Pulsed oxygenation events drove progressive oxygenation of the early Mesoproterozoic ocean

The Mesoproterozoic era has long been considered a time of relative environmental and biological stasis. However, emerging insight suggests that this period may have more dynamic than previously considered, both in terms oxygenation potential consequences for evolution. Nevertheless, our understanding immense remains limited. To provide detailed constraints on dynamics, we report multiproxy geochemical study an early (∼1600–1540 million years ago, Ma) carbonate-dominated succession from the North China craton. We include inorganic carbon isotope (δ13Ccarb), iron-speciation, major trace element data, addition to molybdenum isotopic compositions (δ98/95Mo). These data support previous inferences persistent anoxic ferruginous deeper water conditions earliest ocean, with limited surface waters. behaviour these redox-sensitive proxies reveals pulsed events, each event increasing maximum depth oxygenation, leading overall progressive ocean. During events find lightest Mo signatures ever measured rock record, which attribute initial drawdown isotopically light association extensive Mn Fe (oxyhydr)oxide precipitation, followed by diagenetic recycling. shallower sediments deposited after pulses faithfully record composition coeval seawater. For samples, utilise single reservoir cycling model, constrained updated estimate seawater concentration, scaled using function associated differential organic flux between shelf basin. When modern rates accumulation under variable marine redox conditions, modelling estimates suggest minimum oxic seafloor area ∼30% total at ∼1540 Ma. It unclear whether observed across ∼60 year interval represents transition persistently oxygenated or oceanic oxygen levels fluctuated considerably through later Mesoproterozoic.