Prediction of atmospheric δ 13 CO 2 using fossil plant tissues

Reconstruction of the carbon isotope composition of atmospheric CO 2 is critical to the understanding of long‐term global carbon cycling. We have suggested that the δ 13 C value of land plant carbon ( δ 13 C p ) preserved in the geologic record should reflect the δ 13 CO 2 at the time during which the plants grew ( δ 13 C a ), based on a meta‐analysis of modern plant data. Here we present the results of laboratory experiments designed to quantify the relationship between plant tissue δ 13 C and δ 13 CO 2 values under varying environmental conditions, including differential p CO 2 ranging from 1 to 3 times today's levels. As predicted, plants grown under elevated p CO 2 showed increased average biomass compared to controls grown at the same temperature. Across a very large range in δ 13 C a (≈24‰) and p CO 2 (≈740 ppmv) we observed a consistent correlation between δ 13 C a and δ 13 C p ( p < 0.001). We show an average isotopic depletion of −25.4‰ for aboveground tissue and −23.2‰ for belowground tissue of Raphanus sativus L. relative to the composition of the atmosphere under which it formed. For aboveground and belowground tissue, grown at both ∼23°C and ∼29°C, correlation was strong and significant ( r 2 ≥ 0.98 and p < 0.001); variation in p CO 2 level had little or no effect on this relationship. These results validate our initial conclusion that in the absence of environmental stress, plant δ 13 C primarily reflects atmospheric δ 13 CO 2 linearly across p CO 2 levels; the demonstrated excellent correlation in δ 13 C a and δ 13 C p suggests a high level of predictive power across varying environmental conditions.