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

• Published in: Nature (London) 2008-05, Vol.453 (7194), p.504-506
• Main Authors: Bao, Huiming, Lyons, J. R., Zhou, Chuanming
• Format: Article
• Language: English
• Subjects: Atmosphere; Atmosphere - chemistry; Barium Sulfate - chemistry; Biosphere; Cambrian; Carbon dioxide; Carbon Dioxide - analysis; Carbon Dioxide - metabolism; Carbonates; Fossils; Geologic Sediments - chemistry; Geological time; Geologists; Geology; Glaciation; History, Ancient; Humanities and Social Sciences; Hydrosphere; Ice Cover; Isotopes; letter; Lithosphere; Methane; multidisciplinary; Oxygen; Oxygen Isotopes; Ozone - analysis; Ozone - chemistry; Partial Pressure; Photochemical reactions; Photochemicals; Photochemistry; Photosynthesis; Sample size; Science; Science (multidisciplinary); Standard deviation; Surface water
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature06959

Ancient atmospheres: Snowball Earth exit by proxy Information about the past composition of the Earth's atmosphere on geological timescales is hard to come by. So the debut of a new stable isotope proxy for ancient atmospheric condition is a notable event. The proxy, the triple oxygen isotope composition of sulphate from ancient evaporites and barites, exhibits variable negative oxygen-17 anomalies over the past 750 million years. The anomalies track atmospheric oxygen and in turn reflect the partial pressure of carbon disoide via a stratospheric ozone/carbon dioxide/oxygen photochemical reaction network. In line with modelling results, the proxy data point to a high-carbon dioxide atmosphere in the Early Cambrian compared to earlier eras. Significantly, the oxygen-17 anomalies of barites from Marinoan cap carbonates (∼ 635 million years ago) display a distinct negative spike, suggesting that carbon dioxide was still high when barite was precipitating in the cap carbonate sequences. This supports the Neoproterozoic 'snowball' Earth hypothesis and/or massive methane release after the Marinoan glaciation. Historical information about the atmosphere is scarce. Huiming Bao and colleagues show that the triple oxygen isotope composition of sulphate from ancient evaporites and barites exhibits variable negative oxygen-17 isotope anomalies over the past 750 million years. 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 past 1 , 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 oxygen 5 , 6 . Our results suggest that was much higher in the early Cambrian tha