Tracking the evolution of seawater Mo isotopes through the Ediacaran-Cambrian transition

The rapid increase in animal diversity and abundance over the Ediacaran-Cambrian boundary has been attributed to, among other parameters, pronounced Earth surface oxygenation at that time. However, whether atmospheric oxygen elevated to modern levels, or whether the extent was smaller and oxygen remained relatively muted until the late Paleozoic is currently debated. Here, we present new Mo isotope datasets of drill core material for two key black shale units (the Ediacaran Doushantuo Formation and the lower Cambrian Jiumenchong Formation), located on the Yangtze Block. These data capture a 0.7 parts per thousand isotope fractionation between sediments experiencing quantitative and non-quantitative Mo removal. By identifying this offset, we provide integrated estimates for global seawater Mo isotope compositions during the deposition of these two formations (similar to 1.2-1.4 parts per thousand and similar to 1.8-2 parts per thousand, respectively). When paired with a mass balance model, our results indicate that a large proportion of the early Cambrian ocean was bathed under oxic waters. We also discuss how recent discoveries have provided insights into the geochemical processes underpinning the oxygenation of the oceans.