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Seasonal, latitudinal, and secular variations in the abundance and isotopic ratios of atmospheric CO 2 : 2. Results from oceanographic cruises in the tropical Pacific Ocean

1984; American Geophysical Union; Volume: 89; Issue: D3 Linguagem: Inglês

10.1029/jd089id03p04615

2156-2202

Charles D. Keeling, Alane F. Carter, Willem G. Mook,

Climate variability and models

Between March 1979 and June 1980, air was collected on the Hawaii‐Tahiti shuttle expedition of the Global Atmospheric Research Program. A north‐south track was repeatedly sampled and the air analyzed to provide eight sets of data on the concentration, 13 C/ 12 C, and 18 O/ 16 O ratios of atmospheric carbon dioxide between 20°N and 17°S latitude near 155°W. In the northern hemisphere the 13 C/ 12 C ratio of the CO 2 seasonally added to and withdrawn from the atmosphere was found to be nearly constant at circa 30‰ less than standard PDB. This ratio is consistent with photosynthesis and respiration of terrestrial vegetation as the cause of the seasonal cycle. The ratio was progressively less negative southward, reaching about −13‰ at 17°S. This shift in ratio suggests that air‐sea exchange is important in producing the seasonal variation in the southern hemisphere, since this exchange probably occurs with little kinetic isotopic fractionation. The seasonally adjusted 13 C/ 12 C ratio of atmospheric CO 2 increased steadily southward from −7.55‰ near 16°N to −7.47‰ near 17°S, as expected from a predominantly northern hemisphere source of a CO 2 from fossil fuel combustion and a barrier to atmospheric mixing in the tropics. A peak in CO 2 concentration near the equator was not found in the north‐south trend in 13 C/ 12 C ratio. This lack of an isotopic peak suggests that the high concentrations of CO 2 near the equator is produced by the release of CO 2 from sea water and is not related to biological activity of land vegetation.