Research On The Paleo-ocean Environments Of Gaoyuzhuang Formation,Yanqing Area,North China

Lies between two significant oxygenation events,the middle Proterozoic is a special period of time in earth's evolving history.Due to the low atmospheric oxygen level,the ocean was mildly oxic in the surface,anoxic and sulfidic in the middle part,and anoxic and iron-rich in the deep part.Recently,many multicellular fossils were found in Gaoyuzhuang Formation.And the geochemical analysis of carbonate rocks from Gaoyuzhuang Formation indicates a small scale increase in the surface ocean oxygen level at 1.54Ga.Based on these findings,some researchers thought the fluctuated increased oxygen level might have facilitated eukaryotes'early evolving;However,it still lacks direct evidence for the increase of seawater oxygenation.Aimed at this scientific question,this study take the Gangou and Malugou section of Gaoyuzhuang Formation,Yanqing area,North China as study object to explore Gaoyuzhuang Formation's depositional period's ocean chemistry state,in particular the trend of the redox condition.And further understand the environmental conditions of early eukaryotes'evolving and the interplay between early life evolving and the environment.This study has analysized C-O isotopes,whole rock major elements,trace elements,total organic carbon and iron speciation of the carbonate rocks and the interlayered black shales from Gaoyuzhuang Formation of the studied area.Based on the iron speciation data of the black shales from Member Three,Gaoyuzhuang Formation and other geochemical proxies useful in distinguishing depositional environment,propose that the paleo-ocean condition of Gaoyuzhuang Formation's depositional period was dominated by anoxic and ferruginous environment.Combining the?¹³Corg,TOC?Total Organic Carbon?and cross check with C-O isotope analysis,propose that there was a small scale oxygenation event between the Member Two and Member Three,Gaoyuzhuang Formation of the studied area.According to the redox sensitive elements'enrichment and other geochemical proxies capable of reflecting redox coditions,this study propose that the C isotope composition of the carbonate rocks in Gaoyuzhuang Formation were mainly influenced by oxidation during deposition.The?¹³Corgrg of the sedimentary horizon riched in organic carbon of the middle part of Member Three,Gaoyuzhuang Formation,Gangou section decreased to minimum-34.4‰suggest that it might have some kind of relation with some specific chemoautotrophic or anaerobic microbes;The?¹⁸O values on the boundary of Member Two and Menmber Three fluctuated from-3.57‰to-9.51‰.This?¹⁸O value change might reflect the short term sea level change of Gaoyuzhuang Formation's depositional period related to the transgression-regression cycle.