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