The northern seas hold about 19.65 billion tons of organic carbon – roughly 13% of its global reserves in surface marine sediments. At the same time, more than 17 billion tons of this carbon is now virtually unprotected from human activity, which could eventually cause some of this stock to re-enter the atmosphere as CO₂. Scientists at the University of Glasgow in the United Kingdom, the University of Umeo in Sweden, and the Greenland Institute for Natural Resources have reached this conclusion by producing the first-ever detailed map of organic carbon storage in marine sediments in the Arctic and sub-Arctic areas.
The seafloor plays an important role in climate regulation. After the death of plankton, algae and other organisms, some of their carbon deposits sink to the bottom and are gradually buried under new layers of sediment. Carbon can be stored down there for hundreds and thousands of years, effectively falling out of the global carbon cycle.
To find out where such large natural “reservoirs” are located, researchers analyzed more than 13,600 samples of surface seabed sediments and 620 cores of marine sediment collected over decades of research in the North seas. Consequently, using machine-learning techniques, they constructed a distribution map of organic carbon at around 10 km resolution and identified the areas where most organic carbon was stored.
Calculations have shown that only the top 10 cm of bottom sediment north of the 50th parallel contains about 19.65 billion tons of organic carbon, or roughly 13% of the world’s stockpile in surface marine precipitation. More than 60% of this carbon is concentrated on the continental shelf, although it occupies a much smaller area than the deep ocean. On average, the Arctic and sub-Arctic sediments contain almost one and a half times more organic carbon than the marine precipitation of the planet as a whole.
The map created by researchers made it possible to identify areas with the highest concentration of organic carbon, the so-called “hot spots” of its accumulation. The Baltic, Barents and Chukchi seas appeared to be the main zones of concentration. The absolute leader was the Baltic Sea. Despite being relatively small, the seabed accounts for up to 0.21 grams of buried carbon per square centimeter – several times more than in the open ocean.
Why have these seas become the world’s largest natural repositories of carbon? The reasons vary by region. In the Baltic Sea, a large inflow of fresh river water is contributing to carbon accumulation. It forms a less saline layer at the surface, which blends poorly with denser saline water at the bottom. Because of this deposition, oxygen from the surface is almost never released to the bottom, so micro-organisms break down organic residues much more slowly. As a result, carbon can accumulate in sediments for millennia. Humans’ centuries-long economic activities have played an additional role, with land-based nitrogen and phosphorus compounds stimulating the growth of phytoplankton – microscopic algae that form the basis of the marine food chain. The more phytoplankton grew, the more organic matter settled on the seafloor.
In the Barents and Chukchi seas, an entirely different mechanism can be observed. The relatively warm Atlantic waters flow in, wwhile the shrinking area of sea ice increases the amount of sunlight entering the water. As a result, phytoplankton is actively developing here. After its demise, organic residues settle into the ground rapidly, gradually forming new carbon stocks.
Another source of carbon is represented by the major Arctic rivers: the Ob, Yenisei, Lena, and Mackenzie. They carry to the sea organic matter which has been stored in thousands of cells in soils, peatlands and permafrost. As the climate warms, this flow may intensify: thereby the frost is melting, shores are being destroyed, and rivers are carrying more ancient organic particles to the seabed. For example, in the Kara Sea, wherein the Ob and Yenisei rivers flow, carbon is stored not only on the sediment’s surface, but also up to one meter deep. New driftage layers are gradually covering up old organic residues, thereby keeping carbon reliably “sealed” in the bottom sediment.
According to scientists, all of these natural carbon repositories in the northern seas are now more vulnerable. Climate warming accelerates the melting of permafrost and the destruction of Arctic coastlines simultaneously shifting river flows and changing the amount of organic matter entering the sea and how it is transported. At the same time, as sea ice is shrinking in the Arctic, economic activity is increasing – shipping, oil and gas extraction, fishing, submarine cables and pipelines are being laid. Some of these activities are disrupting the upper seabed, which contains a significant amount of organic carbon. For example, in trawling fishing the top of the seafloor is literally ploughed up, causing buried organic matter to decompose faster. As a result, some of the carbon that could remain in the bottom sediment for hundreds or even thousands of years may reenter the atmosphere as CO₂ fueling global warming.
Therefore, scientists believe that bottom sediments should be taken into account when planning economic activities and creating new marine protected areas. Today, about 10% of the Arctic and sub-Arctic’s organic carbon stocks are protected, while areas where the rest is concentrated remain vulnerable.
