Animal-like multicellular fossils appeared towards the end of the Precambrian, followed by a rapid increase in the abundance and diversity of fossils during the Early Cambrian period, an event also known as the ‘Cambrian explosion’1,2,3. Changes in the environmental conditions at the Precambrian/Cambrian transition (about 542 Myr ago) have been suggested as a possible explanation for this event, but are still a matter of debate1,2,3. Here we report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 Myr. We find a transient molybdenum isotope signal immediately after the Precambrian/Cambrian transition. Using a box model of the oceanic molybdenum cycle, we find that intense upwelling of hydrogen sulphide-rich deep ocean water best explains the observed Early Cambrian molybdenum isotope signal. Our findings suggest that the Early Cambrian animal radiation may have been triggered by a major change in ocean circulation, terminating a long period during which the Proterozoic ocean was stratified, with sulphidic deep water.
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This work was financed by grants from the Swiss National Foundation to J.D.K. and T.F.N., from Deutsche Forschungsgemeinschaft to B.L., and from the German Academic Exchange Service to S.S. Thanks to Petroleum Development Oman LLC for financial and logistical support, and to J. Grotzinger and NASA for initial analytical work.
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Wille, M., Nägler, T., Lehmann, B. et al. Hydrogen sulphide release to surface waters at the Precambrian/Cambrian boundary. Nature 453, 767–769 (2008). https://doi.org/10.1038/nature07072
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