Carbon burial in the mid-latitude fjords of Scotland

Fjord sediments are recognized global hotspots for the burial of organic carbon (OC) and as an integral part of the global carbon (C) cycle. Relative to their spatial extent, more OC is trapped and stored in the sediments of fjords than any other marine sedimentary environment. Until recently, our understanding of the rate at which OC accumulates and is buried in mid-latitude fjord sediments was poor, as these systems have largely been overlooked in favour of their high latitude counterparts. In this study, we quantify and explore the drivers of OC burial in the mid-latitude fjords of Scotland. By examining fifteen sediment cores from ten fjords, it is estimated that on average 57.1 ± 10.9 g C m⁻² yr⁻¹ accumulates in the sediments of Scottish fjords, exceeding observed OC burial in other vegetated fjord systems. When combined with an understanding of the spatial heterogeneity of the fjord sediments, it is estimated that Scottish fjords bury 84,000 t of OC annually, which is equivalent to the whole North Sea sedimentary system, despite the area of the latter being approximately 190 times larger. These findings highlight that mid-latitude fjords play a more significant role in global carbon cycling than previously thought, providing highly effective burial and storage of OC in fjord sediments.