Modelling the continental effect of oxygen isotopes over Eurasia

Abstract

δ¹⁸O in CO₂ attracts attention for its ability to decipher the gross fluxes of terrestrial CO₂ exchange. This capacity can only be exploited if one knows the impact of the biology and the water isotope cycle on the isotopic composition of atmospheric CO₂. To study the processes, we built an integrated global model of δ¹⁸O in atmospheric CO₂ that calculates ¹⁸O in the water cycle pools, the CO₂ and the inherent δ¹⁸O-CO₂ fluxes and transports these in the atmosphere. Within the framework of the European Project EUROSIBERIAN CARBONFLUX, we investigated the 18O processes on the continent of Eurasia north of 40 °N. We show that there is a large impoverishment in the water isotopic composition of rain in the continental interior but this is significantly reduced during the growing season. We validated the model directly by comparing it to Net Ecosystem Exchange (NEE) measurements from eddy-flux towers and indirectly by comparing it to atmospheric measurements of CO₂ and δ¹⁸O-CO₂ at 3000 m a.s.l. The model reproduces well the seasonal cycle of CO₂ and the phase for δ¹⁸O-CO₂ but it underestimates the amplitude of that species. Our model further predicts a large diurnal rectifier effect that is different in CO₂ than in δ¹⁸O-CO₂ in eastern Siberia due to very low or even negative leaf discrimination. We analyse the longitudinal gradient in leaf discrimination which reflects primarily the gradient in leaf water isotopic composition. The leaf water δ¹⁸O-H₂O gradient is itself determined 25% by the meridional gradient in source water and 75% by the change of relative humidity with longitude.