Redox Evolution Of The Mesoproterozoic Ocean Indicated By Carbonate-Associated Iodine Abundance

Molecular oxygen is essential for the evolution of early eukaryotes.However,oxygen concentrations during the critical period of eukaryotic origin and early evolution(i.e.mid-Proterozoic:1.8-0.8 Ga)have not been well constrained.In regard to the mid-Proterozoic oxygen levels,there exists two common opinions:'low'(pO2<0.1-1%PAL)or 'high'(pO2>0.1-1%or>4%PAL),but their potential fluctuations in the period were rarely dealt with.The poorly constrained oxygen levels,therefore,have severely hindered our understanding of the relationship between redox conditions and eukaryotic evolution.In order to further constrain atmospheric and shallow seawater redox variations in the mid-Proterozoic,and their potential influences on eukaryote evolution,shallow-water carbonates from the Mesoproterozoic Gaoyuzhuang,Yangzhuang,Wumishan and Tieling formations(1.6-1.4 Ga)of the North China Platform were studied using multiple approaches,including sedimentology,mineralogy,iodine and C-O isotope geochemistry.The results show that I/(Ca+Mg)ratios of most samples(202/366)are below the Proterozoic background value(<0.5 ?mol/mol),but some of them(11/366)can be up to>2.6?mol/mol,which represents oxic conditions compared with that in modern marine waters.The significant positive shift of I/(Ca+Mg)ratio in the Gaoyuzhuang Formation is associated with negative ?13Ccarb and ?13Corg anomalies,and also corresponds to the rapid evolution of eukaryotes.Semiquantitative estimates indicate that the background oxygen concentration of epicontinental sea in the North China Platform was higher than 1?M,but less than the maximum oxygen concentration that could be produced by primary production(10 ?M);while the oxygenation could increase shallow seawater oxygen concentration higher than 20 ?M,Oxidation of dissolved organic carbon is likely the major cause for the quick oxygen consumption after the pulsed oxygenations.The study also shows that the mid-Proterozoic oxygen levels of atmosphere and shallow seawater were highly dynamic,and short-lived pulsed oxygenations could facilitate the evolution of eukaryotes,but overall anoxic background may have hampered the continuous evolution of early eukaryotes during the Mesoproterozoic period.