Oceanic redox conditions during the terminal Cambrian extinction event

Marine animal diversity during the late Cambrian was reduced by a series of extinctions that have generally been attributed to oceanic anoxic events associated with positive carbon isotope excursions. Here we present carbon and uranium isotope ratios (delta 13C and delta 238U values) as proxies for global carbon cycle and oceanic redox conditions, respectively, from carbonate rocks of the Wa'ergang section, South China. The dataset spans an interval that includes the last major negative delta 13C excursion (TOCE) of the Cambrian Period. The TOCE is a globally documented event, recovery from which corresponds to the terminal Cambrian extinction event. The delta 13C and delta 238U values covary through the section, shifting initially to lower values, with delta 238U falling below the modern open-ocean seawater value from the start to the middle of the profile, followed by a shift to higher values towards the end of the Cambrian. Neither the co-occurrence of delta 13C and delta 238U negative excursions, nor the association of rising delta 238U with extinction have been commonly reported. Here we argue that robust positive coupling of delta 13C and delta 238U relates to the existence of extensive intermediate reducing settings (from low-O2 suboxia to intermittent anoxia) during the late Cambrian alongside low atmospheric pO2 and a greenhouse climate. Similarly, a stepwise increase in the delta 238U baseline in carbonates across the Ediacaran-Cambrian boundary is consistent with the growing importance of an intermediate reducing sink through that interval. We propose further that divergent trends in lower and upper ocean redox conditions could have driven the parallel isotope excursions. An expansion of intermediate reducing conditions, rather than persistent anoxic euxinia, is consistent with the recovery of delta 13C and delta 238U to higher values, as well as the presence of benthic fauna and shoreward extension of deeper-water fauna that may have had a greater tolerance against hypoxia.