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 new study conducted by the GEOMAR Helmholtz Centre for Ocean Research Kiel. Based on sediment samples from Kiel Bight in the Baltic Sea, 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.
“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.
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.
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.
