Carbon exchange between a shelf sea and the ocean: The Hebrides Shelf, west of Scotland

Global mass balance calculations indicate the majority of particulate organic carbon (POC) exported from shelf seas is transferred via downslope exchange processes. Here we demonstrate the downslope flux of POC from the Hebrides Shelf is approximately 3‐ to 5‐fold larger per unit length/area than the global mean. To reach this conclusion, we quantified the offshore transport of particulate and dissolved carbon fractions via the “Ekman Drain,” a strong downwelling feature of the NW European Shelf circulation, and subsequently compared these fluxes to simultaneous regional air‐sea CO2 fluxes and onshore wind‐driven Ekman fluxes to constrain the carbon dynamics of this shelf. Along the shelf break, we estimate a mean offshelf total carbon (dissolved + particulate) flux of 4.2 tonnes C m−1 d−1 compared to an onshelf flux of 4.5 tonnes C m−1 d−1. Organic carbon represented 3.3% of the onshelf carbon flux but 6.4% of the offshelf flux indicating net organic carbon export. Dissolved organic carbon represented 95% and POC 5% of the exported organic carbon pool. When scaled along the shelf break the total offshelf POC flux (0.007 Tg C d−1) was found to be 3 times larger than the regional air‐sea CO2 ingassing flux (0.0021 Tg C d−1), an order of magnitude larger than the particulate inorganic carbon flux (0.0003 Tg C d−1) but far smaller than the DIC (2.03 Tg C d−1) or DOC (0.13 Tg C d−1) fluxes. Significant spatial heterogeneity in the Ekman drain transport confirms that offshelf carbon fluxes via this mechanism are also spatially heterogeneous. Key Points Cross‐shelf transports and carbon fluxes are reported for the Hebrides Shelf The mean Ekman drain downslope transport was 1.81 m2 s−1, but important 4‐fold spatial variability was identified The Hebrides Shelf may export 5 times more POC per unit area via the Ekman drain than the global mean