Hydrogen sulphide release to surface waters at the Precambrian/Cambrian boundary

Changes in environmental conditions at the Precambrian–Cambrian transition (around 542 million years ago) have been suggested as a possible explanation for the apparent rapid increase in abundance of multicellular organisms known as the 'Cambrian explosion'. The nature of the environmental changes is still a matter of debate, however. Wille et al. now report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of about 542 million years. With the help of a box model of the oceanic molybdenum cycle, they find that intense upwelling of hydrogen sulphide-rich deep ocean water best explains the observed early Cambrian molybdenum isotope signal. This suggests that the early Cambrian animal radiation may have been triggered by a major change in ocean circulation, following a long period during which the ocean was stratified, with sulphidic deep water.