New research suggests that Arctic seabed sediments are trapping much of the ancient carbon released by thawing permafrost before microbes can convert it into climate-warming gases.
Date: August 3, 2026
Source: Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research
The Arctic holds one of the planetโs largest reservoirs of ancient organic carbon. For thousands of years, enormous quantities of plant-derived material have remained frozen in permafrost soils. But as the Arctic warms and its coastlines erode, some of that long-stored carbon is being released into rivers and coastal seas.
Scientists have been concerned about what happens next.
Once ancient carbon enters the ocean, microorganisms can consume it and transform it into carbon dioxide and other gases that may eventually reach the atmosphere, potentially reinforcing climate warming.
A new study, however, suggests that the story may be more complicated โ and somewhat less alarming โ than previously assumed.
Researchers from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, and MARUM โ Centre for Marine Environmental Sciences at the University of Bremen found that most of the land-derived organic carbon reaching coastal waters near Qikiqtaruk, or Herschel Island, in northern Canada becomes buried in marine sediments.
Only about 10% of the organic carbon present in those sediments appears to be converted by microorganisms into gases.
The findings, published in Nature Geoscience, provide new insight into the fate of ancient Arctic carbon as rapidly thawing permafrost increasingly connects land and ocean.
A vast frozen carbon reservoir is beginning to move
Arctic terrestrial permafrost ecosystems contain an estimated 1,300 gigatonnes of organic carbon, much of it originating from ancient plant material.
Another roughly 400 gigatonnes of carbon is stored in marine sediments and river deltas.
These reservoirs matter because the Arctic is warming exceptionally rapidly. As temperatures rise, once-permanently frozen soils are thawing, while waves, storms and melting ground are accelerating coastal erosion.
That allows carbon that had remained locked away for centuries or millennia to enter the Arctic Ocean through rivers and collapsing coastlines.
According to lead author Dr. Manuel Ruben of the Alfred Wegener Institute, as much as 0.02 gigatonnes of organic carbon may currently enter the sea each year.
Climate projections suggest that this transfer could increase dramatically by the end of the century, potentially rising by approximately 70% to 150% by 2100.
But simply knowing how much carbon reaches the ocean is not enough.
The crucial question is what happens to it afterward.
If microorganisms rapidly break down the old organic material, a portion of its carbon could return to the atmosphere as greenhouse gases. If instead the material becomes buried in marine sediments, much of it may remain isolated from the active carbon cycle for considerably longer periods.
Until now, scientists have had relatively little direct information on how these competing processes play out along eroding Arctic coastlines.
Sediment cores reveal the carbonโs fate
To investigate, the researchers studied sediments deposited offshore from Herschel Island.
They collected marine sediment cores containing approximately five decades of accumulated material. Layer by layer, these cores provided a historical record of carbon entering the coastal ocean from both land and marine sources.
The results revealed that large quantities of land-derived organic carbon were preserved in the seafloor.
Despite substantial amounts of material being transported from the eroding coastline, relatively little appeared to enter the rapidly cycling marine carbon system.
The researchers estimated that microorganisms converted only around 10% of the sedimentary organic carbon into gaseous products.
Those gases can move through sediment pore water, reach the overlying ocean and, under some circumstances, eventually enter the atmosphere.
The majority of the carbon, however, remained within the sediment.
This indicates that Arctic coastal seabeds may act as an important burial zone for some of the ancient carbon mobilized by permafrost thaw and coastal erosion.
Carbon isotopes reveal what microbes are eating
The team did more than measure how much carbon accumulated.
They also wanted to identify which types of organic material microorganisms were actually consuming.
To do this, the scientists examined dissolved inorganic carbon within the sedimentโs pore water โ the tiny volumes of water trapped between sediment particles.
When microorganisms consume organic matter, they produce dissolved inorganic carbon, including carbon dioxide. By studying its chemical and isotopic composition, scientists can trace where the consumed organic material originally came from.
Carbon isotopes provided particularly valuable clues.
Carbon-13 helped distinguish between organic matter originating on land and material produced within the marine environment.
Carbon-14, meanwhile, helped researchers determine the age of the carbon being consumed.
Together, these measurements revealed a striking pattern.
The microbes were not feeding equally on all available carbon.
