Geochemistry Of Selenium Of Doushantuo Formation In Younzte Platform

The second Great Oxidation Event(GOE2) began at Neoproterozoic- Early Cambrian, where the redox-sensitive elements of sedimentary rocks appear gradually enriched in this period. Selenium is a typical redox-sensitive element, and selenium-enriched sedimentary rocks may indicate changes in ocean redox environment. In order to discuss ocean redox conditions of this period, we systematically analyzed Se and other redox-sensitive elements in the black shales of Member IV, upper Doushantuo Formation in Yangzte Platform, South China. With the system work of Se value, major elements values,trace elements values and Se isotopes researches, we get the following understandings:(1) The average Se values of the six sections are 19.02 ppm, 14.97, 4.8ppm, 7.7ppm, 2.18 ppm and 3.31 ppm,which are much higher than the Se crustal abundance(0.05ppm). Se values and Co values correlation reflects the relationship between the biological effects and Se enrichment; the relations of Se values and Ag values, Se values and the V values implies a combination of Se(-?) with Ag and V. Se enrichment have different degrees of relevance with elements of Co, Ag, V, etc.(2) Mo-U conversation indicates the anoxic environment of Ediacaran Doushantuo Formation in Yichang and Guizhou, and fluctuation of Co and Mo values indicates the fluctuating redox conditions. Mo-U system indicates varying degrees of anoxic conditionon the Ediacaran ocean of the Yangtze plate: Baiguoyuan section is located neritic mainly anoxic environment with the most abundant land-based sources and the highest degree of Se enrichment; Jiulongwan section was in deeper stable sulfide water location, have a more stable anoxic(sulfide) environment, and the reduced state Se is fully converted towith Ag, V combine to form enrichments; Chenjiayuanzi section was at the lower edge of the sulfide water, and there are fluctuations in the degree of anoxic marine environment, and the degree of Se enrichment is slightly lower than Jiulongwan section; Wuhe section in Guizhou was located in continental shelf at Ediacaran, and the seawater was intermittent sulfidic, and the anoxic environment is relatively stable, and the Se enrichment is similar to Chenjiayuanzi's; Guizhou Nangao section was located in continental shelf, and the biological effect is weak,and the water may be affected by hydrothermal action, while the Se enrichment is in low degree; Guizhou Sandu section, with the minimum degree of Se enrichment, was located in phase transition zone between shelf and deep-sea, andthe marine environment was influenced by the ironic hydrothermal ingredients.(3) The Se isotopes show that there is a the fraction of Se isotopes in different ocean space deeps. Combined with predecessors' and contemporaries selenium isotope data, we make a simple analys on the ocean stratification of selenium isotope fractionation. The ?82/76 Se range of Chenjiayuanzi section Doushantuo Formation Member ? black shale was 0.30‰~1.60‰,with an average of 0.73‰ and heavy selenium isotope fractionation, which is due to the reduction of seawater curing priority to restore the weight selenium isotopes. Srtandmann et al.(2015) researched ?82/76 Se of Doushantuoan black shale on the continental shelf,ranging from-0.84 ~ 0.48 ‰, which indicates the average value of the marine environment of ancient anaerobic and iron-rich marine environment, and the lightweight selenium isotope fractionation indicates the potential anoxic marine environment and ironic hydrothermal ingredients.(4) Considered the previous data and the new data in this paper, inorganic selenitea and selenate partially turn to the organic selenium by biological assimilation effects in shallow water; inorganic Se(?) and Se(?) partly deposit into the deep ocean sediments by adsorption in oxic environment, while in anoxic/euxinic environment; Se(?) and Se(?) in sediments are also able to convert to Se(0) or Se(-II) in anoxic/euxinic environment.Selenium enrichment of Ediacaran Doushantuo Formation related to transgression, biological effects, sulfidic water, ironic and instability anoxic marine environment. Regional factors of different sections also contributed to the different degrees of Se enrichment.