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Quantifying marine sedimentary carbon stocks is key to improving our understanding of long-term storage of carbon in the coastal ocean and to further constraining the global carbon cycle. Here we present a methodological approach which combines seismic geophysics and geochemical measurements to quantitatively estimate the total stock of carbon held within marine sediment. Through the application of this methodology to Loch Sunart, a fjord on the west coast of Scotland, we have generated the first full sedimentary carbon inventory for a fjordic system. The sediments of Loch Sunart hold 26.9 ± 0.5 Mt of carbon split between 11.5 ± 0.2 and 15.0 ± 0.4 Mt of organic and inorganic carbon respectively. These new quantitative estimates of carbon stored in coastal sediments are significantly higher than previous estimates. Through an area-normalised comparison to adjacent Scottish peatland carbon stocks, we have determined that these mid-latitude fjords are significantly more effective as carbon stores than their terrestrial counterparts. This initial work supports the concept that fjords are important environments for the burial and long-term storage of carbon and therefore should be considered and treated as unique environments within the global carbon cycle.

Continental shelf sediments are recognized as long-term stores of globally significant quantities of carbon (C) and potentially provide an important, yet largely overlooked climate regulation service via the Earth’s C cycle. Current understanding of the spatial distribution of sedimentary C across continental shelves remains poor, inhibiting the targeted management and potential inclusion of these globally significant C stores into national C budgets. Further understanding of the spatial heterogeneity of continental shelf sediments and associated C provides a foundation to quantify the organic carbon (OC) stock and better understand the role that marine sediments play in regulating the global climate and the potential for CO2 to be released through anthropogenic disturbance of these C stores. Utilizing a spectrum of available marine data, we have created bespoke sediment maps that quantify the surficial (top 10 cm) OC stock and highlight significant spatial heterogeneity in the distribution of sediments and their associated C content across the United Kingdom’s Exclusive Economic Zone (EEZ). The surficial sediments within the UK EEZ are estimated to store 524 ± 68 Mt of organic carbon (OC) and 2,582 ± 168 Mt of inorganic carbon (IC). The spatial mapping of this C highlights well-defined OC accumulation hotspots in fjords, estuaries and coastal muds, while large accumulations of IC are found in the tidally swept areas around Orkney, Shetland and the South West of England. Within the well-defined OC hotspots, muddy sediments store the greatest quantity of OC; the muds offer potentially valuable opportunities for targeted future management and protection of sedimentary C stores within the UK EEZ. In the future, if areas of the seafloor were to be managed to include the protection of these valuable sedimentary C resources, we recommend an initial focus on hotspots of high sedimentary OC density.

Fjords are recognized as globally important sites for the burial and long‐term storage of carbon (C) within sediments. The proximity of fjords to the terrestrial environment in combination with their geomorphology and hydrography results in the fjordic sediments being subsidized with organic carbon (OC) from the terrestrial environment. It has been well documented that terrestrial OC (OCterr) is an important component of coastal sediments, yet our understanding of the quantity of OCterr stored in these sediments remains poorly constrained. Utilizing Bayesian isotopic sediment fingerprinting techniques to the surface sediments of Loch Sunart, we estimate that 42.0 ± 10.1% of the OC is terrestrial in origin. Through combining these outputs with sedimentary OC stock estimates, we have calculated that the surface sediments (0–15 cm) hold 0.1 megaton (Mt) OCterr and estimate that the postglacial sediment held within the fjord contains 3.96 Mt OCterr. When these totals are compared to the quantity of OC stored in the adjacent terrestrial environment, it is clear that the fjord's catchment stores a greater amount of OCterr in the form of vegetation and soil. Though when normalized for area the results suggest that the marine sediments are a more effective long‐term store of OCterr than the adjacent terrestrial environment. This striking result highlights the importance of the terrestrial environment as a source of OC to the coastal ocean and that the OCterr subsidy to the marine sediments is a significant mechanism for the long‐term storage of OC in coastal marine sediments.

Abstract. Fjords are recognised as hotspots for the burial and long-term storage of carbon (C) and potentially provide a significant climate regulation service over multiple timescales. Understanding the magnitude of marine sedimentary C stores and the processes which govern their development is fundamental to understanding the role of the coastal ocean in the global C cycle. In this study, we use the mid-latitude fjords of Scotland as a natural laboratory to further develop methods to quantify these marine sedimentary C stores on both the individual fjord and national scale. Targeted geophysical and geochemical analysis has allowed the quantification of sedimentary C stocks for a number of mid-latitude fjords and, coupled with upscaling techniques based on fjord classification, has generated the first full national sedimentary C inventory for a fjordic system. The sediments within these mid-latitude fjords hold 640.7 ± 46 Mt of C split between 295.6±52 and 345.1 ± 39 Mt of organic and inorganic C, respectively. When compared, these marine mid-latitude sedimentary C stores are of similar magnitude to their terrestrial equivalents, with the exception of the Scottish peatlands, which hold significantly more C. However, when areanormalised comparisons are made, these mid-latitude fjords are significantly more effective as C stores than their terrestrial counterparts, including Scottish peatlands. The C held within Scotland's coastal marine sediments has been largely overlooked as a significant component of the nation's natural capital; such coastal C stores are likely to be key to understanding and constraining improved global C budgets. Highlights-Scottish fjords are a more effective store of C than the terrestrial environment.-A total of 640.7 ± 46 Mt of C is stored in the sediment of Scotland's 111 fjords.-An estimated 31 139-40 615 t yr −1 of C is buried in the sediment of Scotland's fjords.-Fjord sediments are potentially the most effective store of C globally.