Characteristics Of Trace Elements About Mesoproterozoic Carbonate Rocks In Shennongjia And Palaeoenvironmental Significance

Study on sedimentary redox conditions of carbonates in geologic history has important paleoenvironmental implications.However,few proxies have been built for tracing such redox conditions.This study sampled fresh carbonates for analyses from the Yemahe and Wenshuihe Formations of the upper sub-group of Shennongjia Group,which is located at the Northern Yangtze Block.Organic matters were observed and detected by optical microscopy,scanning electron microscopy and transmission electron microscopy.Metamorphic temperature was determined by the Raman spectra of graphite and the crystalline degree of chlorite.The degree of pyritization(DOP)was determined by sequential extraction method.The contents of carbonate-associated trace elements(As,Ce,Mo,U,Ni,V and Cr)and Fe were determined by acid dissolution and ICPMS analyses.The correlations between trace elements and Fe were calculated and their implications to sedimentary redox environments were discussed.The results showed that many eukaryotic fossils were presented in Yemahe and Wenshuihe Formations,which probably indicate that the oxygen level in the meso-Proterozoic(~1200 Ma)was high.Metamorphic temperature was determined to be about 300~o C both for Yemahe and Wenshuihe Formation,which indicates that both formations have gone through the same metamorphic history.The effects of metamorphism on elemental speciation and transfer could be identical.Furthermore,it is found that metamorphic temperature can be correctly determined both by graphite and chlorite thermometers.DOP results showed that the sedimentary environment for Wenshuihe Formation is much sulfidic than that of Yemahe Formation.For Yemahe Formation,carbonate-associated As,Ce,Mo and Ni were positively correlated to Fe,and no obvious correlation was observed between carbonate-associated Fe and U,V and Cr.For Wenshuihe Formation,no obvious correlation was observed for carbonate-associated Fe and trace elements.Such results indicates that the correlations between carbonate-associated Fe and As,Ce,Mo and Ni can be used to trace the sedimentary redox conditions,and such tracing effect is derived from the constraints of the geochemistry behavior of trace elements by iron(hydr)oxides.This study found that the sedimentary condition was more oxidative for Yemahe Formation than that of the upper Wenshuihe Formation.Since the paleogeographic conditions were similar for both formations,it is considered that this sedimentary difference in redox condition was attributed to the difference of atmosphere oxygen level.About 1200 Ma before present,a decrease of atmospheric oxygen level probably has happened.