From fjord to shelf: drivers of organic carbon transformation and storage across connected marine sedimentary systems

Continental shelf sediments play a key role in the global carbon cycle through the long-term storage of organic carbon (OC). However, outside of a few continental shelves dominated by large rivers our understanding of the mechanisms that drive the remineralization, burial and storage of OC across these sedimentary systems is underdeveloped. In the mid- to high-latitudes, fjords are a common feature of many coastlines. Here, we focus on the fjords and adjacent continental shelf of the West coast of Scotland, where we have complied a new data set including OC content, bulk isotopic composition (δ13Corg, δ15N and Δ14C), organic matter (OM) reactivity, particle size and mineral surface area of surficial sediments to investigate the dominant processes driving OC storage across the fjord continental shelf transition. This new data combined with spatial modeling identifies five geographically distinct sedimentary depositional regions, each with contrasting region-dependent drivers of OC dynamics and composition. In the outer shelf regions, the sediments are characterized by low quantities of unreactive OC, derived from aged marine sources, while in the fjords, the sediments are characterized by high quantities of reactive OC, derived from modern terrestrial sources of OM. The results demonstrate that fjord–continental shelf transitions impose strong, spatially structured controls on the drivers of OC reactivity and preservation, shaping long-term OC storage potential within these contrasting depositional zones.