We investigated the seasonal changes and the interactions of the CO 2 system in a marsh-dominated estuary, the Duplin River, on Sapelo Island, Georgia. Surface water pCO 2 , total alkalinity (TA), and total dissolved inorganic carbon (DIC) showed a pronounced seasonal progression in this system. The estuary released 256-306 g C m Ϫ2 (of water) yr Ϫ1 of CO 2 to the atmosphere and exported 109 g C m Ϫ2 (of water) yr Ϫ1 of DIC to adjacent coastal waters. There was a clear seasonal pattern in both fluxes with spring minima, and late summer and fall maxima. Release of inorganic carbon as a respiratory product from surrounding salt marshes is the primary process supporting these two carbon export terms. Concurrently, TA was exported from the Duplin River mainly in summer and fall as a result of anaerobic respiration in marshes. Based on data from the upper Duplin River, export of inorganic carbon from salt marshes was 156 g C m Ϫ2 (of marsh) yr Ϫ1 . Extrapolation of this value to all southeastern U.S. salt marshes indicates that they cumulatively export 0.7 ϫ 10 12 g inorganic carbon annually to coastal waters. This export rivals that of riverine inorganic carbon flux. We propose that the CO 2 fixation of marsh grasses and the subsequent export of inorganic and organic carbon is one major mechanism that causes the marsh-influenced nearshore and offshore waters in the southeastern U.S. to be annual net sources of atmospheric CO 2 . We also suggest that this process provides an efficient and unique means for ocean carbon sequestration of atmospheric CO 2 .The marsh-influenced estuaries, tidal creeks, and coastal waters adjacent to Sapelo Island, Georgia, have been the subjects of pioneer ecological studies on marsh-water interaction since the 1950s (e.g., Ragotzkie and Bryson 1955;Teal 1962;Odum 1968). Based on studies in the Sapelo marshes, it was proposed that intertidal marshes are a net source of organic matter and nutrients that fertilize adjacent estuarine and coastal waters (Teal 1962;Odum 1968). For decades this controversial outwelling hypothesis has fascinated the scientific community and triggered intensive debate (e.g., Nixon 1980;Wiegert et al. 1981;Chalmers et al. 1985;Hopkinson 1985; Dame et al. 1986;Dame 1994;Childers et al. 2000). The current paradigm is that marsh-estuary interaction of a particular system is inherently complicated and depends on climatological, biogeochemical, and hydrological factors such as geological age, developmental status, and tidal range (Childers 1994;Dame 1994 stimulus of ideas and not as a strict statistical hypothesis that must be proven or disproven '' (Childers et al. 2000).Until recently, inorganic carbon had not been a major topic in the discussion of the outwelling hypothesis. However, the concentrations, temporal changes, and spatial variations of total dissolved inorganic carbon (DIC) and surface water pCO 2 provide critical data for revealing how these systems function at the biogeochemical level (Gattuso et al. 1998;Cai et al. 2000). Not only do inorganic ...

Distributions of total alkalinity (TA), dissolved inorganic carbon (DIC), and other parameters relevant to the marine inorganic carbon system were investigated in shelf and adjacent ocean waters during a U.S. Gulf of Mexico and East Coast Carbon cruise in July-August 2007. TA exhibited near-conservative behavior with respect to salinity. Shelf concentrations were generally high in southern waters (Gulf of Mexico and East Florida) and decreased northward from Georgia to the Gulf of Maine. DIC was less variable geographically and exhibited strongly nonconservative behavior. As a result, the ratio of TA to DIC generally decreased northward. The spatial patterns of other CO 2 system parameters closely followed those of the TA : DIC ratio. All sampled shelf waters were supersaturated with respect to aragonite (saturation state V A . 1). The most intensely buffered and supersaturated waters (V A . 5.0) were in northern Gulf of Mexico river-plume waters; the least intensely buffered and least supersaturated waters (V A , 1.3) were in the deep Gulf of Maine. Due to their relatively low pH, V A , and buffer intensity, waters of the northeastern U.S. shelves may be more susceptible to acidification pressures than are their southern counterparts. In the Mid-Atlantic Bight, alongshore mixing tended to increase DIC concentrations southward, but this effect was largely offset by the opposing effects of biogeochemical processing. In the Gulf of Mexico, downstream increases in Loop Current DIC suggested significant contributions from shelf and gulf waters, estimated at 9.1 3 10 9 mol C d 21 . Off the southeastern U.S., along-flow chemical changes in the Florida Current were dominated by mixing associated with North Atlantic subtropical recirculation.

