A 3-dimensional study of δ18O in atmospheric CO2:: contribution of different land ecosystems

Land biospheric carbon exchange associated with respiration and photosynthesis exerts a major control on the oxygen isotope composition (delta(18)O) of atmospheric CO2 especially with respect to the seasonal cycle. In particular, an important feature that requires our attention is the phase of the seasonal cycle of delta(18)O which lags CO2 by one month in the Arctic. We have developed a global parameterization of the land biotic exchange of O-18 in CO2, which has been prescribed in an atmospheric 3-D transport model in order to simulate the global atmospheric distribution of delta(18)O. Furthermore, we have separated in the model the specific contribution of different regions of the globe to the seasonal and latitudinal variation of delta(18)O. The model simulated values are compared in detail with atmospheric observations made at 22 different remote stations. The respective role of respiration vs, photosynthesis in determining the phase and amplitude of the delta(18)O seasonal cycle is also analyzed. Based on a good agreement between our model simulation and the atmospheric observations, we observe that the large seasonal cycle of delta(18)O at high latitudes is mainly due to the respiratory fluxes of all extra-tropical ecosystems while for CO2 the relative contributions of photosynthesis and respiration to the overall seasonal cycle are similar. Geographically, the CO2 exchanges with the northern Siberian ecosystem dominate the delta(18)O seasonality at all remote stations of the northern hemisphere, reflecting the strongly continental climate of that region.