Paleozoic deep-ocean oxygenation and early animal evolution

While atmospheric oxygen levels increased during the Great Oxidation Event, the deep ocean likely remained anoxic. Only 2 billion years later in the Paleozoic Era did animals appear in benthic habitats, unambiguously pointing to oxygenated deep waters. We've used selenium isotopes to track the timing of permanent deep ocean oxygenation, finding that deep-water habitats did not become permanently oxygenated until around 400 Myr ago. This coincides with fossil occurrence data showing that early vertebrates did not inhabit deep-water habitats until that time (below, from Sallan et al. 2018 Science). This timing also corresponds to the rise of terrestrial biomass deposition during the Devonian expansion of forests.