The resuspension of seafloor sediments – a process triggered by human activities such as bottom trawling as well as natural processes like storms and tides – can significantly increase the release of carbon dioxide into the atmosphere, a new study looking into the CO2 processes in the Baltic Sea has concluded.

Confirming the belief held by conservation groups and ocean campaigners across the world for many years, a new study from the GEOMAR Helmholtz Centre for Ocean Research Kiel has taken a deep-dive into the disruption of seafloor sediments and just what impact the process can have on the ocean’s role on carbon dioxide storage.

Among its conclusions is that when trawl nets are dragged across the seabed, stirred up sediment not only releases organic carbon, but also intensifies the oxidation of pyrite – leading to an additional release of carbon dioxide. 

In an examination of the geochemical consequences of sediment resuspension using samples from Kiel Bay, researchers from GEOMAR were able to distinguish that in seabed areas with fine-grained sediments – the type particularly crucial for the storage of carbon dioxide in the Baltic Sea – urgent protection is now needed.

The study has now been published in the journal Communications Earth & Environment.

“The fine-grained, muddy sediments are important reservoirs for organic carbon and pyrite,” said the study’s lead author, Habeeb Thanveer Kalapurakkal, a doctoral student in the Benthic Biogeochemistry group at GEOMAR Helmholtz Centre for Ocean Research Kiel.

“We know that sediment resuspension, for example through the use of trawls, leads to a strong release of carbon dioxide in the water column.”

While it was previously assumed that the release of carbon dioxide was due to the oxidation of organic carbon, this new study demonstrates that in actuality the majority of carbon dioxide release during sediment resuspension is due to the oxidation of pyrite.

The study focused on Kiel Bight, a coastal region in the western Baltic Sea located between the German island of Fehmarn and the Danish islands. It’s an area made up of a range of sediment types, from the coarse sandy sediments in shallower waters to the fine-grained mud in deeper regions. These muddy sediments are rich in organic matter and play a crucial role in the carbon cycle of the Baltic Sea.

They are affected routinely by natural forces such as storms as well as by anthropogenic impact, including bottom trawling.

So, what’s all this about the oxidation of pyrite? Pyrite is an iron-coating mineral, typically found in oxygen-poor, muddy seafloor sediments. When it is disturbed – through resuspension – this pyrite reacts with oxygen in the water, generating an acid that converts climate-neutral biocarbonate into the greenhouse gas, carbon dioxide.

A large proportion of the carbon dioxide that is generated by pyrite oxidation is subsequently released into the atmosphere. Modeling results also suggest that these processes ‘could significantly reduce the region’s ability to absorb carbon dioxide from the atmosphere.’

In other words, resuspension can turn the seafloor temporarily from a carbon sink into a carbon source.

“Kiel Bight, like other parts of the Baltic Sea, acts as an important sink for atmospheric carbon dioxide,” said Kalapurakkal. “Our experiments and model simulations show that activities such as bottom trawling significantly reduce this capacity by promoting pyrite oxidation and acidification.”

The findings, the researchers have stated, underscore the need to protect seafloor areas with fine-grained, muddy sediments – regions that are typically rich in pyrite.

“These areas need to be protected to maintain the carbon dioxide uptake capacity of the Baltic Sea,” said Kalapurakkal.

The paper – titled Sediment resuspension in muddy sediments enhances pyrite oxidation and carbon dioxide emissions in Kiel Bight – is now published in the scientific journal Communications Earth and Environment.

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