The diversification of metazoans during the latest Neoproterozoic and early Cambrian has been attributed to, among other factors, a progressive rise in surface oxygen levels. However, recent results have also questioned the idea of a prominent rise in atmospheric oxygen levels or a major or unidirectional shift in the marine redox landscape across this interval. Here, we present new carbonate-associated uranium isotope data from upper Ediacaran to lower Cambrian marine carbonate successions. These data provide evidence for short-lived episodes of widespread marine anoxia near the Ediacaran-Cambrian transition and during Cambrian Age 2 (ca. 525 Ma). We suggest that biotic turnover and resulting ecological restructuring, triggered by marine redox fluctuations rather than progressive oxygenation, were the dominant drivers of the Cambrian explosion. Episodes of harsh environmental conditions against a backdrop of Proterozoic–Phanerozoic oceanic oxygenation on the eve of the Cambrian explosion could have, by promoting ecosystem restructuring, spurred the diversification of the Cambrian Evolutionary Fauna.

The evolution of oxygen cycles on Earth’s surface has been regulated by the balance between molecular oxygen production and consumption. The Neoproterozoic–Paleozoic transition likely marks the second rise in atmospheric and oceanic oxygen levels, widely attributed to enhanced burial of organic carbon. However, it remains disputed how marine organic carbon production and burial respond to global environmental changes and whether these feedbacks trigger global oxygenation during this interval. Here, we report a large lithium isotopic and elemental dataset from marine mudstones spanning the upper Neoproterozoic to middle Cambrian [~660 million years ago (Ma) to 500 Ma]. These data indicate a dramatic increase in continental clay formation after ~525 Ma, likely linked to secular changes in global climate and compositions of the continental crust. Using a global biogeochemical model, we suggest that intensified continental weathering and clay delivery to the oceans could have notably increased the burial efficiency of organic carbon and facilitated greater oxygen accumulation in the earliest Paleozoic oceans.