Multicellular eukaryotes followed:MOE or life stagnation

The discovery of the fossils of decimetre-scale multicellular eukaryotes in the Mesoproterozoic from the 1.56-Gyr-old Gaoyuzhuang Formation in the North China Craton indicates that eukaryotes have evolved to a high level, but the evolution of life after it is still unclear due to the lack of definitive fossil evidence. Multi-proxies suggests that a pulsed oxygenation event was recorded during1.56-1.57 Ga, but the subsequent ocean oxygen levels are unequivocal. Here we report I/(Ca+Mg) ratios, carbonate C-O isotopes, and Ce anomaly across the ca. 1.56-1.50 Ga in the North China Platform. The results showed that the evolution of ocean oxygen content could be divided into four stages: (1)The δ13Ccarb values remained stable at 0‰ , and the I/(Ca+Mg) ratios was near detection limit, with no Ce anomaly from 1.56 to 1.53Ga, indicating that the ocean was anoxic; (2)The I/(Ca+Mg) ratios increased to 1.2μmol/mol with an excursion from 0‰ to -2.6‰ negative δ13Ccarb anomaly, and a negative Ce anomaly to 0.56 in 1.52Ga, which may be the result of the oxidation of the dissolved organic carbon(DOC) in the ocean, and the ocean changed from anoxic to oxic;(3) The C isotope composition 0‰ , with I/(Ca+Mg) ratios maintained at 0μmol/mol-0.5μmol/mol, and there was no Ce anomaly in 1.51Ga. Oxygen consumption through oxidation of DOC may have quickly lowered marine O2 levels to suboxic.(4) A positive shift in  δ13Ccarb from 0 ‰ to +2.1‰, and the I/(Ca+Mg) increased to 1μmol/mol, without obvious Ce anomaly, which may be a certain degree of biological flourishing leading to the increase of oxygen content in the ocean. Although the ocean in 1.52Ga was oxic, the oxygen levels were low, which may be a key factor restricting the evolution of eukaryotes.