Paper No. 1
Presentation Time: 9:00 AM-6:00 PM

THE EFFECT OF ATMOSPHERIC CO2 CONCENTRATION ON CARBON ISOTOPE FRACTIONATION IN C3 LAND PLANTS


JAHREN, A. Hope, Geology and Geophysics, University of Hawaii, 1680 East-West Road, POST 701, Honolulu, HI 96822 and SCHUBERT, Brian A., School of Geosciences, University of Louisiana at Lafayette, 611 McKinley St 44530, Hamilton Hall #323, Lafayette, LA 70504, schubert@louisiana.edu

Experiments growing plants under elevated pCO2 in chamber and field settings, as well as meta-analyses of published data, have yielded a wide range of estimates for the effect of pCO2 on the net isotopic discrimination (Δδ13Cp) between plant tissue (δ13Cp) and atmospheric CO213CCO2). Here we present new data from plant growth chambers in which we grew a total of 191 C3 plants (128 Raphanus sativus plants and 63 Arabidopsis thaliana) across fifteen levels of pCO2 ranging from 370 to 4200 ppm. Three types of plant tissue were harvested and analyzed for carbon isotope value: above-ground tissues, below-ground tissues, and leaf-extracted nC31-alkanes. We observed strong hyperbolic correlations (R ≥ 0.94) between the pCO2 level and Δδ13Cp for each type of plant tissue analyzed; the linear relationships previously suggested by experiments across small (10 to 350 ppm) changes in pCO2 (e.g., 300 to 310 ppm or 350 to 700 ppm) closely agree with the amount of fractionation per ppm increase in pCO2 calculated from our hyperbolic relationship. In this way, our work is consistent with, and provides a unifying relationship for, previous work on carbon isotopes in C3 plants at elevated pCO2. Our results suggest that it may be possible to reconstruct changes in paleo-pCO2 level using our new hyperbolic relationship if δ13CCO2 value and initial pCO2 level can be independently quantified. Several implications also arise for the reconstruction of water availability and water-use efficiency in both ancient and recent plant Δδ13Cp values across periods of changing pCO2 level. For example, the change in Δδ13Cp implied by our relationship for the rise in pCO2 concentration observed since 1980 is of the same magnitude (= ~0.7‰) as the isotopic correction for changes in δ13CCO2 required by the input of 13C-depleted carbon to the atmosphere. For these reasons, only the portion of a terrestrial carbon isotope excursion that persists after accounting for changes in pCO2 concentration should be used for the interpretation of a change in paleo-environmental conditions.