The southeastern continental shelf of the United States as an atmospheric CO2 source and an exporter of inorganic carbon to the ocean

The US southeastern continental shelf, also known as the South Atlantic Bight (SAB), is a strong source of CO2 to the atmosphere, which is in direct contrast to recent reports regarding other major continental shelves. Both spatial (cross-shelf) and seasonal variations of the CO2 system were pronounced in the SAB. Sea surface pCO2 in winter was undersaturated relative to the atmosphere, while oversaturation of pCO2 dominated the entire shelf water in all other seasons. Annually, the SAB releases CO2 to the atmosphere at an average rate of 30 g C m−2 (2.5 mol C m−2). This system also discharges dissolved inorganic carbon to the open ocean (30 g C m−2 yr−1). Methods of estimating CO2 flux and DIC flux are critically evaluated and compared. A carbon mass balance model in the SAB is presented based on inorganic carbon fluxes from this study and organic carbon fluxes from literature. The carbon budget is much closer to balance by using this carbon flux approach than by direct measurements of primary production and respiration. It is concluded that the SAB is a net heterotrophic system annually. Intensified heating, elevated input of inorganic carbon from coastal salt marshes, microbial respiration of marsh-exported organic carbon and the lack of annual spring blooms all contribute to maintaining the SAB as a strong CO2 source to the atmosphere during the warm seasons. In winter, the primary factor that governs the CO2 sink in the SAB is likely the cooling process. Strong heterotrophy during warm seasons also sustains the SAB to be an exporter of inorganic carbon to the open ocean annually. The SAB shelf functions differently from the East China Sea, the North Atlantic European Shelves, and the Mid-Atlantic Bight as a source or sink of atmospheric CO2. The SAB is classified as a “marsh-dominated” shelf as compared to other shelves in terms of carbon dynamics. Further work to study carbon dynamics in coastal margins is warranted to interpret their roles in the global CO2 budget.