The relationship between atmospheric carbon dioxide (CO2) and climate in the Quaternary period has been extensively investigated, but the role of CO2 in temperature changes during the rest of Earth’s history is less clear1. The range of geological evidence for cool periods during the high CO2 Mesozoic ‘greenhouse world’2,3 of high atmospheric CO2 concentrations, indicated by models4 and fossil soils5, has been particularly difficult to interpret. Here, we present high-resolution records of Mesozoic and early Cenozoic atmospheric CO2 concentrations from a combination of carbon-isotope analyses of non-vascular plant (bryophyte) fossils and theoretical modelling6,7. These records indicate that atmospheric CO2 rose from ∼420 p.p.m.v. in the Triassic period (about 200 million years ago) to a peak of ∼1,130 p.p.m.v. in the Middle Cretaceous (about 100 million years ago). Atmospheric CO2 levels then declined to ∼680 p.p.m.v. by 60 million years ago. Time-series comparisons show that these variations coincide with large Mesozoic climate shifts8,9,10, in contrast to earlier suggestions of climate–CO2 decoupling during this interval1. These reconstructed atmospheric CO2 concentrations drop below the simulated threshold for the initiation of glaciations11 on several occasions and therefore help explain the occurrence of cold intervals in a ‘greenhouse world’3.
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We thank H. Elderfield, D. Royer, P. Wilson and I. Woodward for helpful comments, M. Katz for the δ13Ccarb data sets, A. Ridgwell for the pH-corrected δ18O data sets and H. Walker for stable-carbon-isotope analyses. We also thank the following for kindly providing fossil materials for isotopic analysis: A. Herman and V. Krassilov (Russian Academy of Sciences, Moscow), S. Wing and J. Wingerath (Smithsonian Institution, Washington), I. Daniel (University of Canterbury, Christchurch, New Zealand), A. Crame (British Antarctic Survey, Cambridge), P. Kenrick (Natural History Museum, London), W. G. Chaloner (University of London), D. Royer (Wesleyan University), J. McElwain (Trinity College, University of Dublin) and J. Francis (University of Leeds), who also provided Fig. 1d. We gratefully acknowledge financial support of this research through a University of Sheffield studentship to B.J.F., a University of Sheffield Divisional Directors award and a Leverhulme Trust award to D.J.B., and a DOE grant to R.A.B.
Supplementary tables 1-3 and supplementary figure 1
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Scientific Reports (2013)