Carbon–oxygen–barium Isotope Constraints on Early Cambrian Barium Cycling Recorded in a Witherite Deposit in Chongqing

The early Cambrian represents a critical transitional interval in Earth history, characterized by fluctuations in atmospheric-oceanic oxygen levels, rapid diversification of animal phyla, and widespread mineralization of critical elements. Stratiform sedimentary barium deposits are widely developed within early Cambrian strata of South China. Barite-only (BaSO4) deposits in the southern Yangtze Block have been extensively studied, whereas the witherite-dominated (BaCO3) deposits in the northern Yangtze Block remain debated with respect to their formation mechanism and the coupling between paleoenvironmental change and barium mineralization. Here we present a systematic multi-proxy investigation (major and trace elements, δ¹³Ccarb, δ¹⁸Ocarb, and δ¹³⁸Ba) of ZK603 drill core in Chengkou. BaO contents indicate that barite was deposited together with witherite in the main ore horizon. The δ¹⁸Ocarb values range from -18 to -7 ‰ and show no distinct excursion across the ore interval. In contrast, a pronounced negative carbon isotope excursion (-10 to -16 ‰) occurs within the main ore layer and recovers to ~ -12 ‰ above the ore, indicating substantial perturbations to the carbon cycle during mineralization. For carbonate fractions, δ¹³⁸Ba varies from +0.07 to +0.37 ‰ and shows no relationship with δ¹⁸Ocarb. In contrast, silicate fractions display δ¹³⁸Ba values ranging from -0.16 to +0.44 ‰ and show a moderate correlation with δ¹⁸Ocarb (R²≈ 0.39). Additionally, δ¹³Ccarb values correlate with Y/Ho ratios in both fractions (R²≈ 0.40), suggesting links between productivity changes, seawater chemistry, and barium enrichment. This study provides the first integrated C-O-Ba isotope characterization of early Cambrian witherite deposits from the Chongqing region. By combining elemental and isotopic signatures from carbonate and silicate fractions, we reconstruct redox conditions, productivity changes, and Ba cycling during mineralization. Our results highlight the role of evolving marine Ba cycling in controlling the formation of sedimentary barium deposits and demonstrate that witherite mineralization may record broader changes in early Cambrian ocean chemistry and stratified basin dynamics.