New data from sediment core analysis reveals that much of the organic carbon entering the Arctic Ocean from eroding permafrost coasts remains locked in the seabed rather than releasing into the atmosphere as greenhouse gases, according to researchers from the Alfred Wegener Institute and MARUM.
Global warming is taking a heavy toll on polar regions, where temperatures rise faster than anywhere else on the planet. Permafrost ecosystems in the Arctic store approximately 1,300 gigatonnes of carbon from organic sources such as plant remains, while ocean sediments and river deltas hold another 400 gigatonnes. As the ground thaws and coastal sections erode, this material enters the northern seas. According to research from the Alfred Wegener Institute, up to 0.02 gigatonnes enter the sea each year, with forecasts suggesting this outflow could surge by 70 to 150 per cent by 2100.
The findings are published in Nature Geoscience.
Sediment Cores Reveal What Happens to Arctic Carbon
For years, scientists understood that thawing land masses pushed massive quantities of carbon toward the water, but the exact destination of that material remained largely a mystery. To answer this question, researchers retrieved and analyzed sediment cores off the coast of the Canadian island of Qikiqtaruk, also known as Herschel Island. These deposits span roughly 50 years of accumulation.
Instead of fueling atmospheric warming, the major share of this eroded material settles securely into the seabed. Microorganisms convert only about ten per cent of the organic carbon from the sediments into gases that rise through the water column, while the rest remains sequestered.
Bacterial Dietary Preferences Shape Carbon Release
To determine how microorganisms process the sediment, scientists examined pore water—the tiny liquid cavities trapped within sediment layers. This water reveals how much carbon dioxide microbes exhale after consuming organic matter. By analyzing isotopic signatures, the team traced the origin of the carbon.
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.” Prof. Gesine Mollenhauer, Alfred Wegener Institute
The analysis showed that seabed microorganisms behave like selective eaters. The sediment houses what researchers describe as gourmet bacteria that bypass the ancient permafrost carbon in favor of fresh carbon derived from recent algal remains. Because of this dietary preference, land-derived carbon contributes far less to atmospheric greenhouse gases than many scientists initially feared.
New Modeling Tracks Inland Runoff and River Loads
While marine sediments lock away a significant portion of coastal carbon, inland dynamics tell a different story. New data computed by the Permafrost Water Balance Model and published in the journal Global Biogeochemical Cycles highlights how thawing ground alters northern rivers. Researchers at the University of Massachusetts Amherst analyzed a tract of land comparable to the size of Wisconsin, finding that runoff from thawing permafrost is increasing significantly and enlarging carbon loads in northern Alaska rivers.
The model, developed over 25 years and powered by a supercomputer at the Massachusetts Green High Performance Computing Center, fills critical data gaps for daily river flows across a 44-year period. The findings indicate that as the active soil layer thaws deeper each year, some dissolved organic carbon reaches calm coastal waters where it outgasses into the atmosphere, creating a warming feedback loop that extends the thaw deeper into autumn.
Related reading
