The Mass Balances

The most recent Joint Global Ocean Flux Study (JGOFS) synthesis, however, suggests that the marginal seas are sinks of CO2 (Chen et al. 2003). This synthesis, with slight modifications to reflect some recent developments, indicates that the global average new production of phytoplankton on the shelf is 0.78 Pg y-1 particulate organic carbon (POC) and 0.25 Pg y-1 particulate inorganic carbon (PIC) (Figure 18.1, Table 18.1). This new production is only 13 percent of the average primary production rate and is mostly a result of upwelling (Chavez and Toggweiler 1995; Liu et al. 2000; Chen 2000, 2003a). Although net community productivity is rather high on global shelves (Lee 2001), only about 0.2 PgC y-1 of PIC and the same amount of POC out of the total production (0.48 Pg y-1 PIC and 6.2 Pg y-1 POC) is buried and stored on the shelf. Downslope transport of modern particulate carbon is 0.5 PgC y-1, 58 percent of which is organic (Chen et al. 2003). Results from major programs on the eastern United States, western European, East China Sea, and Mediterranean continental shelves all show that most of the biogenic particulate matter is remineralized over the shelves. Only a small proportion (< 8 percent) is exported to the adjacent slopes (Wollast and Chou 2001; de Haas et al. 2002; Chen 2003b), but higher export ratios have occasionally been reported.

The downslope transport of POC is only 23 percent of the offshore transport of dissolved organic carbon (DOC). Recent studies based on 13C, 14C, and 15N show that a large portion of the off-shelf transport of POC may be old terrestrial or relic matter.

Table 18.1. Fluxes relevant to continental margins

Values

Table 18.1. Fluxes relevant to continental margins

Values

Rivers plus groundwater and ice

• DIC: 32 (1, 6)

• DOC: 30 (1), 27 (6)

• PIC: 15 (1, 6)

• POC: 20 (1), 18 (6)

• IC: 37 (7), 13 (9)

• OC: 34 (2), 30 (7), 31 (8)

Air-to-sea (gaseous)

• CO2: 25 (1), 20 (2), 49 (3), 46-75 (4), 30 (6),

8.3 (7), 62 (10), 83 (11);

• CH4: -0.1 (6);

• DMS: -0.07 (6)

Precipitation plus dust

• PIC: 0.3 (1, 6); OC: 0.2 (8)

Net burial plus fish catch

• PIC: 15 (1, 6, 9), 14.5 (9); POC: 15 (1, 6, 9),

14 (2)

• Total: 12.5 (7)

Gross upwelling plus surface inflow

• DIC: 2800 (1), 2827 (6);

• DOC: 80 (1), 70 (6);

• POC: 4 (1, 6)

Downslope export of particulates

• PIC: 20 (1, 6, 9);

• POC: 20 (1, 9), 27 (6);

• Total: 167 (7)

Gross surface water outflow

• DIC: 2800 (1,6);

• DOC: 120 (1, 6);

• PIC: 1.0 (1, 6);

• POC: 12 (1), 22 (6);

• Net: 58 (7)

Gross offshelf export (downslope

• DIC: 2800 (1, 6);

+ surface outflow)

• DOC 120 (1, 6);

• PIC: 21 (1, 6);

• POC 32 (1), 49 (6);

• Net: 225 (7)

Net offshore export (downslope +

• DIC: 0 (1), -27 (6);

surface outflow-upwelling plus

• DOC: 40 (1), 50 (6), 33 (13);

surface inflow)

• PIC: 21 (1, 6);

• POC: 28 (1), 45 (6);

• IC 4 (9);

• OC: 40 (2), 38 (12);

• Total: 58 (7)

(continued)

Table 18.1. (continued)

Category Values

Primary productivity

• PIC: 40 (1, 6, 9), 24.5 (9);

• POC: 516 (1, 6,9);

• OC: 368 (2), 789 (5);

• Total: 830 (7)

New productivity

• DOC: 23 (6);

• PIC: 6 (1), 21 (6);

• POC: 75 (1), 42 (6);

• OC: 43 (2), 231 (5), 158 (13),

• Total: 167 (7)

f-ratio

• 0.15 (1), 0.12 (2), 0.29 (5), 0.2 (7), 0.13 (6)

Note: All values except f-ratio are in 1012 moles C y-1; numbers in parentheses are reference numbers.

Traditionally these values are given in moles C and are not converted to PgC.

Sources: 1. Figure 17 and Table 8 of Chen et al. (2003) and the 27 references therein; 2. Rabouille et al. (2001); 3. Yool and Fasham (2001); 4. extrapolated from data on the European shelves by Frankignoulle and Borges (2001); 5. Gattuso et al. (1998); 6. this study; 7. Liu et al. (2000);

8. Smith et al. (2001); 9. Milliman (1993) and Wollast (1994) ; 10. Walsh and Dieterle (1994);

11. Tsunogai et al. (1999); 12. Alvarez-Salgado et al. (2001a) and 13. Hansell and Carlson (1998).

Precipitation

& Dust Gaseous

Figure 18.1. Schematic diagram for the annual carbon budget (in 1012 mol PgC y-1) for the continental margins of the world (modified from Chen et al. 2003)

There have been reports of high PIC contents on the slopes (Chen 2002; Epping et al. 2002), but whether or not these are relic is unknown (Bauer et al. 2001). Net off-shelf transport of DOC is 0.60 PgC y-1, about twice terrestrial input of 0.32 PgC y-1 (Figure 18.1). The remaining 0.28 Pg y-1 DOC is produced on the shelves and represents 35 percent of new organic carbon production, or 27 percent of total new carbon production, a finding that is consistent with the results of Hansell and Carlson (1998).

Marginal seas are also sources of CH4 and DMS (Sharma et al. 1999; Marty et al. 2001). For CH4, shelf sediments are likely to be the principal source, whereas for DMS, biological production in the water column is probably the main source. These fluxes are small (Figure 18.1) compared with the very large CO2 fluxes, but they play an important role, based on their activity in absorbing solar energy and reactivity in the atmosphere.

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