Ocean oxygenation in the aftermath of the origin of multicellular eukaryotes: Evidences from Ce anomaly and I/Ca of the Yangzhuang Formation at 1.50 Ga

Multi-proxies and decimeter-scale multicellular eukaryotes fossils in the North China Craton indicate a pulsed oxygenation event at 1.56–1.57 Ga. It has been widely proposed that oxygen concentration is the most important control factor governing the evolution of life. However, the subsequent evolution of life and ocean redox condition are less constrained. We here report I/(Ca + Mg) values, carbonate C isotopes (carbon) and cerium (Ce) anomalies across the ca. 1.56–1.52 Ga Gaoyuzhuang-Yangzhuang Formation in the North China Platform, in order to reconstruct the ocean redox conditions and its relationship with the evolution of life. Our geochemistry proxies demonstrate that after the pulsed oxygenation event at 1.57 Ga, the content of oxygen rapidly decreased to lower levels and continued until 1.55 Ga. Negative Ce anomaly (as low as 0.56) and high I/(Ca + Mg) values (up to1.2 μmol/mol) are identified at 1.54 Ga, indicating a possible pulsed oxygenation event. Furthermore, the negative δ¹³C_carb excursion from 0.5 ‰ to −2.6 ‰ recorded by the Yangzhuang formation supports oxidation of the dissolved organic carbon (DOC) in the early Mesoproterozoic Ocean. No Ce anomaly and low I/(Ca + Mg) values (＜0.5 μmol/mol) are observed at 1.53 Ga, suggesting that the shallow ocean oxygen levels decreased to lower levels again. Thus, the Yangzhuang Formation may have recorded a transient oxidation event at 1.54 Ga, but the fossil record indicative of eukaryotic evolution is missing until Neoproterozoic. Therefore, we concluded that besides the redox condition of ocean, the limited input of key trace metal elements and higher seawater temperature may have hindered the evolution of eukaryotes in the Mesoproterozoic.