Ni-Mo isotopes track marine metal cycling through early Cambrian redox shifts: Metalliferous black shales as paleo-environmental archives

Highly metalliferous black shales, enriched in trace metals like nickel and molybdenum, are valuable archives for reconstructing ancient marine environment and metal cycling. Despite various metallogenic models, the mechanisms driving NiMo mineralization remain unclear, with limited attention to continuous stratigraphic profiles and environmental context. To address this, we apply the coupled δ98Mo-δ60Ni isotopic approach, along with multi-proxy geochemical data, to a continuous early Cambrian section (Songlin section) and compares with three other Ni-Mo-enriched deposits across South China (Zunyi, Nayong and Zhangjiajie). δ98Mo values range from −0.3 ‰ to +2.1 ‰ and δ60Ni from −0.4 ‰ to +0.8 ‰, with a strong positive correlation (R2 = 0.51). Consistent trend between ore layers and host rocks suggests co-fractionation of Mo and Ni isotopes through adsorption onto FeMn (hydro)oxides. Stratigraphic trends suggest a three-stage paleoenvironmental evolution, with intensified euxinia and upwelling during Member II deposition favoring NiMo precipitation. While euxinia and organic matter burial contributed to metal retention in the sediments, these factors alone cannot fully explain the isotope variations. We propose that redox-sensitive FeMn (hydro)oxide cycling, modulated by variable redox, upwelling and primary productivity conditions, serves as the primary control on the metal accumulation. Comparable isotopic patterns in other metalliferous shales along the earth's history suggest a common mechanism during global redox transitions. This study underscores the potential of coupled δ98Mo-δ60Ni as powerful tracers for marine metal cycling and paleoredox reconstruction, offering new insights into the role of ocean chemistry in metal deposition during key redox transitions in the Earth's past.