Atmospheric carbon dioxide is the raw material for the biosphere. Therefore, changes in the carbon isotopic composition of the atmosphere will influence the terrestrial δ13C signals we interpret. However, reconstructing the atmospheric δ13C value in the geologic past has proven challenging. Land plants sample the isotopic composition of CO2 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 δ13C value of atmospheric CO2 can be reconstructed from the isotopic composition of plant tissue. Over a range of pCO2 (198–1300 ppmv), the δ13C value of plant tissue does not vary systematically with atmospheric carbon dioxide concentration. However, environmental factors, such as water stress, can influence the δ13C value of leaf tissue. These factors explained a relatively small portion of variation in the δ13C 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 δ13C 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 δ13C value of the atmosphere under which it was fixed. We use a subset of our data spanning a geologically reasonable range of atmospheric δ13C values (−6.4‰ to −9.6‰) and excluding C3 Poaceae to develop an equation to reconstruct the δ13C value of atmospheric CO2 based on plant values. Reconstructing the δ13C value of atmospheric CO2 in geologic time will facilitate chemostratigraphic correlation in terrestrial sediments, calibrate pCO2 reconstructions based on soil carbonates offer a window into the physiology of ancient plants.

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Unusually large and locally variable carbon isotope excursions coincident with mass extinctions at the end of the Permian Period (253 Ma) and Guadalupian Epoch (260 Ma) can be attributed to methane outbursts to the atmosphere. Methane has isotopic values (d 13 C) low enough to reduce to feasible amounts the carbon required for isotopic mass balance. The duration of the carbon isotopic excursions and inferred methane releases are here constrained to !10,000 yr by counting annual varves in lake deposits and by estimating peat accumulation rates. On paleogeographic maps, the most marked carbon isotope excursions form linear arrays back to plausible methane sources: end-Permian Siberian Traps and Longwood-Bluff intrusions of New Zealand and end-Guadalupian Emeishan Traps of China. Intrusion of coal seams by feeder dikes to flood basalts could create successive thermogenic methane outbursts of the observed timing and magnitude, but these are unreasonably short times for replenishment of marine or permafrost sources of methane. Methane released by fracturing and heating of coal during intrusion of large igneous provinces may have been a planetary hazard comparable with bolide impact.

Reconstruction of changing C isotopic composition of Early Cretaceous atmospheric CO 2 from fossilized C3 vascular land-plant tissue revealed a brief and striking negative excursion (⌬ ഠ ؊5‰) in atmospheric ␦ 13 C, followed by a rapid positive compensation (⌬ ഠ ؉5‰) during the Aptian (ca. 117 Ma). Mass-balance calculations show that dissociation of a small amount of methane gas hydrate is the most tenable cause of the negative excursion; this would also result in an increased CO 2 :O 2 mixing ratio as O 2 is consumed during CH 4 oxidation to CO 2 , spurring the exponential phase of angiosperm biogeographic expansion.

A new technique, in which consumption of these foods may be estimated in humans by measuring the natural abundance stable-carbon-isotope profile of corn- and sugar cane-sweetened or sugar-containing foods as tracked in tissue or blood, could potentially provide an objective assessment of dietary intake and offer new opportunities for the study of diet-disease relations.