Limited physical protection leads to high sedimentary organic carbon reactivity under anoxic conditions

Marine sediments bury ~160 Tg organic carbon (OC) yr-1 globally, with ~90% of 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 indicated that physical protection of OC is limited. Overall, these results suggest that the WGB sediments receive large amounts of OC relative to the total supply of sedimenting particles, far exceeding the potential for physical protection. As a result, OC reactivity is high, even though bottom waters are anoxic. Our results suggest that bottom-water anoxia does not always lead to reduced OC mineralization rates and increased burial efficiencies.