Geochemical Characteristics Of Mesoproterozoic The Third Member Of Gaoyuzhuang Formation,the North Of Hebei Province And Its Paleoenvironment Significance

The mid-Proterozoic?ca.1.8-0.8 Ga?lies between the Great Oxygenation Event?GOE?and the Neoproterozoic Oxygenation Event?NOE?.As an important period in the geological history,the mid-Proterozoic witnessed the appearance of eukaryote and their early evolution.Studying Mesoproterozoic's ocean chemistry condition has great scientific significance in revealing ocean environment's influence in eukaryote's early evolving.The Mesoproterozoic Gaoyuzhuang Formation in North China platform preserved abundant microscopic eukaryote fossil and macroscopic multicellular eukaryote fossil.However,the ocean chemistry condition that the eukaryote lived is not well studied.This paper has chosen the carbonate rocks and shales from the third Member of Gangyuzhuang Formation,Pingquan Wangzhangzi section and Kuancheng Jianshanzi Bridge section,northern Hebei Province as study objects.Using sedimentary facies analysis and geochemical data,this thesis mainly deals with the ocean chemical conditions of the third Member of Gangyuzhuang Formation deposition and discusses the potential cause for the eukaryotic evolution and their delayed diversification at that time.?¹³C_carb??¹³C_org??C(?C=?¹³C_org-?¹³C_carb)and TOC comprehensive analysis suggest that North China epicontinental sea has a shallow chemocline and low oxygen concentration,and its subtidal zone and below are dominated by chemoautotrophic microbes.This indicates that the early eukaryote evolving North China epicontinental sea has a common low oxygen feature.According to Mn/Fe ratios,from the bottom to top of Member Three,Gaoyuzhuang Formation,the seawater oxygen concentration decreased and then increased slowly;The shale from the lower part has a deeper depositional depth,a low and stable Mn/Fe ratios,representing an anoxic environment?The P/Al ratios analysis indicates that the atmospheric oxygen level during deposition of Member Three,Gaoyuzhuang Formation,of the studied section has a almost constant,slowly increased and then abruptly decreased trend.This implies that the atmospheric and seawater oxygen level was low.And it might be one of the important reasons preventing eukaryote's early evolving.Based on RSE distribution analyze,the seawater's anoxic degree increased gradually from the lower to upper part of Member Three,Gaoyuzhuang Formation and has a sulfidic trend at the top part;The atmospheric oxygen level increased gradually.There is a small oxygenation event at the upper part of Member Three,leads to increased terrestrial weathering input and BSR,finally result in short-lived sulfidic and delayed eukaryote evolving.The P and Mo abundance increased simultaneously from the upper part of Member Three of the studied section.This coincides with the macroscopic multicellular eukaryote's horizon,suggests that the increased bioessential elements has promoted the macroscopic multicellular eukaryotes and microbes growth.