1. Institute of Geological Sciences, University of Bern, Baltzerstrasse 3, 3012 Bern, Switzerland
2. Institute of Mineralogy and Mineral Resources, Technical University of Clausthal, 38678 Clausthal-Zellerfeld, Germany
3. Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
4. Present address: Total E&P, Avenue Larribau, F-64018 Pau, France.

Correspondence to: Martin Wille¹ Correspondence and requests for materials should be addressed to M.W. (Email: martin.wille@anu.edu.au).

Abstract

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 debate^{1, 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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