Protracted ocean oxygenation in the early Mesoproterozoic

Emerging evidence suggests that, against a backdrop of generally low atmospheric oxygen levels, the mid-Proterozoic (ca. 1.8–0.8 Ga) ocean may have experienced transient oxygenation events, potentially peaking at ca. 1.4 Ga. However, whether these events were short-lived or represent enhanced longer-term oxygenation remains uncertain. Here we present molybdenum isotope (δ98/95Mo), iron speciation, redox-sensitive trace element and nutrient data for the ca. 1.33 Ga Baishugou Formation on the southern margin of the North China Craton. Our data document a stable oxygen-minimum zone featuring an euxinic to ferruginous core surrounded by dysoxic to oxic waters. In the euxinic interval, δ98/95Mo values are notably high (up to +1.62‰), indicating expanded ocean oxygenation. Combined with existing geochemical records, we suggest that the early Ectasian (1.4–1.3 Ga) was characterized by a sustained interval of relatively high oxygen availability in the oceans rather than episodic pulses, likely creating conditions conducive to early eukaryotic diversification. Sustained ocean oxygenation from 1.4–1.3 Ga, rather than transient pulses, may have facilitated early eukaryotic evolution, according to molybdenum isotope values and redox proxies from ca. 1.33 Ga North China shales.