Global marine redox evolution from the late Neoproterozoic to the early Paleozoic constrained by the integration of Mo and U isotope records

The emergence and diversification of early animals is commonly thought to have coincided with atmosphere and ocean oxygenation across the terminal Neoproterozoic and early Paleozoic, during which oxygen levels on Earth's surface were sufficient to support the metabolism of early multicellular metazoans in the ocean. Although surface oxygen levels are likely to have broadly risen through the Paleozoic, ocean oxygenation levels during this period are still disputed and poorly constrained. While the community is actively developing high time-resolution records of redox proxies in marine sediments, uncertainties remain about how these records can be used to reconstruct the global marine redox landscape. In this review, we compile newly published molybdenum and uranium isotope data from the late Neoproterozoic to the early Paleozoic (ca. 680-480 Ma) to provide an updated look at the secular changes in global ocean redox state and the potential drivers of these shifts. Integrations of Mo and U isotope records suggest a gradual transition from a pervasive anoxic condition to a highly dynamic condition for global marine redox state from the late Neoproterozoic to the Cambrian. We further concentrate on the marine redox landscape of early Cambrian, by comparing the carbon, sulfur, nitrogen and uranium isotope records and reproducing the variations in carbon-sulfur-uranium isotope records with biogeochemical box models. Changes in marine primary productivity under a relatively low atmospheric oxygen level, are proposed to play a first-order control on the early Cambrian ocean redox dynamics.

Automated summary

The emergence and diversification of early animals are believed to coincide with atmospheric and ocean oxygenation, supporting the metabolism of early multicellular organisms; though surface oxygen levels might have risen in the Paleozoic, marine oxygenation levels remain disputed. New molybdenum and uranium isotope data sheds light on the global ocean redox changes from the late Neoproterozoic to the early Paleozoic.