Genesis of manganese-rich deposits in the Mesoproterozoic Gaoyuzhuang Formation, North China: Implications for atmospheric redox state and early eukaryote evolution

Precambrian Mn deposits account for more than 77 % of global Mn resources, with most formed during periods of significant atmospheric oxygenation: the Paleoproterozoic Great Oxidation Event (GOE) and the Neoproterozoic Oxidation Event (NOE). The intervening interval, between 1.8 and 0.8 Ga, has historically been labelled as the “boring billion” without significant sedimentary Mn accumulation. However, recent studies have revealed episodic oxygenation events during the Mesoproterozoic, accompanied by distinct Mn-rich sediments. This study reports on the petrography, mineralogy, whole-rock, and carbonate fraction geochemistry of the ∼1.56 Ga Mn-rich sediments within the lower 2nd Member of the Gaoyuzhuang Formation in the Yanliao Basin, North China. Sedimentological evidence indicates that these Mn-rich deposits formed along the continental shelf margin during a transgression in the early Mesoproterozoic. Geochemical data, including weakly positive Eu anomalies (up to 1.22) and a positive correlation between Mn contents and εNd(t) (n = 10, r = +0.74, p(α) = 0.01), suggest that dissolved Mn²⁺ was derived from distal hydrothermal vents. This conclusion is further supported by discrimination diagrams constructed from both whole-rock and carbonate fraction data. The Mn(II)-bearing minerals are dominated by rhodochrosite and kutnohorite. Microscopic features and negative δ¹³C_carb values (−6.72 ‰ to −1.08 ‰, avg. = −4.43 ‰) suggest that rhodochrosite formed during early diagenesis, while kutnohorite precipitated from both anoxic seawater and sediment porewater. Subsequent Mn(IV)-oxide reduction occurred during diagenesis (affecting all rhodochrosite and some kutnohorite) or near the redoxcline (for portions of kutnohorite), reflecting a pulsed oxygenation event. A sudden increase in Fe and S concentrations and the presence of layered pyrite indicate a transient euxinic environment immediately following Mn deposition. This euxinia was probably driven by enhanced oxidative weathering on land, which increased sulfate fluxes to the ocean. Thereafter, δ¹³C_carb became decoupled from δ¹³C_org, likely due to the expansion of the oceanic dissolved organic carbon (DOC) reservoir. This expanded DOC reservoir may have been oxidized immediately prior to the appearance of macroeukaryotes in the overlying 3rd Member of the Gaoyuzhuang Formation. Notably, sustained oxygenated surface conditions capable of supporting a diversified eukaryotic ecosystem likely emerged only after the oxidation of excess DOC and electron donors (e.g., Mn²⁺ and Fe²⁺). Therefore, we propose that the rising atmospheric oxygen levels during the deposition of the lower 2nd Member of the Gaoyuzhuang Formation, combined with the expanded DOC reservoir, created conditions favourable for macroeukaryotes evolution.