Phanerozoic concentrations of atmospheric oxygen reconstructed from sedimentary charcoal

Abstract

Varying concentrations of atmospheric oxygen have affected the development of animals and the role of wildfire in ecosystems. Reconstructions of past oxygen concentrations from fossil charcoal constrain atmospheric oxygenation over the past 400 million years. Variations of the Earth’s atmospheric oxygen concentration (pO2) are thought to be closely tied to the evolution of life, with strong feedbacks between uni- and multicellular life and oxygen1,2. On the geologic timescale, pO2 is regulated by the burial of organic carbon and sulphur, as well as by weathering3. Reconstructions of atmospheric O2 for the past 400 million years have therefore been based on geochemical models of carbon and sulphur cycling4,5,6. However, these reconstructions vary widely4,5,6,7,8,9,10, particularly for the Mesozoic and early Cenozoic eras. Here we show that the abundance of charcoal in mire settings is controlled by pO2, and use this proxy to reconstruct the concentration of atmospheric oxygen for the past 400 million years. We estimate that pO2 was continuously above 26% during the Carboniferous and Permian periods, and that it declined abruptly around the time of the Permian–Triassic mass extinction. During the Triassic and Jurassic periods, pO2 fluctuated cyclically, with amplitudes up to 10% and a frequency of 20–30 million years. Atmospheric oxygen concentrations have declined steadily from the middle of the Cretaceous period to present-day values of about 21%. We conclude, however, that variation in pO2 was not the main driver of the loss of faunal diversity during the Permo–Triassic and Triassic–Jurassic mass extinction events.