The importance of the Arctic Ocean as a carbon sink may be underestimated: Researchers have found that a previously unknown current is transporting carbonaceous biomass from the Arctic marginal seas to the depths of the Arctic Ocean. According to the model calculations, the lateral inflow from the Barents and Kara Seas into the Nansen Basin could bind up to 3.6 million tons of carbon dioxide annually in the Arctic deep sea for thousands of years. There are probably other similar tributaries, so the importance of the system in the role of the Arctic deep sea as a carbon sink could be much greater, the scientists say.
It is bound from the atmosphere in the form of CO2, built into biomass and then released again through degradation processes or combustion: carbon runs through a cycle on our planet that is of fundamental importance for life and the climate. This is why understanding the complex transport and conversion processes within this system is so important. This is the only way to estimate global carbon dioxide budgets and create forecasts for global warming. The oceans play a central role in the carbon cycle: on the sea surface, unicellular algae absorb CO2 from the atmosphere and sink to the depths after they die. When the carbon bound in this way reaches the deep water, it is only released again when the water returns to the sea surface through circulation currents. Sometimes, however, the biomass is also deposited in sediments and can therefore remain bound for very long periods of time.
Inquiring look at the arctic ocean
This process, also known as the biological carbon pump, can thus remove carbon from the atmosphere for very long periods of time and thus represents an important sink in the carbon cycle of our planet. Up to now, the Arctic Ocean has been assigned comparatively little importance. Because its biological productivity is limited compared to other oceans: Due to the long polar night and the extensive sea ice cover, there is often a lack of light and the nutrient supply is also limited. As a result, the tiny algae in the upper water layers have poor growth conditions compared to other oceans and can build up less biomass.
That is why the measurements by the research team led by Andreas Rogge from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI) in Bremerhaven initially caused astonishment: During the "ARCTIC2018" expedition in summer 2018 with the research ship Akademik Tryoshnikov, the Scientists found large amounts of carbon in the form of particles from biomass in the Nansen Basin in the central Arctic Ocean. More detailed investigations then showed that the carbon-rich water body extends to a depth of up to two kilometers. It finally became apparent that the bottom water was from the Barents Sea and the Kara Sea – two shallow marginal seas of the Arctic Ocean that adjoin the basin to the south. As the researchers explain, it is water that is formed when sea ice forms in winter. The cold, heavy water sinks there and then eventually flows from the shallow coastal shelf down the continental slope into the deep Arctic basin. The plume with the carbon-rich water extends from the shelf of the Barents and Kara Seas to about 1000 kilometers into the Arctic deep sea basin, say the researchers.
A carbon sink is emerging
Model calculations showed that the outflow occurs in seasonal pulses, since the uptake of CO2 by the phytoplankton in the Arctic coastal seas occurs exclusively in summer. "Based on our measurements, we were also able to calculate that this water mass transport causes more than 2000 tons of carbon to flow into the Arctic deep sea per day, which corresponds to 8500 tons of atmospheric carbon dioxide. Extrapolated over a year, that is 3.6 million tons of CO2, which is about the same as Iceland’s annual emissions,” says Rogge. According to the scientists, this newly described mechanism removes around 30 percent more carbon from the atmosphere than previously assumed, primarily through the Barents Sea.
As they point out, the carbon conveyor belt discovered so far may be just one of several. Because there are other little explored areas in the polar shelf seas where bottom water is formed and could flow into the deep sea. The scientists say that the global importance of this system as a carbon sink and thus in the climate system could be much greater than has been shown so far.
As Rogge finally points out, however, it seems questionable how the system will continue to develop as a result of the strong climate warming in the far north: "Increasing global warming is contributing to less ice and thus less ground water being formed. On the other hand, it also means that more light and nutrients are available for the phytoplankton, so that more CO2 is bound. Thus, the future development of this carbon sink cannot be estimated at the moment and the identification of possible tipping points urgently requires further research," says the scientist.
Source: Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, specialist article: NatureGeoscience. Doi: 10.1038/s41561-022-01069-z