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

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 bulk 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.