Arctic seabed microbes appear to prefer the fresh stuff
The sediment communities appeared to favor relatively young, fresh organic matter produced in the ocean โ such as recently deposited algal material โ rather than ancient carbon released from permafrost.
Prof. Gesine Mollenhauer, a geochemist at the Alfred Wegener Institute and co-spokesperson for the Cluster of Excellence โThe Ocean Floor โ Earthโs Uncharted Interface,โ compared the organisms to selective diners.
The researchers describe them as behaving somewhat like โgourmetโ bacteria: when given a choice, they preferentially consume fresh marine carbon instead of older, more degraded permafrost-derived material.
That preference could have important consequences.
If ancient permafrost carbon is less attractive to sediment microbes, a larger proportion of it may remain buried rather than being rapidly converted into carbon dioxide.
This would mean that not all carbon released by thawing coastal permafrost contributes equally or immediately to atmospheric greenhouse-gas concentrations.
The finding does not mean that thawing permafrost is harmless, nor does it eliminate concerns about the Arctic carbon cycle.
Some permafrost-derived material may already be decomposed before it reaches the seabed, for example during erosion, river transport or movement through coastal waters.
The researchers therefore stress that additional work will be needed to determine the full climate impact.
Permafrost carbon can affect Arctic ecosystems even if it stays out of the atmosphere
The consequences of coastal permafrost erosion also extend beyond greenhouse-gas emissions.
When large quantities of soil, sediment and organic matter enter nearshore waters, they can transform the physical and biological environment.
Suspended sediment can make coastal waters cloudy, limiting the amount of sunlight that penetrates through the water column.
Dissolved organic carbon can also darken the water, further reducing light availability.
That matters for microscopic algae and other photosynthetic organisms, which depend on sunlight to produce biomass and oxygen.
These organisms form the foundation of many Arctic coastal food webs.
Changes in their productivity could therefore influence animals higher up the food chain, including crustaceans, fish and seals โ species that are ecologically important and, in many Arctic communities, closely connected with local food resources.
The transport of thawed terrestrial material into the ocean may therefore reshape Arctic coastal ecosystems even when much of its carbon ultimately becomes buried.
Scientists will investigate the wider effects during Arctic Pulse
Researchers hope to better understand these interconnected processes during the international Arctic Pulse research campaign planned for 2027.
The campaign will combine observations from the German research icebreaker Polarstern, Alfred Wegener Institute research aircraft and land-based monitoring stations.
Scientists will examine how rapid environmental changes are affecting interactions between Arctic land, ocean, atmosphere and ecosystems.
By coordinating measurements across these environments, researchers hope to capture the movement of carbon and other materials as they travel from thawing permafrost into rivers, coastal waters, marine sediments and potentially the atmosphere.
Better data for future climate models
The new study provides an important piece of that larger puzzle.
By estimating how much land-derived carbon becomes buried and determining which types of organic carbon are actually consumed by microorganisms, researchers can improve models of the Arctic carbon cycle.
Such models are essential for predicting whether thawing permafrost will produce strong additional climate feedbacks as warming continues.
The results suggest that nearshore marine sediments can preserve a substantial fraction of carbon released from Arctic coastal permafrost.
Rather than being immediately transformed into greenhouse gases, much of this ancient material appears to settle to the seafloor, where it becomes incorporated into sediments.
At the same time, scientists caution that seabed burial represents only one stage in a much larger system. Carbon can undergo transformations before reaching the sediment, and future warming, erosion and ecological changes could alter how efficiently Arctic seas retain it.
Still, the study offers an important refinement to scientistsโ understanding of the permafrost-carbon problem.
The thawing Arctic is unquestionably releasing ancient carbon.
But according to these findings, a surprisingly large proportion of the carbon reaching some coastal seabeds may remain buried there instead of being rapidly returned to the atmosphere.
Journal reference
Manuel Ruben, Bingbing Wei, Anabel von Jackowski, Jens Hefter, Torben Gentz, Florence Schubotz, Heidi Taubner, Bo Liu, Michael Fritz, Anna Irrgang, Walter Geibert, Maarten Boersma, Gabriel A. Juma, Silla Thomsen, and Gesine Mollenhauer. Limited remineralization of Arctic permafrost-derived organic carbon in nearshore marine sediments. Nature Geoscience (2026). DOI: 10.1038/s41561-026-02060-8.