Abstract
Abstract. A quantification of carbon fluxes in the coastal ocean and across its boundaries with the atmosphere, land, and the open ocean is important for assessing the current state and projecting future trends in ocean carbon uptake and coastal ocean acidification, but this is currently a missing component of global carbon budgeting. This synthesis reviews recent progress in characterizing these carbon fluxes for the North American coastal ocean. Several observing networks and high-resolution regional models are now available. Recent efforts have focused primarily on quantifying the net air–sea exchange of carbon dioxide (CO2). Some studies have estimated other key fluxes, such as the exchange of organic and inorganic carbon between shelves and the open ocean. Available estimates of air–sea CO2 flux, informed by more than a decade of observations, indicate that the North American Exclusive Economic Zone (EEZ) acts as a sink of 160±80 Tg C yr−1, although this flux is not well constrained. The Arctic and sub-Arctic, mid-latitude Atlantic, and mid-latitude Pacific portions of the EEZ account for 104, 62, and −3.7 Tg C yr−1, respectively, while making up 51 %, 25 %, and 24 % of the total area, respectively. Combining the net uptake of 160±80 Tg C yr−1 with an estimated carbon input from land of 106±30 Tg C yr−1 minus an estimated burial of 65±55 Tg C yr−1 and an estimated accumulation of dissolved carbon in EEZ waters of 50±25 Tg C yr−1 implies a carbon export of 151±105 Tg C yr−1 to the open ocean. The increasing concentration of inorganic carbon in coastal and open-ocean waters leads to ocean acidification. As a result, conditions favoring the dissolution of calcium carbonate occur regularly in subsurface coastal waters in the Arctic, which are naturally prone to low pH, and the North Pacific, where upwelling of deep, carbon-rich waters has intensified. Expanded monitoring and extension of existing model capabilities are required to provide more reliable coastal carbon budgets, projections of future states of the coastal ocean, and quantification of anthropogenic carbon contributions.
Highlights
Along ocean margins, the atmospheric, terrestrial, sedimentary, and deep-ocean carbon reservoirs meet, resulting in quantitatively significant carbon exchanges
Carbon export, referring to the flux of organic and inorganic carbon from coastal waters to the deep ocean, can occur through the so-called continental shelf pump – a term coined by Tsunogai et al (1999) after they observed a large uptake of atmospheric CO2 in the East China Sea
Previdi et al (2009) investigated opposite phases of the North Atlantic Oscillation (NAO) and found that the simulated air–sea flux in the Mid-Atlantic Bight (MAB) and Gulf of Maine (GOM) was 25 % lower in a high-NAO year compared to a low-NAO year
Summary
The atmospheric, terrestrial, sedimentary, and deep-ocean carbon reservoirs meet, resulting in quantitatively significant carbon exchanges. This review aims to summarize recent findings with respect to coastal carbon uptake and ocean acidification for the ocean margins of North America (Fig. 1) and was conducted as part of the second State of the Carbon Cycle Report (SOCCR-2). The review builds on and extends several previous activities, including a report by the North American Continental Margins Working Group (Hales et al, 2008), the first State of the Carbon Cycle Report (SOCCR-1; King et al, 2007), and activities within the North American coastal interim synthesis (Benway et al, 2016; Benway and Coble, 2014; Najjar et al, 2012). Along passive margins with broad shelves like the Atlantic coast, the continental shelf is the relevant spatial unit for discussing carbon fluxes. Along active margins with narrow shelves, such as the Pacific coast, a larger region than just the shelf needs to be considered to meaningfully discuss coastal carbon dynamics. We discuss climate-driven trends in coastal carbon fluxes and coastal ocean acidification (see Sect. 4), followed by conclusions
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