Abstract

The Tonian–Cambrian interval (c. 0.9–0.5 Ga) witnessed major tectonic, climatic and chemical changes to the Earth system and culminated in the Ediacaran–Cambrian radiation of animals. Negative carbon isotope (δ¹³C) excursions of extraordinary magnitude form the backdrop to all these events and are consistent with the presence of a vast marine organic carbon reservoir that changed its size due to periodic imbalances between organic production (as an oxidant source) and terrestrial oxidant sinks. Prior to both Cryogenian glaciations, this pool of long-lived organic carbon may have become substantially depleted, leading to a weakening of the regulation of climate and oxygen. The late Ediacaran Shuram anomaly probably represents a third depletion event, which was followed by oxygenation and the Ediacaran–Cambrian radiation. The evolutionary diversification of animals shortened the ocean residence time of organic carbon and introduced new carbon cycle feedbacks that together helped to mould the modern Earth system.