Major changes in the oxygenation of the atmosphere and ocean have been suggested to trigger the taxonomic diversification and ecological expansion of complex life, including animals, during the Neoproterozoic–Palaeozoic transition. However, testing this hypothesis is hampered by the paucity of quantitative constraints on the oceanic oxygen availability at that time. Here we show how the spatial pattern of I/Ca ratios in marine carbonates—a proxy for dissolved oxygen in the local upper ocean—provides a fingerprint of the oxygenation state of Earth’s surface. Spatial analyses on published I/Ca ratios spanning the past 2,000 million years show that the latitudinal gradient of oxygen concentrations in the upper ocean was reversed in the Proterozoic eon relative to the modern pattern of decreasing oxygen concentrations from the mid-latitudes to the Equator. Using an Earth system model, we identify that the Proterozoic I/Ca latitudinal pattern is associated with a biosphere-controlled distribution of oxygen in the upper ocean at a low atmospheric oxygen level, and the transition to a modern pattern in the I/Ca proxy may correspond to a threshold of around 1% of today’s atmospheric oxygen concentration.
He et al. (Tue,) studied this question.