Arctic holds 1.3 trillion tonnes of ancient carbon, scientists find what warming does to it

Most of the ancient carbon released from Arctic permafrost that reaches coastal sediments may remain buried rather than returning to the atmosphere, according to a new study published in Nature Geoscience. Researchers from the Alfred Wegener Institute (AWI) and MARUM at the University of Bremen found that microorganisms converted only about 10% of the permafrost-derived organic carbon in the studied sediments into gases capable of entering the water and eventually the atmosphere.

The research examined sediment from the waters off Qikiqtaruk, also known as Herschel Island, on Canada's Arctic coast. The findings offer a more detailed picture of what happens to ancient carbon after thawing permafrost and coastal erosion carry it into the ocean, although the researchers said some carbon may be broken down before reaching the seabed.

Arctic permafrost stores a vast carbon reserve

Arctic terrestrial permafrost contains about 1,300 gigatonnes of organic carbon, much of it originating from plant material preserved in frozen ground for thousands of years. Ocean sediments and river deltas hold another estimated 400 gigatonnes.


As Arctic temperatures rise faster than in any other region, thawing permafrost and increasingly eroded coastlines are moving some of that stored material into surrounding waters. Up to 0.02 gigatonnes of land-based carbon currently enters the Arctic Ocean each year, and projections suggest the annual outflow could increase by between 70% and 150% by 2100.

Until now, scientists had limited information about how much of that material would be decomposed and released as greenhouse gases and how much would remain stored in marine sediments.

Scientists use 50-year sediment record to trace carbon

To investigate the carbon's fate, researchers collected sediment cores at several locations off Herschel Island. The layers represented roughly five decades of deposited material, providing a record of how carbon accumulated on the seabed over time.

The team also studied pore water, the liquid held within tiny spaces between sediment particles. By measuring dissolved carbon produced during microbial decomposition, the researchers could estimate how actively organic material was being broken down.

Carbon isotopes then helped identify the source and age of the material being consumed. The researchers used carbon-13 to distinguish between terrestrial and marine sources and carbon-14 to determine whether microorganisms were consuming older permafrost-derived material or newer organic matter associated with algae.

Most carbon remains stored in the seabed

The results showed that only around one-tenth of the organic carbon in the sediments was converted by microorganisms into gases that can move into the water and potentially reach the atmosphere. The majority remained trapped in the seafloor.

“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,” lead author Manuel Ruben of the Alfred Wegener Institute said.

Researchers said the result was partly explained by the feeding preferences of microorganisms living in the sediments.

‘Gourmet’ bacteria prefer newer carbon

According to geochemist Gesine Mollenhauer, microorganisms in the seabed appear to favour fresh marine carbon, such as material from relatively recent algal remains, instead of older organic matter originating from permafrost deposits.

“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,” Mollenhauer said.

That preference means ancient permafrost carbon reaching the seabed may contribute less to atmospheric greenhouse gases than previously feared. However, the researchers cautioned that part of the permafrost-derived material could already have decomposed before it reaches the ocean floor, so the study does not provide a complete accounting of all carbon released from thawing land.

Permafrost runoff could also affect Arctic marine life

The researchers said the movement of carbon from land to sea has consequences beyond greenhouse gas emissions.

Eroded material can make coastal waters more turbid, while dissolved organic carbon can darken the water. Those changes affect the amount of sunlight available to algae, which depend on light to produce biomass and oxygen and form an important foundation for marine food webs supporting fish, crustaceans and seals.

Scientists plan to investigate these wider ecological effects through the international Arctic Pulse campaign scheduled for 2027. The work will involve the Polarstern research icebreaker, AWI aircraft and land-based observations.

The researchers say the new findings provide a stronger basis for climate models seeking to assess the consequences of continued Arctic permafrost thaw.