Temporal and spatial variations of the carbon isotopic ratio of atmospheric carbon dioxide in the western Pacific region

CO2 samples were collected on board container ships between Japan and Australia and between Japan and the United States from April 1984 to December 1991. They were then analyzed for their delta(13)C values with a precision of 0.02 parts per thousand. The seasonal cycle of delta(13)C was clearly observable at latitudes from 55 degrees N to the equator. Peak-to-peak amplitude of the average seasonal cycle of delta(13)C was about 0.8 parts per thousand at latitudes north of 40 degrees N and decreased going southward to 0.2 parts per thousand at the equator. The comparison of the observed seasonal cycles of delta(13)C With those of the CO2 concentration suggests that the seasonal CO2 cycles observed at latitudes north of the equator are due to the seasonal change in terrestrial biospheric activities. However, air transport from the southern hemisphere is also thought to be partly responsible for the seasonal CO2 cycles at northern low latitudes. Values of delta(13)C decreased secularly at a rate of about -0.03 parts per thousand/year, owing mainly to increased amounts of isotopically light CO2 produced in fossil fuel combustion and deforestation. Interannual variations of the long-term trend of delta(13)C in association with the El Nino-Southern Oscillation (ENSO) event and other factors were clearly observed, which were. almost opposite in phase with those of the CO2 concentration. From the comparison of the observed long-term trend of delta(13)C with those calculated using a box diffusion model, it is suggested that an imbalance of the CO2 exchange between the atmosphere and the terrestrial biosphere could be responsible for such interannual variations.