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Abstract: Although oxic surface, ferruginous deep, and euxinic intermediate waters have been documented, the redox heterogeneity of early Cambrian oceans remains largely unclear, precluding our understanding of the relationship between marine redox evolution and early animal diversification. In this study, we analyzed iron species, redox‐sensitive trace elements, and S‐N‐C isotopes of deepwater black shales in the Silikou section in the continental‐margin Nanhua Basin (South China), which represent extensive clastic input mainly from the Cathaysia Block during the early Cambrian (~541–509 Ma). Integrated data reveal a continuous shift in bottom‐water redox conditions at Silikou from euxinic in the lowermost black‐shale interval (BS1, 0–37.4 m) to oxic in the uppermost black‐shale interval (BS4, 296–372.9 m) accompanying a progressive movement of the O2/H2S redoxcline into the sediment. In between, most importantly, the BS2 interval (91–154 m) accumulated under manganous‐ferruginous conditions (i.e., Fe‐Mn reduction zone) characterized by an active Fe‐Mn particulate shuttle, and the BS3 interval (204–260 m) under nitrogenous conditions (i.e., nitrate reduction zone) characterized by strong denitrification processes, suggesting highly complex redox conditions in early Cambrian oceans. The observed sequence of redox conditions supports a strong lateral and depth‐related redox stratification in the early Cambrian ocean, including (from surface to deep and from nearshore to offshore) oxic, nitrate reduction, Fe‐Mn reduction, and euxinic zones, which were sequentially recorded at Silikou likely due to secular expansion of the oxic surface layer and/or long‐term relative sea level fall. Our study highlights the need for continued paleo‐redox studies to explore the redox heterogeneity of early Cambrian oceans. [ABSTRACT FROM AUTHOR]

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]