[1] Underway and in situ observations of surface ocean pCO 2 , combined with satellite data, were used to develop pCO 2 regional algorithms to analyze the seasonal and interannual variability of surface ocean pCO 2 and sea-air CO 2 flux for five physically and biologically distinct regions of the eastern North American continental shelf: the South Atlantic Bight (SAB), the Mid-Atlantic Bight (MAB), the Gulf of Maine (GoM), Nantucket Shoals and Georges Bank (NSþGB), and the Scotian Shelf (SS). Temperature and dissolved inorganic carbon variability are the most influential factors driving the seasonality of pCO 2 . Estimates of the sea-air CO 2 flux were derived from the available pCO 2 data, as well as from the pCO 2 reconstructed by the algorithm. Two different gas exchange parameterizations were used. The SS, GBþNS, MAB, and SAB regions are net sinks of atmospheric CO 2 while the GoM is a weak source. The estimates vary depending on the use of surface ocean pCO 2 from the data or algorithm, as well as with the use of the two different gas exchange parameterizations. Most of the regional estimates are in general agreement with previous studies when the range of uncertainty and interannual variability are taken into account. According to the algorithm, the average annual uptake of atmospheric CO 2 by eastern North American continental shelf waters is found to be between À3.4 and À5.4 Tg C yr À1 (areal average of À0.7 to À1.0 mol CO 2 m À2 yr À1 ) over the period

Dynamic tidal export of dissolved inorganic carbon (DIC) to the coastal ocean from highly productive intertidal marshes and its effects on seawater carbonate chemistry are thoroughly evaluated. The study uses a comprehensive approach by combining tidal water sampling of CO 2 parameters across seasons, continuous in situ measurements of biogeochemically-relevant parameters and water fluxes, with high-resolution modeling in an intertidal salt marsh of the U.S. northeast region. Salt marshes can acidify and alkalize tidal water by injecting CO 2 (DIC) and total alkalinity (TA). DIC and TA generation may also be decoupled due to differential effects of marsh aerobic and anaerobic respiration on DIC and TA. As marsh DIC is added to tidal water, the buffering capacity first decreases to a minimum and then increases quickly. Large additions of marsh DIC can result in higher buffering capacity in ebbing tide than incoming tide. Alkalization of tidal water, which mostly occurs in the summer due to anaerobic respiration, can further modify buffering capacity. Marsh exports of DIC and alkalinity may have complex implications for the future, more acidified ocean. Marsh DIC export exhibits high variability over tidal and seasonal cycles, which is modulated by both marsh DIC generation and by water fluxes. The marsh DIC export of 414 g C m 22 yr 21 , based on highresolution measurements and modeling, is more than twice the previous estimates. It is a major term in the marsh carbon budget and translates to one of the largest carbon fluxes along the U.S. East Coast.

Recent air‐to‐sea CO2 flux measurements at several major continental shelves suggest that shelves may act as a one‐way pump and absorb atmospheric CO2 into the ocean. The U.S. South Atlantic Bight (SAB) contrasts these findings in that it acts as a source of CO2 to the atmosphere while simultaneously exporting dissolved inorganic carbon (DIC) to the open ocean. The shelf‐wide heterotrophy and carbon exports in the SAB are subsidized by the export of organic carbon from the abundant intertidal marshes, which are a sink for atmospheric CO2. It is proposed here that the SAB represents a marsh‐dominated heterotrophic ocean margin as opposed to river‐dominated autotrophic margins. Based on this and other studies, DIC export flux from margins to the open ocean must be significant in the overall global ocean carbon budget.