Carbon cycling in the North American coastal ocean: a synthesis

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.

Seasonal patterns of surface inorganic carbon system variables in the Gulf of Mexico inferred from a regional high-resolution ocean biogeochemical model

“…Simulated patterns in surface Ar showed maximum values in late summer and minimum in late winter, with most values ranging from 3 to 4.4 units. The meridional and crossshore gradients for model surface Ar are consistent with patterns observed by Gledhill et al (2008). Our model results also agree with observations by Guo et al (2012), Wang et al (2013), and Wanninkhof et al (2015), which showed the most buffered surface waters off the MARS delta during summer.…”

A uniform <i>p</i>CO<sub>2</sub> climatology combining open and coastal oceans

“…Likewise, the nearshore domain in the NN open is influenced by the high CO 2 partial pressures offshore. This data sparsity and spatial heterogeneity is a further challenge for model evaluation (Franco et al, 2018). No steep pCO 2 gradient can be identified along the west coast of Australia in the merged product (Fig.…”

“…Therefore, monitoring the full aquatic continuum is essential in these regions. Both are characterized by strong upwelling and significant offshore transport of carbon-rich water from depth (see, e.g., Lovecchio et al, 2018;Franco et al, 2018) resulting in elevated pCO 2 levels exceeding atmospheric levels at the sea surface. Such values are consistent with observations in the Canary upwelling system (Fig.…”

A uniform <i>p</i>CO<sub>2</sub> climatology combining open and coastal oceans

“…Some of the best monitored regions spanning both coastal and near-shore open ocean can be found along the US coast (Fennel et al, 2008;Laruelle et al, 2015;Fennel et al, 2019). Similarly well monitored to the US east coast is the US west coast upwelling system, not the least because its variability is tightly linked to El Nino Southern Oscillation (see e.g.…”