Abstract: The marine animal diversity was generally low during late Cambrian and early Ordovician. Biogeochemical studies suggest that it may have been related with fluctuations in oceanic redox conditions, despite still lack of evidence. Well-preserved marine carbonates could record the geochemical characters of ancient seawater, and their uranium isotope compositions (δ238U) can be used to reconstruct the global oceanic redox conditions. In this study we analyzed the uranium isotope compositions of marine carbonates of the basal Cambrian Stage 10 from Wa’ergang section, South China. Our data show that the δ238U profile has three negative excursions with minima of -0.8‰ , -0.55‰ and -0.60‰ , respectively, which indicates three intervals of oceanic anoxia expansion. Modeling results based on the uranium isotope mass balance model suggest that about 22%, 3.5% and 5% of the global sea-floor were covered by anoxic water during these three intervals, respectively. We speculate that the first anoxic interval may have been related with increase in primary productivity due to enhanced weathering input of nutrients, and the latter two may have been induced by upwelling of deep anoxic water during the transgressions.

Key words: U isotopes, redox, Cambrian Stage 10, oceanic anoxia