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• Negative Ce anomaly in the Gaoyuzhuang and Hongshuizhuang formations provides the evidence of transient oxygenation of the Mesoproterozoic ocean. • The proposed Ce anomaly depth model provide a quantitative constrain the surface environment of the Yanshan Basin. • Transient oxygenations could have provided vital niches for the evolution of eukaryotes. Predominantly anoxic conditions with a heterogeneous stratified water column are widely accepted for the Mesoproterozoic ocean. However, transient and episodic oxygenation during the Mesoproterozoic has been recently hypothesized, which may have been an important trigger for eukaryote evolution. In this study, we analyzed the Ce anomalies of the Gaoyuzhuang and Hongshuizhuang formations in North China, to investigate the Earth's surface environment during the Mesoproterozoic. Our data of negative Ce anomalies in the Gaoyuzhuang and Hongshuizhuang formations, reaching 0.73 and 0.84, respectively, suggest transient oxygenations of the surface environment. We applied a Ce N /Ce N * depth model to quantify these transient oxygenations and the results suggest that the atmospheric oxygen level would have increased by 0.01–0.03% present atmospheric level (PAL), 0.02–0.05% PAL, and 0.1–0.4% PAL, with the respective initial levels of 0.05% PAL, 0.1% PAL, and 1% PAL, in the Gaoyuzhuang and Hongshuizhuang formations. Our results suggest that although oxygen levels in the Mesoproterozoic were low, transient oxygenation may have occurred in the surface ocean, which would have provided vital niches for the evolution of eukaryotes. [ABSTRACT FROM AUTHOR]

• The Fe speciation data help bridge the global data gap of Fe species gap during 1500–1400 Ma. • The global Fe speciation data suggest underestimated oxic depositional conditions during the Mesoproterozoic. • Oxic environment could have provided suitable habitats for complex eukaryotes and their subsequent evolution. The Mesoproterozoic (1600–1000 million years ago, Ma) is thought to have experienced important changes in ocean chemistry and eukaryote evolution. Our understanding of global redox conditions remains in its infancy, and redox data derived from the Mesoproterozoic thus far indicate heterogeneity and complexity in the realm of ocean redox, which hampers our understanding of the potential significance of variations in ocean redox on eukaryote evolution. In this study, we report analyses of Fe species and redox-sensitive trace metals of U and Mo from the Jixian Group, including the Wumishan, Hongshuizhuang and Tieling formations, as well as the Xiamaling Formation, in the Yanshan Basin in North China. The Fe speciation data from the Wumishan, Hongshuizhuang, and Tieling formations first reported in this study help bridge the global data gap of Fe species data during 1500–1400 Ma. Our data suggest likely oxic bottom water conditions for the Wumishan Formation and ferruginous bottom water conditions for the Tieling Formation. The Fe species data can distinguish ferruginous conditions for the upper Hongshuizhuang from sulfidic depositional conditions for the lower-middle Hongshuizhuang Formation. Based on Fe species and trace metal data, redox conditions likely varied from anoxic conditions with transient euxinia in the lower-middle Xiamaling Formation to oxic conditions in the upper Xiamaling Formation. The new Fe speciation data from the Yanshan Basin, when combined with previously published data, show a surprisingly high proportion of middle Proterozoic samples with very low highly reactive to total iron ratios. This suggests either a relatively large extent of oxic seafloor, or non-uniformitarian iron cycling. Resolving this discrepancy will be key to understanding the extent of suitable habitats for complex eukaryotes. [ABSTRACT FROM AUTHOR]