Mid-Devonian ocean oxygenation enabled the expansion of animals into deeper-water habitats

The oxygenation history of Earth’s surface environments has had a profound influence on the ecology and evolution of metazoan life. It was traditionally thought that the Neoproterozoic Oxygenation Event enabled the origin of animals in marine environments, followed by their persistence in aerobic marine habitats ever since. However, recent studies of redox proxies (e.g., Fe, Mo, Ce, I) have suggested that low dissolved oxygen levels persisted in the deep ocean until the Late Devonian, when the first heavily wooded ligniophyte forests raised atmospheric O₂ to modern levels. Here, we present a Paleozoic redox proxy record based on selenium enrichments and isotope ratios in fine-grained siliciclastic sediments. Our data reveal transient oxygenation of bottom waters around the Ediacaran–Cambrian boundary, followed by predominantly anoxic deep-water conditions through the Early Devonian (419 to 393 Ma). In the Middle Devonian (393 to 382 Ma), our data document the onset of permanent deep-ocean oxygenation, coincident with the spread of woody biomass across terrestrial landscapes. This episode is concurrent with the ecological occupation and evolutionary radiation of large active invertebrate and vertebrate organisms in deeper oceanic infaunal and epifaunal habitats, suggesting that the burial of recalcitrant wood from the first forests sequestered organic carbon, increased deep marine oxygen levels, and was ultimately responsible for the “mid-Paleozoic marine revolution.”