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24 April 2025/Kiel. When bottom trawls are dragged across the seafloor, they stir up sediments. This not only releases previously stored organic carbon, but also intensifies the oxidation of pyrite, a mineral present in marine sediments, leading to additional emissions of carbon dioxide (CO2). These are the findings of a study conducted by GEOMAR. Based on samples from Kiel Bight, the researchers investigated the geochemical consequences of sediment resuspension. Their conclusion: areas with fine-grained sediments, which play a crucial role in CO2 storage in the Baltic Sea, should urgently be placed under protection. The study has been published in Communications Earth & Environment.
The resuspension of seafloor sediments – 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 (CO2) into the atmosphere. When these sediments are exposed to oxygen-rich seawater, large-scale oxidation of pyrite occurs. This reaction plays a much greater role in CO2 emissions than previously assumed, exceeding the contribution from the oxidation of organic carbon. The new study provides the first quantitative evidence of this effect in the western Baltic Sea.
“Fine-grained, muddy sediments are important reservoirs of organic carbon and pyrite,” says lead author Habeeb Thanveer Kalapurakkal, a PhD student in the Benthic Biogeochemistry working group at GEOMAR. “We already knew that sediment resuspension can release significant amounts of CO2 into the water column. But until now, it was believed that this was mainly due to organic carbon oxidation.” The new study now shows that the major part of the CO2 release is caused by pyrite oxidation.
Kiel Bight: A Critical Carbon Sink at Risk
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. This area features a range of sediment types: coarse sandy sediments in shallower waters and fine-grained mud in deeper regions. These muddy sediments are rich in organic matter and play a central role in the carbon cycle of the Baltic Sea. They are affected both by natural forces such as storms and by anthropogenic impacts like bottom trawling.
Laboratory Experiments Reveal New Insights
To study the effects of sediment resuspension, the researchers conducted sediment slurry incubations. They collected sediment samples from different sites in Kiel Bight — ranging from coarse sandy to fine grained muddy sediments — and stirred them in laboratory containers filled with seawater. The experiments simulated both oxygen-rich and oxygen-poor conditions. During the incubation period, the team monitored changes in key chemical parameters, including CO₂ concentrations, pH, sulfate, nutrients and isotope concentrations. These measurements allowed them to identify the underlying processes and assess their impact on the local carbon cycle. The laboratory data were then integrated into a biogeochemical model to better understand the effects of sediment resuspension and oxygen availability.
Pyrite Oxidation: A Key Factor in CO2 Release
The results show that sediment resuspension leads to substantially greater CO₂ emissions than previously thought — mainly due to the oxidation of pyrite. When this iron-containing mineral, typically found in oxygen-poor, muddy seafloor sediments, is disturbed it reacts with oxygen in the water. This reaction generates acid that converts climate-neutral bicarbonate into the greenhouse gas CO2. A large fraction of the CO2 generated by pyrite oxidation is subsequently released into the atmosphere. Modeling results suggest that these processes could significantly reduce the region’s CO2 uptake capacity. In other words, resuspension can turn the seafloor temporarily from a carbon sink into a carbon source.
Protecting Sensitive Seafloor Areas to Preserve CO2 Uptake
“Kiel Bight, like other parts of the Baltic Sea, acts as an important sink for atmospheric CO₂,” says 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 underscore the need to protect seafloor areas with fine-grained, muddy sediments — regions typically rich in pyrite. Kalapurakkal: “These areas need to be protected to maintain the CO2 uptake capacity of the Baltic Sea.”
Kalapurakkal, H.T., Dale, A.W., Schmidt, M. et al. (2025): Sediment resuspension in muddy sediments enhances pyrite oxidation and carbon dioxide emissions in Kiel Bight. Commun Earth Environ 6(1), 156.
https://doi.org/10.1038/s43247-025-02132-4
http://www.geomar.de/n9849 Images for download
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