Oceanic redox conditions during the terminal Cambrian extinction event

Abstract

Marine animal diversity during the late Cambrian was punctuated 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 compositions (δ13C and δ238U) as proxies for organic matter burial and oceanic redox, respectively, from carbonate rocks of the Wa’ergang section, South China. The dataset spans an interval that includes the last major negative δ13C excursion (TOCE) of the Cambrian Period. The TOCE is a globally documented event, recovery from which corresponds to the terminal Cambrian extinction event. δ13C and δ238U values covary through the section, shifting initially to lower values, with δ238U 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. The co-occurrence of δ13C and δ238U negative excursions, as well as extinctions associated with rising δ238U, has seldom been reported. Here we argue that positive δ13C vs. δ238U covariation can be produced by expanded intermediate reducing settings (from low-O2 suboxia to intermittently sulfidic anoxia), which are likely to have been widespread during the late Cambrian with low atmospheric pO2 and a greenhouse climate. A stepwise increase in the δ238U baseline in carbonates across the Ediacaran−Cambrian boundary is consistent with the growing importance of an intermediate reducing sink through this interval. We propose further that divergent trends in deep and upper ocean redox conditions, as well as the waxing and waning of the accommodation space for organic carbon burial, in response to climate and sea-level changes, could have driven the parallel isotope excursions. An expansion of intermediate reducing conditions, rather than persistent anoxic euxinia, is more consistent with the presence of benthic faunas and shoreward extension of deeper-water fauna that may have had a greater tolerance against hypoxia.