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Climate change and the associated rising temperatures are melting more and more frozen ground in the Arctic. This dissolved matter contains large amounts of organic carbon which is flowing into the central Arctic Ocean.

In a new study, scientists led by Alfred-Wegener-Institute quantified how much terrestrial organic matter accumulates in the central Arctic Ocean. Using chemical fingerprints, they were able to assess how fast it degrades, thus releasing additional CO₂ to the ocean. These findings are an important basis to project how inputs from land affect Arctic marine ecosystems and the ability of the ocean to store CO₂ in a warming climate. The results are in the journal Nature Geoscience.

Melting permafrost releases ancient carbon

When permafrost thaws in the Arctic, it releases very old organic matter from plants, microorganisms or animals, containing carbon that was frozen in the soil for hundreds or thousands of years. Rivers transport this material into the Arctic Ocean, where it dissolves and turns into "dissolved organic matter (DOM)."

"This matter constitutes a large reservoir of organic carbon in the ocean, rivaling the amount of atmospheric CO₂ in scale," says Dr. Xianyu Kong, scientist at Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI) and first author of the study.

"Compared to most other oceans, the Arctic Ocean receives more freshwater and a disproportionally large amount of terrestrial organic matter from extensive inputs from permafrost thaw, river discharge, and coastal erosion."

Tracking carbon from land to ocean depths

Together with German, Norwegian and Danish colleagues, the AWI scientist has now quantified, how much organic carbon is accumulated in the central Arctic Ocean.

"Our study reveals that about 16% of all dissolved organic carbon here originates from land, much of it persisting even in deep waters, where we surprisingly found a consistently high terrestrial contribution," says Kong.

"This suggests that some of land-derived organic matter is chemically stable enough to survive long transport and move from the central Arctic Ocean to the North Atlantic Deep Water. This connects Arctic processes with the global carbon cycle."

Terrestrial dissolved organic carbon is also transported in surface waters: The so-called Transpolar Drift is a surface current that transports freshwater, sea ice as well as nutrients across the Arctic Ocean towards the North Atlantic. The amount of organic carbon in regions that are affected by the Transpolar Drift was about twice as high in neighboring regions. From this, the research team estimated that around 39 million tons of terrestrial carbon are transported from the Arctic to the Atlantic every year.

Impacts on the Arctic carbon cycle

DOM from terrestrial sources impacts the organic carbon cycle in the Arctic Ocean by changing light attenuation, nutrient availability, or microbial processes.

"Previous studies show that the concentration of dissolved organic carbon increases in freshwater environments as a response to climate change," says Prof. Boris Koch, co-author of the study and chemical oceanographer at AWI. "For the Arctic Ocean, however, there is no data available that shows similar trends, partly due to the lack of appropriate methods."

With their results, the AWI researchers are filling a knowledge gap in the understanding of how much carbon enters the Arctic Ocean from land, how it is distributed, and how it changes in the ocean.

"As Arctic warming accelerates, inputs of terrestrial organic matter are expected to increase, potentially altering carbon cycling and broader biogeochemical processes in the Arctic Ocean," says Kong. Previous climate models do not yet reflect these findings. "These findings are an important basis for predicting how inputs from land affects Arctic marine ecosystems and the Arctic carbon inventory in a warming climate."

Innovative methods reveal carbon's journey

For the analysis, the researchers, in collaboration with the Helmholtz Center for Environmental Research (UFZ), developed a new analytical approach over several years to analyze seawater samples from across the central Arctic Ocean that they have collected during the MOSAiC expedition in 2019 /2020. To measure the organic compounds in the samples, they used ultrahigh-resolution Fourier-transform mass spectrometry (FT-ICR MS).

"This method allowed us to identify and quantify thousands of individual organic molecular formulas in seawater and distinguish whether they originated from the ocean and sea ice or terrestrial sources," says Kong. "We were not only able to quantify the concentration of terrestrial carbon, but also to estimate how far the degradation of the organic material has already progressed."

Using this method, the researchers were able to produce the first depth-resolved map of dissolved terrestrial organic carbon in the Arctic Ocean.