In the midst of much conversation about rising carbon emissions and their effect on the atmosphere and climate of Earth, a new study has identified a hitherto unrecognized carbon sink at the bottom of the Arctic Ocean. This involves a previously unknown transport route that makes use of currents to pump carbon, absorbed from the atmosphere by plant plankton at the ocean surface, down to the deepest, darkest depths.
Scientists from several research institutes, including the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, took part in the Arctic expedition ARCTIC2018, between August and September 2018, on the Russian research vessel Akademik Tryoshnikov. During their investigations, they were surprised to discover large quantities of particulate carbon (i.e., stored in plant remains) deep down in the Nansen Basin in the central Arctic Ocean.
The biological productivity of the central Arctic Ocean is relatively low, due to the short supply of sunlight that restricts photosynthesis. Light is limited due to the Polar Night in winter months, and to the presence of sea-ice cover at times during the year. This means that microalgae (phytoplankton) in the surface layers are not able to photosynthesize as much in this ocean as in other oceans. In addition, nutrients are relatively scarce, making algal proliferation less likely.
Analysis of the waters in the Nansen Basin, however, revealed the presence of a body of water with large amounts of particulate carbon in dead organic matter. This carbon was present even down to depths of two kilometers, in bottom water that had originated in the Barents Sea. Bottom water is produced when winter sea ice melts and sinks, subsequently flowing from the shallow coastal shelf down the continental slope and into the deep Artic Basin.
“Based on our measurements, we calculated that through this water-mass transport, more than 2,000 metric tons of carbon flow into the Arctic deep sea per day, the equivalent of 8,500 metric tons of atmospheric CO2. Extrapolated to the total annual amount revealed even 13.6 million metric tons of CO2, which is on the same scale as Iceland’s total annual emissions,” explains Dr. Andreas Rogge, first author of the Nature Geoscience study and an oceanographer at the Alfred Wegener Institute.
According to the research findings, this plume of carbon-rich water spans from the Barents- and Kara Sea shelf to roughly 1,000 kilometers into the Arctic Basin. In light of this newly discovered mechanism, the Barents Sea – already known to be the most productive marginal sea in the Arctic – would appear to effectively remove roughly 30 percent more carbon from the atmosphere than previously believed. Moreover, model-based simulations determined that the outflow shows seasonal pulses related to the fact that absorption of CO2 by phytoplanktonin the Arctic’s coastal seas only takes place in summer.
It is important to understand the ways in which carbon is transported and transformed during the carbon cycle, as this is crucial to developing carbon dioxide budgets for the planet and projecting the extent and consequences of global warming.
Marine phytoplankton (single-celled algae) absorb CO2 from the atmosphere during photosynthesis and use this to build organic molecules in their bodies. When they die, they sink towards deeper waters, carrying the carbon with them. Once carbon, bound in this manner, reaches the deep water, it stays there until overturning currents bring the water back to the ocean’s surface, which takes several thousand years in the Arctic. And if the carbon is deposited in deep-sea sediments, it can even be trapped there for millions of years, as only volcanic activity can release it.
This process, also known as the biological carbon pump, can remove carbon from the atmosphere for long periods of time and represents a vital sink in our planet’s carbon cycle. The dead organic matter can also provide a source of food for deep-sea fauna, like sea stars, sponges and worms, meaning that not all the sinking particulate carbon ends up being sequestered in the deep ocean water or sediments. The actual percentage of carbon absorbed by the ecosystem in the Nansen Basin is not known, but certainly warrants further research.
There are many regions of the Arctic shelf that remain, as yet, unexplored, and it is possible that these may also harbor systems in which bottom water is formed from ice melt that carries surface organisms down into the deep sea. Therefore, it is likely that the importance of this mechanism as a carbon sink is actually far greater. However, its significance going into the future is unknown, as it will be affected by different drivers under the influence of global warming.
Andreas Rogge explains that, due to the ongoing global warming, less ice and therefore less bottom water is formed. “At the same time more light and nutrients are available for the phytoplankton, allowing more CO2 to be bound. Accordingly, it’s currently impossible to predict how this carbon sink will develop, and the identification of potential tipping points urgently calls for additional research.”
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