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

<jats:p>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<jats:sup>13</jats:sup>C signals we interpret. However, reconstructing the atmospheric δ<jats:sup>13</jats:sup>C value in the geologic past has proven challenging. Land plants sample the isotopic composition of CO<jats:sub>2</jats:sub>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<jats:sup>13</jats:sup>C value of atmospheric CO<jats:sub>2</jats:sub>can be reconstructed from the isotopic composition of plant tissue. Over a range of pCO<jats:sub>2</jats:sub>(198–1300 ppmv), the δ<jats:sup>13</jats:sup>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<jats:sup>13</jats:sup>C value of leaf tissue. These factors explained a relatively small portion of variation in the δ<jats:sup>13</jats:sup>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<jats:sup>13</jats:sup>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<jats:sup>13</jats:sup>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<jats:sup>13</jats:sup>C values (−6.4‰ to −9.6‰) and excluding C3 Poaceae to develop an equation to reconstruct the δ<jats:sup>13</jats:sup>C value of atmospheric CO<jats:sub>2</jats:sub>based on plant values. Reconstructing the δ<jats:sup>13</jats:sup>C value of atmospheric CO<jats:sub>2</jats:sub>in geologic time will facilitate chemostratigraphic correlation in terrestrial sediments, calibrate pCO<jats:sub>2</jats:sub>reconstructions based on soil carbonates offer a window into the physiology of ancient plants.</jats:p>