A 300-million-year record of atmospheric carbon dioxide from fossil plant cuticles

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Abstract
To understand better the link between atmospheric CO2 concentrations and climate over geological time, records of past CO2 are reconstructed from geochemical proxies1,2,3,4. Although these records have provided us with a broad picture of CO2 variation throughout the Phanerozoic eon (the past 544 Myr), inconsistencies and gaps remain that still need to be resolved. Here I present a continuous 300-Myr record of stomatal abundance from fossil leaves of four genera of plants that are closely related to the present-day Ginkgo tree. Using the known relationship between leaf stomatal abundance and growing season CO2 concentrations5,6, I reconstruct past atmospheric CO2 concentrations. For the past 300 Myr, only two intervals of low CO2 (2,000 p.p.m.v.) concentrations. These results are consistent with some reconstructions of past CO2 (refs 1, 2) and palaeotemperature records7, but suggest that CO2 reconstructions based on carbon isotope proxies3,4 may be compromised by episodic outbursts of isotopically light methane8,9. These results support the role of water vapour, methane and CO2 in greenhouse climate warming over the past 300 Myr.