Determining the nature and drivers of Earth’s first metazoan radiation and subsequent extinction: The Cambrian ‘Explosion’ and Sinsk Event

The Cambrian Radiation, starting ca. 540 million years ago (Ma), marks the appearance of abundant and diverse animals (metazoans) in the fossil record. This radiation may have been driven by oscillating ocean oxygenation and productivity events, but was terminated by the first mass extinction, the ‘Sinsk Event’, ca. 513 Ma1. The Sinsk Event is considered to have been a hyperthermal interval of widespread ocean anoxia that reset the trajectory for all subsequent metazoan life. However, despite its significance for the course of Earth’s habitability and the evolution of complex life, little is known about the drivers of this event, or which environmental factors controlled extinction selectivity. We hypothesise that it was low oceanic sulphate levels that promoted oscillating oxygenation, so creating pulsed metazoan radiations, but paradoxically this also made the early Cambrian shallow ocean highly susceptible to anoxia in response to global warming, thereby terminating these radiations with a profound extinction. We propose an ambitious integrated palaeobiological, geochemical and modelling approach to unravel the controlling processes of radiations, and the consequences of the enigmatic, but highly significant, Sinsk Event. By comparing the drivers of the Cambrian Radiation with those of its demise, we will address a fundamental motivating question: Did low sulphate seas both promote radiations and drive extinctions through Earth History?