Melting sea ice in the East Siberian Sea. As sea ice melts, the mineral ikaite dissolves and carbon dioxide is consumed. This process is an important carbon pump, alongside the biological pump. The picture was taken during a research expedition with the icebreaker Oden in the summer of 2014. Photo: Michael Tjernström

Published 2024-09-11.

The ocean is crucial to our planet's carbon cycle – and therefore to the climate. 20–30 per cent of the carbon dioxide emitted by humans since the 1980s has been absorbed by the ocean, which thereby mitigates the ongoing climate warming.

Next to sedimentary rocks in the Earth's crust, the deep oceans are the most carbon-rich part of our planet. Only a fraction of the Earth’s total carbon stock can be found in the air, or sequestered in plants and animals on land. Without the long-term storage of carbon in the deep ocean, the average temperature of the Earth would not be 15 degrees Celsius, as it is now, but around 50 degrees Celsius.

The biological carbon pump in the ocean is well known. It is based on the uptake of carbon dioxide by phytoplankton in the surface waters, through their photosynthesis. When the plankton die, they sink into the deep sea, where they decompose. During decomposition, the carbon dioxide is released back into the water, but is then so deep down that it takes many hundreds of years for the carbon to re-enter the atmosphere. The carbon has been ‘captured’ by the ocean through biological production and sedimentation.

Lesser-known carbon pump 

But the ice-covered polar seas also harbour another, much lesser-known carbon pump, in the form of the mineral ikaite. Researchers at Stockholm University have now found that its contribution to carbon dioxide uptake may be at least as large as that of the biological pump, during the growth season in summer.

Ikaite is a hydrated form of calcium carbonate, which means that each carbonate molecule binds to six water molecules. It usually forms in carbonate-rich water, with high pH and near the freezing point – conditions that often prevail when sea ice forms.

Sinks with ‘brines’ to the seabed

When the mineral precipitates, carbon dioxide is formed, and some of it is released into the atmosphere. However, some of the carbon dioxide also sinks to the seabed with so-called ‘brines’. Brines are concentrated saline solutions created in the water when sea ice forms, and sink quickly because of their high density.

However, ikaite is an unstable mineral, and as soon as summer arrives and the water temperature rises, it quickly dissolves again. This means that when the sea ice melts, the opposite chemical reaction takes place: ikaite dissolves, consuming carbon dioxide.

Expedition in the Arctic Ocean

In the study, the researchers used data collected during an expedition in the East Siberian Sea, with the icebreaker Oden in the summer of 2014. The expedition travelled over 1200 km of the shallow sea – the largest shelf sea in the Arctic – which forms part of the Arctic Ocean and is still mostly covered with ice even in summer.

Using the WEGAS (Water Equilibration Gas Analyzer System) constructed at the Baltic Sea Centre, coupled to a spectrometer, the researchers continuously sampled the water and air to measure the amount of carbon dioxide. The alkalinity and carbonate content of the water was also analysed at 22 stations.

Almost half of the carbon dioxide uptake

The researchers found a large spatial variation in the amount of ikaite dissolved in different parts of the ocean. But overall, the dissolution of ikaite, which occurs in summer when sea ice melts, accounted for almost half of the ocean’s total carbon dioxide uptake, with the primary production of phytoplankton accounting for the rest.

The study is the first large-scale field investigation of the role of ikaite as a carbon pump in cold oceans during summer. Previously, the role of ikaite has only been studied in individual ice plugs or in lab experiments. 

“We can now conclude that the dissolution of ikaite is an important summertime carbon sink. More research is needed to know what happens on a large scale during winter, when the ikaite is formed and carbon dioxide is released”, says Professor Christoph Humborg at the Baltic Sea Centre. “We don't know yet how much of that carbon dioxide is expelled into the atmosphere and how much is exported via ‘brines’ down to the seabed and then to the deeper parts of the polar oceans, where the carbon can remain for hundreds of years.” 

“If less sea ice is formed in the future due to climate change, the potential carbon sequestration via these brines would also be lost. But this ‘export to the seabed’ is not yet quantified, so we don't know how much carbon is involved”, says Professor Humborg.

The data on which the study is based is archived and freely available in the Bolin Centre database.

Text: Michaela Lundell

Read the scientific article

Sun et al. 2024: Large-Scale Summertime Variability of Carbonate Chemistry Across the East Siberian Sea: Primary Production Versus Ikaite Dissolution. Journal of Geophysical Research: Oceans.