Limited Organic Carbon Burial by the Rusty Carbon Sink in Swedish Fjord Sediments

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 plays 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.