Isotopic diagnosis of processes governing interannual variability of CO18O fluxes in the tropics

David Noone

Program in Atmospheric and Oceanic Sciences, Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA.

C. Still

University of California, Geography Department, Santa Barbara, CA

W. Riley

Lawrence Berkeley Laboratory, 1 Cyclotron Road, Berkeley, CA 94720

L. Welp

California Institute of Technology, Division of Environmental Engineering, Pasadena, CA 91125 United States

J. Randerson

University of California, Earth System Science Department, Irvine, CA

Abstract

Large scale variations in precipitation and temperature in phase with ENSO dominate the interannual variability in gross primary production in the tropics. During El Nino periods, large areas of tropical ecosystems are subject to water limitation and reduction in gross primary production. Because the ¹⁸O content of atmospheric CO₂ reflects a balance between gross uptake and back diffusion of CO₂ that has undergone isotopic exchange in the leaves, details of the processes which govern the interannual variability in carbon exchange can be exposed with isotopic analysis. Using a sophisticated model of biospheric exchange of water and CO₂ isotopes, we contrast interannual variability of modeled CO¹⁸O fluxes for the period 1947-2002 with measurements from the CMDL flask network. Our analysis focuses on attributing the changes in CO$_{18}$O fluxes to changes in the plant physiology during times of drought stress, and changes in the isotopic state of water during those times. Water within leaves, with which the CO2 equilibrates, changes at the same time as the physiological response, the fluxes contain a commingled signal of the two. Specifically, during times of drought, precipitation and soil water are enriched relative to the climatology due to a greater dominance of evaporation, and without consideration can mask the contributions from changes in leaf level exchange. Within the model these two components may be quantified explicitly, though testing the sensitivity to the isotopic content of precipitation. Further complications arise when the net ecosystem exchange is considered due to changes in the partitioning of CO₂O fluxes between plant processes and soil respiration. This partitioning is quantified in the model simulations. In discussion we consider the contribution to the interannual variability associated with enhanced fire activity, also in phase with ENSO, and thus the importance of fire in the global CO¹⁸O variability.

Citation: Noone, D., et al. (2004), Isotopic diagnosis of processes governing interannual variability of CO¹⁸O fluxes in the tropics. Eos Trans. AGU, 85(17), Jt. Assem. Suppl., Abstract B41B-02.