Trace Elements As Redox Paleoenvironments Proxies In Xiamaling Formation Of The Mesoproterozoic In North China And Their Geological Significances

The origin of life and its earlier evolution in the Precanbrian is one of the major and hard concerns in nature science.It has been suggested that,during the mid Proterozoic(ca 1.84 to ca 0.85Ga),the ocean was in a permanent stratified status,with the surface layer moderately oxygenated,middle layer euxinic,and the deep layer anoxic but ferruginous.In order to further understand the environmental evolution of the sulfur-cycle anomalous ocean and the possible geobiological processes in such an oceanographical condition,This study attempts to find some more new evidence from the Mesoproterozoic sediments in the North China platform,and to further explore into the interactions between early-life evolution and environments.A total of 44 samples were collected from the Xiamaling Formation(1.53-1.6Ga)at Pingquan sections,Hebei province.Most of the samples are from sediments and sedimentary rocks,Major elements,trace elements,especially redox-sensitive elements,TOC and S isotope composition of these samples were carefully analyzed by aids of various suitable instruments.A number of geochemical proxies that are useful for the determination of depositional environments,such as those for paleoclimate(Mg/Ca),Paleobathymetry(Sr?Sr/Ba)etc,were selected as major indicators in concert with TOC data and facies analysis.Based on the Mesoproterozoic euxic ocean model,and compared with those in the modern Black Sea and Cariaco Basin,as well as those in some of the typical Mesoproterozoic anoxic basins,the proxies of Mo/TOCetc.In Xiamaling Formation show the characteristics of episodic euxinia paleoceanographic chemistry during its deposition.According to the analysis of U,V,and Mo concentrations of the tested 37 samples and their covariation relationships with Th and Al contents in the Xiamaling black shale,we thought that the black shale samples are suitable for the study,and the obtained results can be used to reflect the ancient marine environment and the redox sate.The enrichment factors show clearly enrichment of the redox sensitive elements Mo,V and U in this formation,especially Mo.A weak correlation of enrichment factors between the three elements and TOC suggests that the black shale in the third member of the Xiamaling Formation was possibly deposited in an anoxic or even euxinic environment.The test results of redox sensitive element ratios V/(V +Ni),V/Cr,Mo/TOC,and ?U content may have reflected an anoxic depositional environment,with strong reducing conditions,or even a euxinic environment.Hydrothermal activities were likely present in the basin.during the deposition.The present study has provided insights into the analysis of Mesoproterozoic depositional environments and paleocean chemistry in the North China Platform.In addition,the study also shows that in the analysis of ancient depositional environments,some of the geochemical proxies can not be used independently,and should be combined with some other proxies together in order to get a more reliable result.