Limited organic carbon burial by the rusty carbon sink in Swedish fjord
sediments
Abstract
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.