctic permafrost ecosystems hold roughly 1,300 gigatonnes of organic carbon on land, much of it made up of ancient plant remains, with a further 400 gigatonnes already stored in ocean sediments and river deltas. Because the Arctic is warming faster than any other region on the planet, this frozen carbon store has become increasingly vulnerable, as thawing ground and eroding coastlines allow carbon to escape into the Arctic Ocean via rivers and coastal breakdown. Study lead author Dr Manuel Ruben of the Alfred Wegener Institute said up to 0.02 gigatonnes of this carbon are already entering the sea each year, and that forecasts suggest this outflow could rise by 70 to 150 per cent by 2100. Until now, Ruben said, how much of that carbon eventually returns to the atmosphere as a greenhouse gas, versus how much stays locked in the ocean, had remained largely unknown, despite being essential information for building accurate climate models.

To answer this question, researchers collected sediment cores from several locations along the coast of Herschel Island, each containing layers of material built up over roughly 50 years. According to the study published in Nature Geoscience titled Limited remineralisation of Arctic permafrost-derived organic carbon in nearshore marine sediments, the results showed that only a relatively small share of the carbon swept into the ocean actually becomes part of the ocean's active carbon cycle, meaning most of it is not immediately available for microorganisms to break down and release as gas. Ruben said that although the sea carries away huge quantities of organic carbon from the coast, surprisingly little of it ends up cycling actively through the system. Microorganisms convert roughly 10 per cent of the organic carbon found in the sediments into gases that rise into the surrounding water and can eventually reach the atmosphere, while the remaining carbon stays buried within the seabed itself.

To work out precisely where the carbon in the sediment had originated, the team analysed dissolved inorganic carbon trapped in pore water, the tiny spaces between individual sediment particles, since this reveals how much carbon dioxide microorganisms have released after consuming organic material. According to the Alfred Wegener Institute, the isotopic makeup of that pore water then allowed researchers to trace exactly where the consumed material had come from. Professor Gesine Mollenhauer, a geochemist at AWI and co-spokesperson for the Cluster of Excellence The Ocean Floor: Earth's Unexplored Interface, explained that carbon isotopes function as atomic indicators of an organism's food source. The 13C isotope reveals whether microorganisms had consumed carbon originating from land or from the sea, while the 14C isotope allowed the team to determine whether these single-celled organisms preferred old organic carbon from permafrost or fresher organic carbon from more recent algae remains.

The isotope analysis revealed a clear pattern in what the seabed's microorganisms actually preferred to eat. According to Mollenhauer, the sediment is home to what she described as gourmet bacteria that clearly prefer fresh carbon, such as that from recent algal remains, over the older carbon released from thawing permafrost deposits. Because these microbes favour fresher marine material over ancient permafrost carbon, the researchers concluded that permafrost carbon may end up contributing less to atmospheric greenhouse gas levels than scientists had previously feared. The research team was careful to note that this reassuring picture remains incomplete. Some of the organic carbon released from permafrost may already be broken down before it ever reaches the seabed, meaning further research is still needed to build a full accounting of the carbon's journey from thawing land to the ocean floor.

Beyond its implications for greenhouse gas emissions, the movement of carbon from land into the ocean carries other consequences for coastal Arctic ecosystems. According to the Alfred Wegener Institute, sediment released through coastal erosion can reduce how much sunlight reaches the water, since freshly eroded material makes coastal water cloudier while dissolved organic carbon can darken it further. That reduced light availability can in turn affect single-celled algae that rely on sunlight to produce biomass and oxygen, organisms that sit at the base of a food web supporting fish, crustaceans and seals relied upon by local communities. Researchers plan to investigate these broader ecosystem effects further during the international Arctic Pulse campaign scheduled for 2027, which will involve coordinated observations from the Polarstern research icebreaker, AWI research aircraft and monitoring sites on land, aimed at understanding how rapidly changing Arctic conditions are reshaping the region's ecosystems as a whole. In the meantime, Ruben said the current findings provide a far more precise picture than previously available of exactly how much carbon is being safely stored in the seabed, and how much of the carbon being broken down genuinely originates from ancient permafrost, offering climate modellers a stronger foundation for predicting the true consequences of continued permafrost thaw.