Protracted Ocean Oxygenation in the Early Mesoproterozoic

The mid-Proterozoic (ca. 1.8–0.8 Ga) is generally regarded as a period of persistently low atmospheric oxygen, yet mounting geochemical evidence indicates that this interval was punctuated by discrete oxygenation events, with peak oxygen availability potentially reached at ca. 1.4 Ga. A critical unresolved question for both Ectasian chronostratigraphy and the reconstruction of Earth-system evolution is whether these geochemical signals record short-lived, episodic redox excursions or a sustained, long-term increase in surface oxygen levels. In this study, we present an integrated geochemical dataset, comprising Mo isotopes (δ⁹⁸/⁹⁵Mo), Fe speciation, redox-sensitive trace elements, and nutrient proxies that are from three stratigraphic sections of the ca. 1.33 Ga Baishugou Formation on the southern margin of the North China Craton. The data consistently document a vertically stratified water column during Baishugou deposition, with an oxygen-minimum zone (OMZ) characterized by a euxinic-to-ferruginous core surrounded by dysoxic-to-oxic waters. Within the euxinic interval, δ⁹⁸/⁹⁵Mo values reach up to +1.62‰, significantly elevated relative to coeval global average seawater, and indicative of expanded ocean oxygenation beyond the local depositional setting. Integration of these new records with published geochemical data demonstrates that elevated oceanic oxygen availability was sustained over a stratigraphically significant interval of several tens of millions of years (ca. 1.4–1.3 Ga). This prolonged oxygenation horizon constitutes a potentially valuable chemostratigraphic marker for inter-basinal and global correlation of Ectasian strata, complementing existing lithostratigraphic and biostratigraphic frameworks. Furthermore, the persistence of expanded oxygenation throughout the early Ectasian likely established the ecological conditions including increased nutrient availability and expanded aerobic habitats which facilitated early eukaryotic diversification, linking Mesoproterozoic redox evolution directly to the biological record preserved in contemporaneous strata.