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

Abstract

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 CO 2 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 1 Ekman drain transport confirms that offshelf carbon fluxes via this mechanism are also spatially heterogeneous.

Sources, Sinks, and Subsidies: Terrestrial Carbon Storage in Mid‐latitude Fjords

“…The additional ~90% of missing OC terr could be removed from the system through multiple pathways (Figure ). The primary processes most likely to account for this missing OC terr are (i) some of the OC terr export may be in a dissolved rather than particulate form (Bauer et al, ; Bianchi, ); (ii) some of the eroded soils may not reach the fjord and may be deposited within the catchment (e.g., floodplains) (Wang et al, ); (iii) some of the OC terr may be degraded and lost to the atmosphere during transport in the fluvial system (Dinsmore et al, ; Leith et al, ), within the water column of the fjord (Burt et al, ) and within the sediment itself (Arndt et al, ; Glud et al, ); and (iv) some of the OC terr may be exported further offshore to the continental shelf and beyond (Bischoff et al, ; Haas et al, ; Painter et al, ) bypassing the coastal sediment. It is likely that other fjords may be more efficient traps of OC terr than these results suggest; we know that Loch Sunart does not suffer from periods of water column hypoxia (Gillibrand et al, ) and the absence of this OC preservation mechanism likely results in lower OC terr preservation in comparison to sites with hypoxic conditions (Middelburg & Levin, ; Woulds et al, ).…”

Integrated Observations of Global Surface Winds, Currents, and Waves: Requirements and Challenges for the Next Decade

“…We are currently able to observe the total atmosphere-ocean exchange of CO 2 (e.g., Watson et al, 2009;Woolf et al, 2016), and synoptic scale observations of this exchange require both satellite observations (e.g., sea state, temperature, wind) and in situ observations (e.g., gas concentrations). Existing synoptic scale observations of surface transport predominantly rely upon satellite altimetry or exploit spatially and temporally sparse in situ measurements (e.g., Painter et al, 2016).…”

“…Wind-driven cross-shelf transport is an important mechanism for nearshore-midshelf and shelf-ocean exchange. In broad, shallow shelf seas, cross-shelf transport of water can bring open ocean low nutrient surface waters onto the shelf, and help to force the offshore transport lower in the water column of carbon rich water from the shelf to the deep ocean, sometimes called the "Ekman drain" (Painter et al, 2016). In eastern boundary upwelling systems (Chavez and Messié, 2009) where the mean wind forcing is substantial, upwelling brings nutrient rich, but low pH, low oxygen water to the surface, which can be detrimental to marine ecosystems (Grantham et al, 2004;Chan et al, 2008;Connolly et al, 2010;Siedlecki et al, 2015;Adams et al, 2016).…”

Storm-driven across-shelf oceanic flows into coastal waters

“…Drogued drifters released into the slope current in July 2013 moved on-shelf downstream of a canyon system at 55.5 • N and travelled towards the coast in a coherent current which the authors named the AIC. While on-shelf flow is captured by models of the region (Aleynik et al, 2016;Graham et al, 2018b;Holt et al, 2009;O'Dea et al, 2012;Xing and Davies, 2001;Young and Holt, 2007) and previous drifter studies have provided insight on local circulation (Booth, 1988;Burrows and Thorpe, 1999;Pingree et al, 1999), this study provided the first evidence of a narrow, jet-like current crossing f/ h contours and transporting oceanic water onto the adjacent shelf.…”

“…A topographically steered slope current flows along the shelf edge between Biscay and Norway, becoming increasingly consistent in flow speed and direction north of the Celtic Sea. The stability and persistence of the slope current, particularly north of 55 • N, favour along-slope (poleward) transport and inhibit ocean-shelf exchange Pingree et al, 1999;White and Bowyer, 1997). Despite the reduced cross-shelf flow associated with the slope current, the presence of relatively undiluted oceanic water is detectable many tens of kilometres on-shelf (Inall et al, 2009;Jones et al, 2018; at several persistent locations along the shelf.…”