Partial pressures of CO
2 in surface seawater (pCO
2sea) and the overlying air (pCO
2air) were measured in the North Pacific (mainly 30-40°N) from January 1999 to October 2000 (9 cruises) onboard the voluntary observation ship MS
Alligator Liberty. Distributions of pCO
2air and pCO
2sea showed that the western North Pacific (west of 180°) acted as a sink for atmospheric CO
2 throughout the year except for August, whereas the eastern North Pacific (east of 160°W) acted as a sink for only half the year (November to May). Total dissolved inorganic carbon (TCO
2) estimated from pCO
2sea and total alkalinity showed a larger seasonal decrease (April to June) in the western North Pacific than in the eastern North Pacific, in accordance with the seasonal decrease in nitrate + nitrite (NO
x = NO
3- + NO
2-). The 9.5 C/N ratio calculated from the relationship between decreased TCO
2 and [NO
x] implied a larger influence of biological activity in the western North Pacific. Using pCO
2sea estimated from the relationship between pCO
2sea and sea surface temperature, we assessed the monthly CO
2 fluxes in the western North Pacific for the year 2000. The fluxes ranged from ca. 0 Gt-C yr
-1 in the summer to ca. -0.4 Gt-C yr
-1 in the winter, with an annual average of -0.2 Gt-C yr
-1, which corresponds to about 10% of the annual oceanic CO
2 uptake over the 1990s.
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