A multi-box model study of the role of the biospheric metabolism in the recent decline of δ18O in atmospheric CO2

From around 1993 to 1997, the NOAA-CU δ18O measurements at Pt. Barrow, Mauna Loa, Cape Kumukahi, Cape Grim and the South Pole show a decrease in atmospheric CO₂δ¹⁸O of about 0.5°. Recently,Gillon and Yakir (2001) have attributed this decrease to a conversion of C₃ forests to C₄ grasslands through anthropogenic land-use change. However, their explanation can account for only about 0.02° yr⁻¹ decrease rate. In this paper we offer a viable alternative explanation. We have used a multi-box model of the global carbon cycle and its δ18O to show that an increase in biospheric respiration (CO² flux from plant with lower-than-atmosphere δ¹⁸O), combined with a decrease in the amount of CO₂ (with higher-than-atmosphere δ18O) diffusing back from plant leaves before being assimilated as part of the gross primary production (GPP), could produce the observed decline in the atmospheric CO₂δ¹⁸O. This decrease in the CO₂ back diffusion out of leaves could be interpreted as an overall increase in both biospheric activities of photosynthesis and respiration. Change in the metabolic activities of the biosphere as a possible cause for the observed decrease inδ¹⁸O is a reasonable hypothesis, since isotopic fractionations that occur during CO₂ exchange processes (photosynthesis and respiration) between the atmosphere and the biosphere contribute significantly to the observed variations in atmospheric CO₂δ¹⁸O, while contribution from the net air-sea CO₂ exchange is negligible.