Carbon stocks and isotopic budgets of the terrestrial biosphere at mid-Holocene and last glacial maximum times

The carbon fluxes, stocks and isotopic budgets of the land biosphere at mid -Holocene (6 ka BP) and last glacial maximum (21 ka BP) times are reconstru cted with the CARbon Assimilation In the Biosphere (CARATB) model forced wi th two different sets of climates simulated by the European Centre-HAMburg (ECHAM) and LMD general circulation models. It is found that the trends pre dicted on the basis of both sets of GCM climatic fields are generally consi stent with each other, although substantial discrepancies in the magnitude of the changes may be observed. Actually, these discrepancies in the biosph eric results associated with the use of different GCM climatic fields are u sually smaller than the differences between biospheric runs performed while considering or neglecting the CO2 fertilization effect (which might, howev er, be overestimated by the model due to uncertainties concerning changes i n nutrient availability). The calculated changes with respect to the presen t of the biosphere carbon stock range from - 132 to + 92 Gt C for the mid-H olocene and from -710 to +70 Gt C for the last glacial maximum. It is also shown that the relative contribution of the material synthesized by C-4 pla nts to the total biomass of vegetation, litter and soils was substantially larger at mid-Holocene and last glacial maximum times than today. This chan ge in the relative importance of the C-3 and C-4 photosynthetic pathways in duced changes in the C-13 fractionation of the land biosphere. These change s in the average biospheric fractionation resulting from the redistribution of C-3 and C-4 plants were partly compensated for by changes of opposite s ign in the fractionation of C-3 plants due to the modification of the inter cellular CO2 pressure within their leaves. With respect to present times, t he combination of both processes reduced the C-13 discrimination (i.e., les s negative fractionation) of the land biosphere by 0.03 to 0.32 parts per t housand during the mid-Holocene and by 0.30 to 1.86 parts per thousand at t he last glacial maximum. (C) 1999 Elsevier Science B.V. All rights reserved .