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 (δ⁹⁸/⁹⁸Mo), 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, δ⁹⁸/⁹⁸Mo 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.