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The early Cambrian Period was a key interval in Earth history with regard to changes in both ocean chemistry and animal evolution. Although increasing ocean ventilation has been widely assumed to have played a key role in the rapid appearance, diversification and spatial colonization of early animals, this relationship is in fact not firmly established. Here, we report a high-resolution Fe-C-S-Al-Ti geochemical study of the lower Cambrian Wangjiaping section from an outer-shelf setting of the Yangtze Sea of South China. Iron speciation data document a redox transition from dominantly euxinic to ferruginous conditions during Cambrian Age 3 (c. 521–514 Ma). Interpretation of coexisting pyrite sulphur isotope (δ34Spy) records from Wangjiaping reveals relatively high marine sulphate availability at Wangjiaping. Furthermore, Wangjiaping section shows lower δ34Spy (‒2.1±5.3‰) and lower TOC (2.4±1.1%) values but higher positive correlation (R2 = 0.66, p < 0.01) between TOC and Fepy/FeHR relative to deeper sections reported previously, suggesting that euxinia developed at Wangjiaping in response to increasing marine productivity and organic matter-sinking fluxes. Our reconstructed redox conditions and fossils at Wangjiaping in comparison with previously well-studied strata in the inner-shelf Xiaotan and Shatan sections suggest that planktonic and benthic planktonic trilobites with bioturbation appeared in the oxic water columns, whereas only planktonic trilobites without bioturbation occurred within the anoxic (even euxinic) water columns during Cambrian Age 3. This finding indicates that spatial heterogeneity of redox conditions in the shelves had an important effect on early animal distribution in the Yangtze Block. [ABSTRACT FROM PUBLISHER]

The Cambrian explosion is represented by a rapid diversification of early animals in which the role of marine primary productivity remains obscure. In this study, we analyzed multiple geochemical data, including TOC, major, and trace elements, in the basinal Yuanjia section, South China. Covariations among TOC, P/Al, Cu EF , and Ni EF suggest that they could be taken as effective marine productivity proxies in the early Cambrian Nanhua Basin. The similarities of Cd EF and Cd/Mo in the Nanhua Basin and modern upwelling settings suggest that they might be effective to track upwelling, where Cd and Mo were mainly controlled by plankton biomass and redox conditions, respectively. Our results indicate that Co EF and Co × Mn were invalid in evaluating upwelling because of the significant effects of water-column redox conditions on Co enrichments in the Nanhua Basin. The decreased TOC, P/Al, Cu EF , and Ni EF reflect a long-term decline in marine productivity from late age 2 to age 3. In comparison with the published results in the outer shelf (Jinsha, TZS drill core, YJK drill core, and GDM-1 well) and slope areas (TX-1 well), the fall in marine productivity might be common in the early Cambrian Nanhua Basin. Our results exhibit that the reduced marine productivity was accompanied by weakened upwelling, quiet hydrothermal activities, and enhanced local terrestrial fluxes, indicating that variations in marine productivity might be mainly driven by the development of upwelling in the early Cambrian Nanhua Basin. Comparison of marine productivity with fossil records suggests that food availability was sufficient to sustain the Cambrian explosion in the Nanhua Basin. We infer that marine productivity might indirectly stimulate early animal evolution through its significant impact on water-column oxygen levels in the early Cambrian Nanhua Basin., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)