The Arctic’s permafrost stores enormous amounts of organic carbon. As climate change accelerates permafrost thaw and coastal erosion, some of this carbon enters the Arctic Ocean. Marine microorganisms can decompose it and release greenhouse gases, potentially intensifying global warming.
Scientists have long had limited data on how much permafrost carbon returns to the atmosphere and how much remains buried beneath the ocean. Researchers from the Alfred Wegener Institute and MARUM – Centre for Marine Environmental Sciences at the University of Bremen investigated this process along the permafrost coastline of Qikiqtaruk, also known as Herschel Island, in Canada.
By analyzing sediment cores, the research team found that significant quantities of land-based carbon remain preserved on the seafloor. The study also revealed that marine microorganisms act like selective feeders, favoring recently produced marine carbon over older carbon released by thawing permafrost. The findings were published in Nature Geoscience.
Arctic Permafrost Holds Vast Carbon Reserves
Arctic terrestrial permafrost ecosystems contain approximately 1,300 gigatonnes of organic carbon, much of it derived from ancient plant material. An additional 400 gigatonnes are stored in ocean sediments and river deltas.
The Arctic is warming faster than any other region on Earth. Rising temperatures are thawing permanently frozen ground and accelerating coastal erosion. As a result, carbon once locked in permafrost can reach the Arctic Ocean through rivers and the collapse of coastal land.
“Consequently, up to 0.02 gigatonnes are entering the sea each year, and according to forecasts, this outflow could rise by 70 to 150 percent by the year 2100,” says Dr. Manuel Ruben, lead author of the study from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). “However, 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.”
Determining the fate of this carbon is essential for understanding how Arctic permafrost thaw may influence greenhouse gas emissions and future climate change.
Sediment Cores Show How Permafrost Carbon Is Stored
To trace the movement of carbon, the researchers collected sediment cores from several sites off the coast of Herschel Island. The layers within these cores recorded approximately 50 years of material deposited on the seafloor.
The analysis showed that only a comparatively small proportion of the carbon transported from land becomes part of the ocean’s active carbon cycle.
“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,” says Manuel Ruben. “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.”
Most of the carbon remains buried and preserved in the seabed rather than being released into the atmosphere.
Carbon Isotopes Reveal Microbial Activity
The scientists examined the chemical composition of the sediment cores and calculated how quickly permafrost-derived material accumulated on the ocean floor.
They also analyzed dissolved inorganic carbon in pore water, the water found in the tiny spaces between sediment particles. This carbon provides evidence of how much CO2 microorganisms produce as they break down organic matter.
Carbon isotope measurements helped the researchers identify the original sources and age of the material consumed by the microorganisms.
“Carbon isotopes represent our atomic indicators that can identify the food source of the microorganisms,” says Prof. Gesine Mollenhauer, a geochemist at the AWI and co-spokesperson for the ‘The Ocean Floor – Earth’s Unexplored Interface’ cluster of Excellence. “The 13C isotope, for example, tells us whether they have consumed carbon from land or from the sea. By way of the 14C isotope, we were able to determine whether the single-celled organisms preferred old organic carbon from permafrost or fresh organic carbon from algae remains.”
Marine Microorganisms Prefer Fresh Carbon
The findings indicate that microorganisms living in Arctic marine sediments do not consume every form of carbon equally.
“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,” explains Gesine Mollenhauer.
This preference for fresh marine organic matter suggests that ancient carbon released by thawing permafrost may contribute less to atmospheric greenhouse gas emissions than previously expected.
However, the researchers emphasize that additional studies are needed before drawing definitive conclusions.
“However, we do need further research here. This is because some of the organic carbon from the permafrost may already have been broken down before it reaches the seabed.”
Permafrost Carbon May Transform Arctic Coastal Ecosystems
The transfer of carbon from Arctic land to the ocean could affect more than greenhouse gas emissions. It may also alter the chemistry and biology of coastal waters that provide important food resources for local communities.
Sediment carried into the ocean by coastal erosion can block sunlight and increase water turbidity. Freshly eroded particles make coastal waters cloudy, while dissolved organic carbon can darken the water even further.
Reduced light availability can affect microscopic algae, which rely on sunlight to produce biomass and oxygen. These organisms form the foundation of marine food webs that support fish, crustaceans, seals and other Arctic wildlife.
The research team plans to examine these ecosystem connections during the international ‘Arctic Pulse’ campaign in 2027. The coordinated expedition will include observations from the Polarstern research icebreaker, AWI research aircraft and land-based study sites. Scientists aim to determine how rapid environmental change is reshaping Arctic ecosystems.
New Data Can Improve Arctic Climate Models
“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,” says Manuel Ruben. “This provides an important foundation for climate models that can predict the consequences of permafrost thawing for the global climate.”
Source: www.sciencedaily.com