Marine sediments bury ~160 Tg of organic carbon (OC) annually and represent an essential component of the global carbon cycle. OC burial is inherently multifactorial; however, in the past decade, the role of iron in regulating OC burial via the formation of organo-mineral associations, known as “rusty carbon sink,” has been extensively studied. Despite widespread recognition, the origin of the OC preserved within these associations and the effect of the bottom-water oxygenation on their stability are still debated. Here, we investigate the rusty carbon sink in sediments collected across transects from the head to mouth of three Swedish fjords presenting contrasting bottom-water oxygenation regimes (the oxic Hake fjord, seasonally hypoxic Gullmar fjord, and anoxic By fjord). We found that the oxygenation regimes, the intensity of benthic iron cycling, or the OC origin have little to no influence on the amount of OC bound to Fe (%OC-Fe). The lack of correlation with any of the parameters studied, in combination with an increase in the %OC-Fe in the fjords with riverine input suggest, at least partially, an allochthonous origin of these organo-mineral associations. Our results also show that the rusty carbon sink play a modest role in the OC burial in these fjords (~6% OC is bound to Fe). While these fjords still represent important OC burial hotspots with an average of ~35 g C m-2 buried annually, the OC burial is controlled by other sedimentary processes, such as the high mass accumulation rates found in these fjord systems.

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.