It has been speculated that the partial pressure of carbon dioxide (pCO2) in shelf waters may lag the rise in atmospheric CO2. Here, we show that this is the case across many shelf regions, implying a tendency for enhanced shelf uptake of atmospheric CO2. This result is based on analysis of long-term trends in the air–sea pCO2 gradient (ΔpCO2) using a global surface ocean pCO2 database spanning a period of up to 35 years. Using wintertime data only, we find that ΔpCO2 increased in 653 of the 825 0.5° cells for which a trend could be calculated, with 325 of these cells showing a significant increase in excess of +0.5 μatm yr−1 (p < 0.05). Although noisier, the deseasonalized annual data suggest similar results. If this were a global trend, it would support the idea that shelves might have switched from a source to a sink of CO2 during the last century.

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Recent interest in the ocean's capacity to absorb atmospheric CO2 and buffer the accompanying “ocean acidification” has prompted discussions on the magnitude of ocean margin alkalinity production via anaerobic processes. However, available estimates are largely based on gross reaction rates or misconceptions regarding reaction stoichiometry. In this paper, we argue that net alkalinity gain does not result from the internal cycling of nitrogen and sulfur species or from the reduction of metal oxides. Instead, only the processes that involve permanent loss of anaerobic remineralization products, i.e., nitrogen gas from net denitrification and reduced sulfur (i.e., pyrite burial) from net sulfate reduction, could contribute to this anaerobic alkalinity production. Our revised estimate of net alkalinity production from anaerobic processes is on the order of 4–5 Tmol yr−1 in global ocean margins that include both continental shelves and oxygen minimum zones, significantly smaller than the previously estimated rate of 16–31 Tmol yr−1. In addition, pyrite burial in coastal habitats (salt marshes, mangroves, and seagrass meadows) may contribute another 0.1–1.1 Tmol yr−1, although their long‐term effect is not yet clear under current changing climate conditions and rising sea levels. Finally, we propose that these alkalinity production reactions can be viewed as “charge transfer” processes, in which negative charges of nitrate and sulfate ions are converted to those of bicarbonate along with a net loss of these oxidative anions.

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In this study we estimate sediment carbonate dissolution rates for sandy sea grass sediments on the Bahamas Bank using an inverse pore-water advection/diffusion/reaction model constrained by field observations. This model accounts for sea grass O 2 input to these sediments, and also parameterizes pore-water advection through these permeable sediments as a nonlocal exchange process. The resulting rates of carbonate dissolution are positively correlated with sea grass density, and are comparable with previous rate estimates for Florida Bay sediments. In contrast, the advective uptake of O 2 by these sediments decreased with increasing sea grass density. This suggests that the competing interplay between bottom-water flow, near-seabed pressure gradients, and the presence of a sea grass canopy is important in controlling this type of sediment oxygen uptake. When the carbonate dissolution rates estimated here are examined in the context of carbonate budgets for shallow-water carbonate platforms systems, they suggest that carbonate dissolution may be a significant loss term in these budgets. Sea grass-mediated carbonate dissolution may also exert a negative feedback on rising atmospheric CO 2 , although the magnitude of this effect remains to be quantified.In shallow water environments such as the Bahamas Bank the production of carbonate sediments (both sands and muds) occurs by biogenic and inorganic precipitation (Macintyre and Reid 1992;Milliman 1993;Broecker et al. 2001;. Once formed, this material can be altered by reactions in the water column (e.g., during resuspension), on the sediment surface, and during burial (also see Winland and Matthews 1974;Melim et al. 2002;Morse 2003).The ultimate fate of this carbonate (i.e., net accumulation vs. export or dissolution) is, however, not well constrained. In particular, carbonate dissolution is a poorly quantified component of shallow-water bank and shelf carbonate budgets (Milliman 1993), although studies conducted in Florida Bay (U.S.A.) suggest that carbonate dissolution may be comparable in magnitude with the assumed offshore export of carbonate from such shallowwater environments (Walter and Burton 1990;Ku et al. 1999;Yates and Halley 2006). The role of shallow-water sediment carbonate dissolution as a sink for rising levels of atmospheric CO 2 has also previously been examined by Andersson et al. (2003), although these authors concluded that this process represents an insignificant buffer to this CO 2 increase.Past studies have described the occurrence of carbonate dissolution in shallow-water sediments using chemical, isotopic, and mineralogical techniques (e.g., Berner 1966; Moulin et al. 1985; Morse et al. 1987). However, rates of carbonate dissolution in the context of other sediment biogeochemical processes are less well quantified (also see Morse et al. 2003 for a review). Furthermore, previous attempts to generate carbonate dissolution budgets for shallow-water carbonate sediments have generally not been closed with respect to the observed amount o...