Preprints
https://doi.org/10.5194/egusphere-2025-3020
https://doi.org/10.5194/egusphere-2025-3020
02 Jul 2025
 | 02 Jul 2025

Limited physical protection leads to high organic carbon reactivity in anoxic Baltic Sea sediments

Silvia Placitu, Sebastiaan J. van de Velde, Astrid Hylén, Mats Eriksson, Per O. J. Hall, and Steeve Bonneville

Abstract. Marine sediments bury ~160 Tg organic carbon (OC) yr-1 globally, with ~90% of the burial occurring in continental margin sediments. It is generally believed that OC is buried more efficiently in sediments underlying anoxic bottom waters. However, recent studies revealed that sediments in the central Baltic Sea exhibit very high OC mineralization rates and consequently low OC burial efficiencies (~5-10%), despite being overlaid by long-term anoxic bottom waters. Here, we investigate factors contributing to this unexpectedly high OC mineralization rates in the Western Gotland Basin (WGB), a sub-basin of the central Baltic Sea. We sampled five sites along a transect in the WGB, including two where organic carbon-iron (OC-Fe) associations were quantified. Sulphate reduction rate measurements indicated that OC reactivity (k) was much higher than expected for anoxic sediments. High OC loadings (i.e., OC concentrations normalized to sediment specific surface area) and low OC-Fe associations showed that physical protection of OC is limited. Overall, these results suggest that the WGB sediments receive large amounts of OC relative to the supply of mineral particles, far exceeding the potential for OC physical protection. As a result, a large fraction of OC is free from associations with mineral surfaces, thus the OC reactivity is high, despite anoxic bottom waters. Overall, our results demonstrate that anoxia does not always lead to lower OC mineralization rates and increased burial efficiencies in sediments.

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Silvia Placitu, Sebastiaan J. van de Velde, Astrid Hylén, Mats Eriksson, Per O. J. Hall, and Steeve Bonneville

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  • RC1: 'Comment on egusphere-2025-3020', Anonymous Referee #1, 11 Jul 2025
  • RC2: 'Comment on egusphere-2025-3020', Anonymous Referee #2, 31 Jul 2025
  • RC3: 'Comment on egusphere-2025-3020', Anonymous Referee #3, 01 Aug 2025
Silvia Placitu, Sebastiaan J. van de Velde, Astrid Hylén, Mats Eriksson, Per O. J. Hall, and Steeve Bonneville
Silvia Placitu, Sebastiaan J. van de Velde, Astrid Hylén, Mats Eriksson, Per O. J. Hall, and Steeve Bonneville

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Short summary
Marine sediments store organic carbon and help regulate climate. Oxygen-depleted waters are thought to enhance this, however West Gotland Basin sediments show low carbon despite such conditions. We studied the role of mineral protection, which can shield carbon from microbes, and found it limited. This suggests that without physical protection, carbon remains accessible and gets degraded, making mineral protection a key factor in carbon preservation.
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