The Evolution Of Mesoproterozoic Oceanic Redox Condition In North China

In this study,the evolution of redox conditions and primary productivity in the Mesoproterozoic oceans were investigated on the basis of trace element analyses of samples collected from Jixian outcrops and three dillcores at Kuancheng in Hebei,North China.The co-evolution between oceanic environment and biology were discussed based on changes redox conditions and biology evolution in the Mesoproterozoic.Trace elements compositions indicated that oceanic redox conditions change from anoxic environment to suboxic condition at the interval from bottom to the top of the Changcheng system.Elements signatures imply that primary productivity was at low level at that interval.The accumulation of organic matters in sediments resulted in the rise of atmospheric O2 level.At the depositional interval from Gaoyuzhuang Formation to the lower of Wumishan Formation,oceanic environments change from suboxic condition to oxic condition and show pulsed oxygenations.This patterns of redox conditions suggest a slowly rise in dissolved oxygen level in seawater that may result from high primary productivity in the oceans at that time.The oxygenation of seawater resulted in the first negative Ce anomaly at the subtidal zone of the deposition of Gaoyuzhuang Formation.However,the first negative Ce anomaly only sustained a short time followed by an interval of anoxic environment owing to the consumption of oxygen by the oxygenation of organic matters in seawater.The second negative Ce anomaly occurred at the interval of the deposition of Yangzhuang Formation,and it caused by sea level down.At the depositional interval from the upper of Wumishan Formation to the Hongshuizhuang Formation,oceanic environments change from suboxic condition to sulfidic condition,and primary productivity in the oceans increased gradually.Changes in redox condition and high productivity may be resulted from in increasing inputs of sulfate and nutrient elements.Anoxic environment and high productivity are favourable for the preservation and accumulation of organic matters.At the depositional interval from Tieling Formation to Xiamaling Formation,oceanic environments change from sulfidic condition to oxic condition on account of insufficient nutrient supply and low primary productivity in the oceans.It led to the occurrence of the third negative Ce anomaly in ca.1.41 Ga subtidal zone of Tieling Formation.Those three weak oxidation events occurred at the same times as those were estimated by other methods.The evolution of biology,especially the eukaryotes,is close connected with the level of nutrient supply and oxygen concentration in oceans.The nutrient supply is influenced by the atmospheric pO2 level and oceanic redox conditions.The gradually oxygenation of Mesoproterozoic ocean resulted in the increasing of nutrient supply.Those environmental changes promoted the occurrence of multicellular eukaryotes and the improvement of eukaryotic biodiversity,which was consistent with the fossil records.So those changes in redox conditions may have contributed to eukaryote evolution in the Mesoproterozoic.