'Strangelove ocean' before the Cambrian explosion

The Palaeozoic and Mesozoic eras were terminated by faunal changes involving mass extinction of the old and explosive evolution of the new fauna, but the fossil record shows only a Cambrian Explosion at the end of the Precambrian. Stanley speculated that the explosion was only possible after the ubiquitous algae community had been largely eliminated 1 ; ecological niches were thus liberated for explosive evolution. If the Cambrian Explosion were preceded by a mass mortality (or by a mass extinction), such an event should leave a record in the form of geochemical anomalies. We have undertaken a search for geochemical anomalies at the Precambrian/Cambrian contact. We report here the discovery of a sharp negative carbon-isotope shift in the carbonate of a clay immediately above a marker in the Precambrian/Cambrian boundary, the China C marker, and interpret this signal as evidence of sudden decrease in fertility before the Cambrian explosion of invertebrate evolution. The discovery suggests that the Precambrian/Cambrian boundary might be defined by an event-marker at a palaeontologically correlative horizon.