Can C3 plants faithfully record the carbon isotopic composition of atmospheric carbon dioxide?

Atmospheric carbon dioxide is the raw material for the biosphere. Therefore, changes in the carbon isotopic composition of the atmosphere will influence the terrestrial δ 13 C signals we interpret. However, reconstructing the atmospheric δ 13 C value in the geologic past has proven challenging. Land plants sample the isotopic composition of CO 2 during photosynthesis. We use a model of carbon isotopic fractionation during C3 photosynthesis, in combination with a meta–data set (519 measurements from 176 species), to show that the δ 13 C value of atmospheric CO 2 can be reconstructed from the isotopic composition of plant tissue. Over a range of pCO 2 (198–1300 ppmv), the δ 13 C value of plant tissue does not vary systematically with atmospheric carbon dioxide concentration. However, environmental factors, such as water stress, can influence the δ 13 C value of leaf tissue. These factors explained a relatively small portion of variation in the δ 13 C value of plant tissue in our data set and emerged strongly only when the carbon isotopic composition of the atmosphere was held constant. Members of the Poaceae differed in average δ 13 C value, but we observed no other differences correlated with plant life form (herbs, trees, shrubs). In contrast, over 90% of the variation the carbon isotopic composition of plant tissue was explained by variation in the δ 13 C value of the atmosphere under which it was fixed. We use a subset of our data spanning a geologically reasonable range of atmospheric δ 13 C values (−6.4‰ to −9.6‰) and excluding C3 Poaceae to develop an equation to reconstruct the δ 13 C value of atmospheric CO 2 based on plant values. Reconstructing the δ 13 C value of atmospheric CO 2 in geologic time will facilitate chemostratigraphic correlation in terrestrial sediments, calibrate pCO 2 reconstructions based on soil carbonates offer a window into the physiology of ancient plants.