Variations In Precipitation Pathways Of Mesoproterozoic Shallow Seawater Carbonates From North China Platform: Response In Seawater Redox Fluctuations?

It has been considered that the precipitation pathways of marine carbonates are closely related to seawater redox conditions.Due to the presence of carbonate precipitation inhibitor(Fe²⁺,Mn²⁺)-rich shallow seawaters during Archean and Paleoproterozoic,nucleation of calcite muds in water column was inhibited but formation of seafloor aragonite precipitates was allowed.In contrast,the oxidative removal of carbonate precipitation inhibitors in Neoproterozoic shallow seawaters promoted the direct precipitation of calcite muds from water column.However,it needs more detailed case studies to test the connection between seawater redox and pathways of carbonate precipitation,since the secular variation in the pathways of carbonate precipitation may controlled by other factors.In this study,the fabric and geochemistry of carbonates deposited during Mesoproterozoic,a transitional period of carbonate precipitation,were studied.It has shown that the Member III of the Gaoyuzhuang Formation(?1.56 Ga),the Member IV of the Wumishan Formation(?1.48 Ga),and the Member II of the Tieling Formation(?1.44 Ga)in North China have abundant calcite muds.These water column precipitated calcite muds have relatively high I/(Ca+Mg)ratios(generally>0.5?mol/mol)and negative Ce anomalies(down to 0.8),indicating moderately oxygenated conditions.In contrast,abundant seafloor precipitated aragonite fans occur in the lower Member IV of the Gaoyuzhuang Formation(?1.55 Ga)and the Member II of the Wumishan Formation(?1.50 Ga).These seafloor precipitates have near zero I/(Ca+Mg)ratios,suggesting suboxic to anoxic conditions.Therefore,this study,firstly using detailed cases,proves that the textures of Precambrian carbonates were largely controlled by the redox conditions of seawaters and could be used as a redox proxy to conduct long-term and multi-section study of marine redox conditions directly and efficiently based on outcrop observations in the field.