Large volcanism, enhanced oceanic productivity, and efficient organic matter burial during Mesoproterozoic oxygenation events: Insights from mercury and carbon isotopes in the Jixian Group, North China Craton

During the Mesoproterozoic, atmospheric O₂ levels were persistently low but interrupted by transient oxygenation events. The causes of these oxygenation events remain less clear. Large volcanism can transiently emit massive mercury (Hg), resulting in high Hg peaks in sedimentary successions. Here, we measure whole-rock rare earth elements and yttrium (REY) concentrations, Hg concentrations, and Hg isotope compositions across the Mesoproterozoic Jixian Group, North China Craton. REY concentrations mark two Mesoproterozoic oxygenation events (MOEs) in the Jixian Group, termed here as MOE1 (1.59 to 1.56 Ga) and MOE2 (1.48 Ga). Mercury peaks are observed at the bottom of MOE1 and MOE2 intervals, implying the presence of large volcanism. The two intervals are organic-rich and display positive excursions in Δ¹⁹⁹Hg values, which can be explained by enhanced input of volcanic Hg into the ocean and efficient scavenging of seawater Hg via organic burial. Combined with published carbon and sulfur isotopic data, we conclude that large volcanism during MOE1 and MOE2 could lead to increased input of nutrients into the ocean, which enhanced oceanic productivity to release abundant free O₂ into the atmosphere, and resulted in high organic matter burial rates and proliferation of heterotrophic microorganisms (e.g., sulfate-reducing bacteria), leading to euxinic conditions in deep seawater and enhanced precipitation of seawater-Hg. This work establishes a causal link between large volcanism and paleoenvironmental changes during MOEs.