| The Ediacaran-Cambrian transition period(E-C transition period)is one of the most significant environmental and life form changes in the history of the earth's evolution.During this critical period,with the extinction of Ediacaran biota,a gradual and multi-episode biological radiation process took place,that is“Cambrian Explosion”.The Ediacaran-Cambrian boundary(E-C boundary)serves as a"watershed"to separate the Neoproterozoic mollusk Ediacaran biota from the Cambrian small shell biota.Therefore,it is particularly important to reveal in detail the paleo-ocean environment in this geological historical period where the E-C boundary is located,which is not only the basis for exploring the co-evolution of environment and biology,but also a part of understanding a number of major geological and historical events.At present,some scholars have studied the paleo-ocean environment near the E-C boundary,but because the sections studied by scholars are not supported by the high-precision age of the E-C boundary,as a result,the accurate redox information of the paleo-ocean near the E-C boundary is still vague.In order to reconstruct the paleo-ocean environment in this geological and historical period where the E-C boundary is located,this paper takes the high-precision zircon CA-ID-TIMSU-Pb age-dependent deep-water facies Pingyin section as the research object,carries out a high-resolution Fe speciation geochemical analysis of the chert samples at the top of the Liucapo Formation in this section,and reveals in detail the paleo-oceanic redox information contained in the Fe speciation data of the chert samples near the E-C boundary in this section.On this basis,combined with a large number of paleontological studies carried out by predecessors,the influence of the change of redox state in the paleo-ocean near the E-C boundary on biological evolution is discussed,and the following understanding is obtained:(1)The cherts at the top of Liuchapo Formation in Pingyin section belong to the rocks formed by silicification,retain normal seawater sedimentary signals,and are less affected by burial,weathering and thermal metamorphism.The Fe T content is between 0.20 wt%?14.58 wt%,with an average of 1.48 wt%,which is generally greater than 0.5 wt%,which is suitable for paleo-ocean environment analysis.(2)At the late of the Ediacaran period,the paleo-ocean environment in the Pingyin section gradually changed from anoxic ferruginous to sulphidic conditions,which crossed the E-C boundary until the early Cambrian.In addition,a short oxidation event occurred at the top of the Liuchapo Formation along with the retrogression.(3)The euxinic event near the E-C boundary of the Pingyin section is not only widely developed in the deep-water facies environment(Diben section and Chunye drilling section),but also recorded in slope facies and platform facies sections(Wuhe section,Xugongping section,Yunnan Xiaotan section,Three Gorges Gunziao section),indicating that the euxinic event is a deep-water euxinic extensification event.In addition,the euxinic incident has also been found in other parts of the world,suggesting that the euxinic extensification near the E-C boundary may be have been a global phenomenon.(4)The Ediacaran extinction and the Cambrian explosion are multi-episode radiation and extinction processes,and each stage of biological radiation is separated by corresponding extinction events.Near the Ediacaran-Cambrian boundary,the episodic upwelling of sulphide water and a wide range of anoxic ferruginous environment inhibited the development of organisms,and may be one of the main reasons for the mass extinction of Ediacaran biota.During this period,the Fe speciation data did not show a significant increase in oxygen levels,so the driving mechanism of biological evolution needs to be further studied. |
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