The pCO2 of the Continental Margins

The discussion so far deals with carbon balances rather than the CO2 partial pressure (pCO2) of surface waters. Yet, for surface waters to be a source or sink for atmospheric CO2, their pCO2 must respectively be larger or smaller than atmospheric pCO2. Estuaries are generally supersaturated with CO2 largely as a result of the respiration of organic carbon input from rivers (Frankignoulle et al. 1998; Abril et al. 2002). The shelf systems have not been studied as thoroughly. The first comprehensive study of pCO2 on a large shelf was conducted in the North Sea in May and June 1986 (Kempe and Pegler 1991). According to that study, the North Sea gained 1.4 mol C m-2 y-1. Since that study, many more pCO2 data have become available. Frankignoulle and Borges (2001), for instance, measured pCO2 during 18 cruises in the surface waters of many northwest European shelves. Their results show that these shelves are a sink of 0.09— 0.17 PgC per year. This is an additional, appreciable fraction (45 percent) of the proposed flux for the open North Atlantic Ocean (Keir et al. 2001; Takahashi et al. 2002).

Data on surface water pCO2, temperature, and salinity have been collected over all four seasons in the Yellow Sea and the East China Sea (ECS). These seas are a year-round CO2 sink (Chen and Wang 1999; Tsunogai et al. 1999). Low pCO2 has also been reported for the Bering and Mediterranean Seas, along the Californian coast, the Bay of Bengal, and many other locations (see tables compiled in Chen et al. 2003 and Chen 2003b). Gattuso et al. (1998) compiled all of the available data for coastal ecosystems and concluded that the proximal shelf regions that are directly influenced by the input of terrestrial organic matter are net heterotrophic. That is, they release CO2 into the atmosphere because respiration is greater than biological production. The distal shelves are net autotrophic, owing to the smaller influence of terrestrial inputs and to the larger export of carbon to sediments and across the continental shelf break. The results compiled by Chen et al. (2003) and Chen (2003b) confirm the findings of Gattuso et al. (1998) and show that the global shelves are CO2 sinks.

Proximal

Open Shelves Open Ocean

Coastal

Coastal

Figure 18.2. Organic carbon cycle in global coastal oceans in its preanthropogenic state. The boxes represent the reservoirs, and the arrows represent the fluxes between them. The air-sea fluxes do not include the net flux of CO2 because the carbonate system is not included in the budget (data taken from Rabouille et al. 2001).

Figure 18.2. Organic carbon cycle in global coastal oceans in its preanthropogenic state. The boxes represent the reservoirs, and the arrows represent the fluxes between them. The air-sea fluxes do not include the net flux of CO2 because the carbonate system is not included in the budget (data taken from Rabouille et al. 2001).

The temporal link between high winds and low sea-surface pCO2 leads to a situation in which average sea surface pCO2 can be supersaturated, and the net annual flux is still from the atmosphere to the sea. This situation occurs because the highest fluxes generally occur in winter and spring, during periods of undersaturation when the winds are strong. Exchange rates are lower in summer and fall when surface waters are generally more supersaturated and the winds are weaker (Memery et al. 2002). Many studies assume that an autotrophic system absorbs CO2 from the atmosphere (e.g., Smith and Hollibaugh 1993), but intensive upwelling regions may be autotrophic and still release CO2 to the atmosphere. This process can happen when the pCO2 of shelf waters is reduced as a consequence of a decrease in the ratio of total CO2:alkalinity due to dissolution of relic carbonate deposits and/or increased alkalinity due to sulfate reduction in the sediments (Chen 2002). In a recent study of the East China Sea continental shelf, Tsunogai et al. (1999) suggested that because the shallow seafloor restricts the convection of cooling water, cooling is greater for waters on the continental shelf than for waters in neighboring open oceans. This process leads to a "continental shelf pump," driven by the production of relatively cold and dense water, which, in combination with biological production, increases the absorption of CO2 in the continental shelf zone. Based on a globally distributed sample of 33 shelves and marginal seas, Yool and Fashman (2001) found that the continental shelf pump accounts for a net oceanic uptake of 0.6 PgC y-1. For the situation prior to human effects, Mackenzie and colleagues concluded that, "Before anthropogenic activities, the global coastal ocean was a net autotrophic system with a net export flux to sediments and the open ocean of 20 T mol organic C/yr (0.24 Pg C y-1)" (Rabouille et al. 2001: 3615). These results support the conclusion that the proximal coastal oceans are CO2 sources (0.10 PgC y-1), whereas the distal coastal oceans are CO2 sinks (0.34 PgC y-1) (Figure 18.2; Gattuso et al. 1998).

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