Understanding the Role of Terrestrial and Marine Carbon in the Mid‐Latitude Fjords of Scotland

The sediments within fjords are critical components of the mid‐ to high‐latitude coastal carbon (C) cycle, trapping and storing more organic carbon (OC) per unit area than other marine sedimentary environments. Located at the land‐ocean transition, fjord sediments receive OC from both marine and terrestrial environments; globally, it has been estimated that 55%–62% of the OC held within modern fjord sediments originates from terrestrial environments. However, the mid‐latitude fjords of the Northern Hemisphere have largely been omitted from these global compilations. Here we investigate the mechanism driving the distribution of OC originating from different sources within the sediments of 38 Scottish fjords. From an array of fjord characteristics, the tidal range and outer sill depth were identified as the main drivers governing the proportions of marine and terrestrial OC in the sediments. Utilizing this relationship, we estimate that on average 52% ± 10% of the OC held within the sediments of all Scotland's fjords is terrestrial in origin. These findings show that the Scottish fjords hold equivalent quantities of terrestrial OC as other global fjord systems. However, the analysis also highlights that the sediments within 29% of Scottish fjords are dominated by marine derived OC, which is driven by local fjord geomorphology and oceanography. Plain Language Summary The sediments accumulating in Scottish fjords are rich in organic carbon (OC) and like fjord sediments globally, they are recognized as carbon burial “hotspots” in the coastal environment. While fjords trap and store marine sourced OC, global estimates suggest that they contain significantly more OC derived from terrestrial (catchment) sources. The Scottish fjords are less restricted and have greater connectivity to the wider coastal ocean than many previously studied fjord systems. Though geomorphologically different from other fjords systems, the results show that the majority of OC held within Scottish fjord sediments comes from the terrestrial environment, which is consistent with other global fjord systems. However, in over a quarter of Scottish fjords, geomorphology and local oceanography result in the sediments holding a greater quantity of OC sourced from the marine environment. When compared to other fjords around the world the Scottish systems receive OC from a similar mix of sources and store and bury an equivalent amount of OC. Key Points 28%–72% of the organic carbon (OC) held within