Highly dynamic marine redox state through the Cambrian explosion highlighted by authigenic δ238U records

Abstract

The history of oceanic oxygenation from the late Neoproterozoic to the early Cambrian is currently debated, making it difficult to gauge whether, and to what extent environmental triggers played a role in shaping the trajectory of metazoan diversification. Uranium isotope (δ238U) records from carbonates have recently been used to argue for significant swings in the global marine redox state from the late Neoproterozoic to the early Cambrian. However, geochemical signatures in carbonates—the U isotope archive most commonly employed to argue for redox shifts—are susceptible to diagenetic alteration and may have variable offsets from seawater values. Therefore, there is an impetus to reconstruct seawater U isotopic evolution using another sedimentary archive, in order to verify that these excursions can indeed be linked to global shifts in marine redox landscape. Here we report new U isotope data from two fine-grained siliciclastic upper Ediacaran to lower Cambrian (ca. 551–515 Ma) successions in South China. We find large δ238U swings between −0.63‰ and +0.39‰ for calculated values of authigenic U in the siliciclastic rocks, consistent with correlative records from the carbonates. The replication of these patterns in both carbonate and siliciclastic units provides confirmatory evidence that the early Cambrian seawater was characterized by highly variable U isotope compositions. These new δ238U data also provide higher-resolution records of global oceanic redox conditions during Cambrian Age 3, coeval with a critical interval of the Cambrian explosion. These δ238U data bolster the case that the Ediacaran-Cambrian transition experienced massive swings in marine redox state, providing a dynamic environmental backdrop for and potentially even a key driver of the emergence and radiation of metazoans.