Geochemical evidence of shales in the Tieling Formation, North China Craton: Implications for Mesoproterozoic episodic oxygenation events

Oxygen is one of the most important factors in the evolution of life. Some recent studies have shown that there were episodic oxygenation events during the quiet period between the two great oxidation events in the Proterozoic era, and the oxygen levels of the atmosphere and ocean were greatly increased around 1. 4 billion years ago. However, it is far to confirm the redox state of the epigenetic environment before 1.4 billion years. In this study, molybdenum isotopic composition and characteristics of redox-sensitive elements and rare earth elements of green shales of the similar to 1. 44Ga Tiding Formation of the Yanliao Basin in the North China Craton were analyzed. It was found that the Fe and Mn contents of the green shales were high with enriched light molybdenum isotopic compositions (-1. 00 +/- 0. 07 parts per thousand to - 0. 49 +/- 0. 06 parts per thousand). The Fe-Mn-Mo triadic correlation indicates that the incorporation of Fe-Mn (hydr) oxide by the form of adsorption co-precipitation, resulting in an enrichment of light Mo in Fe-rich sediments isotope, while the Mo isotope is heavier than that in seawater. It indicates these green shales were deposited in oxidized water bodies. In addition, the contents and ratios of redox-sensitive elements also reveal an oxidizing environment for the green shales of Tiding Formation. These results show that an oxygenation event should occur at similar to 1. 44 billion years ago with the expansion of seawater oxidation during this period, and the episodic oxygenation event in Mesoproterozoic may have provided material basis for the evolution of early life.