Indicators Of The Early Cambrian Palaeo-marine Environment In Tongren,Guizhou

The Cambrian is an important period for the evolution of life and marine environment in geological history.The Cambrian black shales,which exist widely in the world,record the important paleo-oceanic information in this critical period.A set of shales with a thickness of about200m and a stable distribution are widely developed in South China,which provide important data for the study of palaeo-marine environment,major geological events and biological evolution.In this paper,the black mudstone of the Lower Cambrian Niutitang Formation in Tongren area of Guizhou Province is taken as the research object.Based on various geochemical data,such as main and trace elements,rare earth elements,total organic carbon(TOC)contents,total sulfur(TS)contents,mercury(Hg)and nitrogen(N)isotopes composition,a detailed analysis is made on the Tongren profile in Guizhou and the core of ZK609 well.The Early Cambrian depositional redox environment,the occurrence phase and anomalous source of Hg,the controlling factors of N isotopic composition and ocean N-cycle are determined in Tongren area,Guizhou.And the indication of the sedimentary environment of the palaeo-marine N-cycle and abnormal accumulation of Hg are discussed.The results show that the Mo and Uranium(U)enrichment factors(EF)indicate that the Early Cambrian sedimentary water gradually changed from reduction to oxidation,but there is a reduction-oxidation-reduction-oxidation cycle fluctuation.There is abnormal enrichment of Hg content in black mudstone of Niutitang Formation,and organic matter is the main occurrence phase of Hg.The anomalous Hg enrichment and Mass independent fractionation(MIF)composition indicate that the strong hydrothermal activity of the Early Cambrian is the anomalous source of Hg in the black mudstone in Tongren,Guizhou.High content of metallic elements(such as Mo,U and Ba)entered the seawater through intense submarine volcanic activity,which changed the composition of metallic elements and nutrient elements in the palaeo-marine environment.Intensive hydrothermal activity may be one of the important factors for the environmental and biological evolution of the Early Cambrian in South China.In addition,there is an obvious negative migration of nitrogen isotopes(?¹⁵N_TN)in the bottom of Niutitang Formation.Combined with organic carbon and oxygen isotopic data,the negative migration of?¹⁵N_TN values are controlled by nitrate assimilation and nitrogen fixation,and the enhancement of nitrogen fixation is the main reason for the negative migration of?¹⁵N_TN.The Marine nitrogen cycle studies have shown that the ocean of early Cambrian is under a water condition of gradually oxidation,along with the organic carbon pool in marine environment gradually reduce and biological proliferation,phosphorus(P)and other nutrients in the water consumption gradually,cause nitrification and nitrogen fixation enhanced gradually,nitrate(NO₃^-)content increased,and ocean gradually in nitrate-rich environment.The variation characteristics of?¹⁵N_TN values reflect the changes of paleo-marine productivity and marine trophic environment,which is of great significance to the study of paleo-marine environment and biological evolution in the Early Cambrian.