The redox properties of the shallow sea in the Longjiayuan Formation of the Middle Proterozoic in western Henan Province

Key words: 
• mesoproterozoic /
• longjiayuan formation /
• carbonatite /
• Ce anomaly /
• pulsed oxygenation

Abstract: Abstract: [Objective] The level of oxygen content in shallow sea and atmosphere during the Mesoproterozoic (about 1.8-1.0 Ga) has been controversial.Due to the slow evolution of eukaryotes during this period, the redox state of the surface ocean and atmosphere has been considered to be a relatively stable hypoxic state. However, in recent years, more and more evidence shows that there are dynamic fluctuations in oxygen content during this period. However, due to the heterogeneity of time and space, people 's understanding of the global redox state during this period is still very limited.[Methods] In this study, the redox state of the shallow sea in the southern margin of the North China Craton was studied by analyzing the Ce anomaly, δ13C and Y/Ho indexes of carbonate rocks in the Longjiayuan ( 1.59-1.54Ga ) Formation.[Results] The results show that there are obvious negative Ce anomalies ( 0.53 ~ 0.94 ) and positive δ13C anomalies before and after 1.54 Ga in this area, accompanied by a significant increase in Y / Ho ( average 64 ), and later returned to normal levels.[Conclusion] It indicated that there might be a transient oxidation event. This oxidation event is consistent with the contemporaneous strata of the Jixian System in the northern margin of the North China Craton, indicating that the fluctuation of shallow sea oxygen content during this period may be widespread in the North China Craton. This study can provide direct evidence for the pulse oxygenation event in the Mesoproterozoic, and help to further determine the redox state of the shallow sea during this period and the effect of oxygen on the evolution of eukaryotes.