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Triple oxygen isotope evidence for elevated CO2 levels after a Neoproterozoic glaciation


Understanding the composition of the atmosphere over geological time is critical to understanding the history of the Earth system, as the atmosphere is closely linked to the lithosphere, hydrosphere and biosphere. Although much of the history of the lithosphere and hydrosphere is contained in rock and mineral records, corresponding information about the atmosphere is scarce and elusive owing to the lack of direct records. Geologists have used sedimentary minerals, fossils and geochemical models to place constraints on the concentrations of carbon dioxide, oxygen or methane in the past1,2,3,4. Here we show that the triple oxygen isotope composition of sulphate from ancient evaporites and barites shows variable negative oxygen-17 isotope anomalies over the past 750 million years. We propose that these anomalies track those of atmospheric oxygen and in turn reflect the partial pressure of carbon dioxide () in the past through a photochemical reaction network linking stratospheric ozone to carbon dioxide and to oxygen5,6. Our results suggest that was much higher in the early Cambrian than in younger eras, agreeing with previous modelling results2. We also find that the 17O isotope anomalies of barites from Marinoan (635 million years ago) cap carbonates display a distinct negative spike (around -0.70‰), suggesting that by the time barite was precipitating in the immediate aftermath of a Neoproterozoic global glaciation, the was at its highest level in the past 750 million years. Our finding is consistent with the ‘snowball Earth’ hypothesis7,8 and/or a massive methane release9 after the Marinoan glaciation.

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Figure 1
Figure 2: How evaporite or barite sulphate records the negative 17 O anomaly of tropospheric O 2 that originated in the stratosphere.
Figure 3: Model-calculated partial pressures of CO 2 based on the lowest sulphate Δ 17 O value for a given period in geological history.


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We thank numerous colleagues who have contributed evaporite and barite samples over the years, including W. T. Holser, M. Tucker, C. Spötl, R. Denison, C. Laughrey, S. C. Morris, S. P. Das Gupta, K. Benison, B.–H. Fu, S. Xiao, G. Retallack, W.-L. Zang, J. Hanor, B. Ellwood, D. Henry, B. Dutrow and A. J. Kaufman. One important barite sample, collected by G. P. Halverson, P. F. Hoffman and A. C. Maloof in Mauritania, West Africa, was obtained from M. H. Thiemens and D. P. Schrag. We thank Huifeng Bao for field assistance, M. Khachaturyan for laboratory assistance, and NSF, LSU, NASA (Planetary Atmospheres), the NNSF of China, the Chinese Academy of Sciences and the Chinese Ministry of Science and Technology for financial support.

Author Contributions H.B. designed the research, developed analytical procedures and performed measurements. J.R.L. did one-dimensional photochemical modelling and C.M.Z. directed fieldwork in South China. H.B. and J.R.L. wrote the manuscript.

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Correspondence to Huiming Bao.

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Bao, H., Lyons, J. & Zhou, C. Triple oxygen isotope evidence for elevated CO2 levels after a Neoproterozoic glaciation. Nature 453, 504–506 (2008).

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