1.44 Ga Oxygenation Event: Evidence From the Southern Margin of North China

Previous studies suggest that the atmospheric and shallow seawater oxygen levels during the mid‐Proterozoic may have remained persistently unbalanced due to the low atmospheric oxygen levels and that, under these conditions, the oxygen concentrations of shallow seawater would largely reflect local oxygenation caused by primary productivity. As a result, in a microbialite‐rich setting, it may be difficult to differentiate oxygenation caused by local primary productivity from overall oxygenation of the surface environment. To address this issue, we conducted an integrative study of the ∼1.44 Ga stromatolitic carbonates of the Fengjiawan Formation in the Xiong’er Basin, southern North China, using techniques of sedimentology, mineralogy and geochemistry. The results show that the Fengjiawan Formation is dominated by water‐column precipitated carbonate mud (now dolomitized), pointing to moderately oxidized marine conditions. A significant and stable negative Cerium (Ce) anomaly (as low as 0.42) is identified in a long interval (>50 m in thickness) of this formation. This Ce anomaly is lower than the minimum values both recorded in the Great Oxidation Event (GOE; minimum 0.76) and in the Neoproterozoic Oxygenation Event (NOE; minimum 0.52), suggesting a significant oxygenation process rather than an oxygen oasis. Some intervals in this formation show I/(Ca + Mg) values higher than the Precambrian background value of 0.5 μmol/mol, providing a further support for shallow seawater oxygenation. This study, together with the enhanced oxygenation identified in the time‐equivalent Tieling Formation, indicates a multi‐basin oxygenation event in North China at ∼1.44 Ga. Plain Language Summary The ∼1.44‐billion‐year‐old Tieling Formation of the Yanliao Basin in the northern part of North China records a significant increase of oxygen levels. However, it remains uncertain whether this represents a localized marine oxygen oasis or an extensive atmospheric oxygenation. To settle this problem, we conducted an integrated study of the stromatolitic carbonates in the time‐equivalent Fengjiawan Formation of the Xiong’er Basin, southern North China, in terms of sedimentology, mineralogy and geochemistry. Our study shows that the Fengjiawan Formation is dominated by water‐column precipitated carbonate mud with negative Cerium anomaly and iodine enrichment indicative of significant increase of oxygen levels in shallow marine environment. Combined with the oxygenation evidence from