Abstract
AbstractGlobal 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.
Highlights
The temperate coastal ocean is an important sink for atmospheric CO2 but changing environmental conditions are likely to alter the effectiveness of this sink in future [Bauer et al, 2013; Regnier et al, 2013]
Limitations and Wider Applications This study presents a detailed observational assessment of the Ekman drain for the Hebrides Shelf and indicates that there is a significant export of organic carbon to the open ocean, as well as significant variability in export fluxes along the shelf break
During October and November 2014, the Hebrides Shelf was a moderate sink for atmospheric CO2, with a mean shelf wide air-sea CO2 flux of 23.2 mmol C m22 d21
Summary
The temperate coastal ocean is an important sink for atmospheric CO2 but changing environmental conditions are likely to alter the effectiveness of this sink in future [Bauer et al, 2013; Regnier et al, 2013]. Continued improvement in observational capability, data synthesis activities, and analytical methodologies, has resulted in a progressive decrease in the magnitude of this sink term (see summary in Chen et al.[ 2013]) with one recent estimate suggesting a global carbon sink in coastal seas of $0.2 Pg C yr21 [Laruelle et al, 2014]. The Hebrides Shelf, to the west of Scotland, makes up the north-western extremity of the NW European Shelf and accounts for $7% of the total area of the NW European Shelf It is, a region of significant exchange of water with the open ocean with models and long-term observations suggesting typical onshore wind-driven exchanges and compensating downwelling offshelf fluxes of $1 Sv [Holt et al, 2009; Huthnance et al, 2009; Huthnance, 2010; Wakelin et al, 2012]. The strong downwelling circulation along the Hebrides Shelf edge, and more widely along the NW European Shelf edge, is considered a vital conduit for the export of carbon to the open ocean but it PAINTER ET AL
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