Carbon cycling in the North American coastal ocean: A synthesis

Abstract

<p><strong>Abstract.</strong> A quantification of carbon fluxes in the coastal ocean and across its boundaries, specifically the air-sea, land-to-coastal-ocean and coastal-to-open-ocean interfaces, is important for assessing the current state and projecting future trends in ocean carbon uptake and coastal ocean acidification, but is currently a missing component of global carbon budgeting. This synthesis reviews recent progress in characterizing these carbon fluxes with focus on the North American coastal ocean. Several observing networks and high-resolution regional models are now available. Recent efforts have focused primarily on quantifying net air-sea exchange of carbon dioxide (CO<sub>2</sub>). 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 CO<sub>2</sub> flux, informed by more than a decade of observations, indicate that the North American margins act as a net sink for atmospheric CO<sub>2</sub>. This net uptake is driven primarily by the high-latitude regions. The estimated magnitude of the net flux is 160 ± 80 Tg C/y for the North American Exclusive Economic Zone, a number that is not well constrained. The increasing concentration of inorganic carbon in coastal and open-ocean waters leads to ocean acidification. As a result conditions favouring 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 and, in combination with the uptake of anthropogenic carbon, leads to low seawater pH and aragonite saturation states during the upwelling season. 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.</p>

Time of Emergence of Surface Ocean Carbon Dioxide Trends in the North American Coastal Margins in Support of Ocean Acidification Observing System Design

“…ToE, calculated with the forced trend (ToE forced ), represent the timescales for the anthropogenic CO 2 signal, only to become detectable out of internal variability (McKinley et al, 2016) assuming no other secular forcing trends are acting on the system. However, it may be different from the calculated ToE based on observed trends (ToE obs ), given that many other processes in addition to atmospheric CO 2 forcing may affect the ocean margins (Fennel et al, 2018). To better understand the combined effect from both atmospheric CO 2 increase and other forcing, we calculated both ToE forced and ToE obs for f (CO 2,sw ) in all 0.5 • × 0.5 • grids in the North American coastal margins using SOCAT coastal data V5 (Bakker et al, 2016) and the GAMM method.…”

An Enhanced Ocean Acidification Observing Network: From People to Technology to Data Synthesis and Information Exchange

“…Other coastal regions exhibit more rapid increases in pCO 2 relative to the open ocean, indicating more rapid acidification due to the additive effects of CO 2 uptake and increased upwelling (Chavez et al, 2017). Coastal seas have been suggested to have changed in the recent past from a net source to a net sink (Bauer et al, 2013;Fennel et al, 2018;Laruelle et al, 2018). The enhanced uptake of CO 2 by the ocean and shelves also changes the rate at which waters acidify, altering local rates of acidification, a process not well simulated by coarse global simulation models nor adequately captured by many direct measurements from the existing observing system.…”

“…In addition to variability in time, the rates of uptake of CO 2 from the atmosphere also vary spatially, especially in coastal and shelf seas (Fennel et al, 2018;Laruelle et al, 2018). The magnitude of the sink of carbon has been shown to vary latitudinally, with high latitude (north of 30 • ) coastal seas providing a sink while low latitude shelves are generally a source or neutral (Cai et al, 2006;Bauer et al, 2013;Chen et al, 2013).…”

Improved representation of river runoff in Estimating the Circulation and Climate of the Ocean Version 4 (ECCOv4) simulations: implementation, evaluation and impacts to coastal plume regions

“…Coastal plume regions represent a small fraction of Earth's surface, and played as an active component in global cycling of carbon and nutrients (Bourgeois et al, 2016;Carroll et al, 2020;Fennel et al, 2019;Lacroix et al, 2020;Landschützer et al, 2020;Roobaert et al, 2019). Recent satellite-based observations with quasi-global coverage has been greatly improved to monitor Sea Surface Salinity, a key tracer for tracking the river plumes.…”