Geochemistry, Geophysics, Geosystems (Aug 2018)
Evidence for Highly Complex Redox Conditions and Strong Water‐Column Stratification in an Early Cambrian Continental‐Margin Sea
Abstract
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.
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