Elucidating Coastal Ocean Carbon Transport Processes: A Novel Approach Applied to the Northwest North Atlantic Shelf

Abstract

A latitudinal pattern in coastal air‐sea CO2 flux has emerged where mid‐and high‐latitude shelves act as net sinks and low‐latitude shelves as net sources to the atmosphere. Regional studies, however, report the mid‐latitude Scotian Shelf (SS) at the eastern Canadian seaboard acts as a large source of CO2, contradicting several global syntheses. Here, we combine observations and a regional biogeochemical model to explain, for the first time, how this net outgassing of CO2 is sustained. We employ a novel approach using passive dye tracers to estimate how carbonate properties change along dominant transport pathways. We show that cold, carbon‐rich subpolar North Atlantic water is a dominant endmember that warms and combines with low alkalinity (TA), carbon‐deplete freshwater from the Gulf of St. Lawrence, becoming oversaturated with CO2 on the SS. Our approach explicitly considers the 3‐dimensional nature of coastal ocean transport processes and should be applied to other shelf regions.

A Synthesis of Global Coastal Ocean Greenhouse Gas Fluxes

“…The small number of models in that subset (4) makes any statistical argument about resolution difficult. Yet, this result suggests that a better representation of fine scale coastal dynamics improves the representation of the CO 2 flux, likely by improving the representation of the physical and biogeochemical processes controlling CO 2 seasonality in the northern hemisphere (Laurent et al., 2021; Rutherford et al., 2021; Rutherford & Fennel, 2022). The horizontal resolution of the global ocean biogeochemical models used in this synthesis (about 1/4° or coarser) is, however, still too coarse to fully capture coastal ocean dynamics (coastal oceans require resolutions of 1/16° or higher, Hallberg, 2013).…”

“…Polar and subpolar coastal oceans, such as the northwest North Atlantic along the Canadian and US coast (Cahill et al., 2016; Fennel & Wilkin, 2009; Gustafsson et al., 2019; Lachkar & Gruber, 2013; Laruelle et al., 2015; Previdi et al., 2009; Signorini et al., 2013; Thomas et al., 2004), the European shelves (Cossarini et al., 2015; Gustafsson et al., 2019; Neumann et al., 2022; Thomas et al., 2004) and Arctic and Antarctic shelf (Arrigo et al., 2008; Ouyang et al., 2022; Pipko et al., 2017, 2021) generally are strong sinks of CO 2 characterized by large seasonal variations, and likely account for about 90% of the annual global coastal CO 2 uptake (while representing ∼45% of the global coastal surface area, see Dai et al., 2022; Laruelle et al., 2014; Roobaert et al., 2019). There are exceptions to subpolar and polar shelves where outgassing has been identified, such as the Scotian Shelf (Rutherford et al., 2021; Rutherford & Fennel, 2022) or the Laptev Sea in the Arctic (Anderson et al., 2009). Coastal upwelling regions, such as the nearshore California Current, are sources of CO 2 to the atmosphere with a marked seasonality that follows the upwelling dynamics (Dai et al., 2013; Damien et al., 2023; Fiechter et al., 2014; Lachkar & Gruber, 2013; Turi et al., 2014).…”

Ocean Carbon Dioxide Uptake in the Tailpipe of Industrialized Continents

“…Rutherford and Fennel (2022) calculated air‐sea fluxes over the Scotian Shelf. If they had used zonal mean $$\chi {\mathrm{CO}}_{2}^{\text{atm}}$$, the regional ocean uptake would have been underestimated by 4.9% for the year 2014 (Table 2), with stronger underestimated uptake in winter.…”

“…The bias is largest closer to land and can be as high as 30% of the flux estimate for a single day (Figure 2), highlighting the need to use an appropriate atmospheric product for local process studies or atmospheric χ CO 2 whenever it is measured along with surface ocean values. Many process studies make use of the nearest location where discrete atmospheric χ CO 2 is measured (e.g., Andersson et al., 2013; Liu et al., 2022; Rutherford & Fennel, 2022). This approach may improve the flux estimates relative to the use of a zonal mean, depending on the proximity to a measurement location.…”

“…Figure 4 shows two salient examples: on the Scotian Shelf east of Canada and in the East China Sea. Rutherford and Fennel (2022) calculated air-sea fluxes over the Scotian Shelf. If they had used zonal mean 𝐴𝐴 𝐴𝐴CO atm 2 , the regional ocean uptake would have been underestimated by 4.9% for the year 2014 (Table 2), with stronger underestimated uptake in winter.…”