The Pyrite Morphology Characteristics And Its Redox Significance Across G-L Boundary Of Penglaitan GSSP Section In Guangxi Autonomous Region

A mass extinction occurred at the end of the Guadalupian,and many creatures suffered a serious defeat.This mass extinction is mainly related to the Emeishan basalt eruption,ocean anoxia and global sea level decline.The ocean anoxia is considered to be the deadly cause of this mass extinction.Some studies have been done on the redox conditions of seawater across the G-L boundary,but most of biology records cross the G-L boundary were incomplete,more research is needed for studying redox conditions change.In this thesis,the systematic field investigation of the stratum from Guadalupian Maokou formation to Lopingian is carried out in the Penglaitan GSSP section,and the habitat types change,morphology of pyrite,the size characteristics of framboidal pyrite and crystalite are analyzed in detail.According to the change of habitat type,analyzing sea level change from Capitanian to early Wuchiapingian,and the redox conditions are analyzed on the basis of the morphology characteristics of pyrite,and then discussed the influence of sea level change and ocean anoxia on extinction.This thesis also analyzes comprehensively the crystalite size characteristics of framboidal pyrite which come from Penglaitan GSSP section in Laibin,Guangxi,Tianfengping section in Enshi,Hubei and Xiajiaba section in Guangyuan,Sichuan,and according to the crystalite size characteristics of framboidal pyrite,the redox conditions conditions are determined.In the Penglaitan GSSP section,four major categories and five subclass habitat types are identified,including ?2,?1,?2,?,and ?.Habitat type reflects that there was a global sea retreat at end of the Guadalupian period,and ocean environment turned into thetransgression stage at early Lopingian.The lower sea level corresponds to the main extinction,resulting in benthic animals died.But the peak of extinction of floating and swimming animals disappeared at the sea level rise period.It indicates that sea level decline is at least not the only reanson of the extinction of this period.The bottom stratum of Maokou formation contains no framboidal pyrite,Pyrite is given priority to with idiomorphic crystal;The average of framboidal pyrite size is5.86~10.20 ?m,the maximum of most sample is greater than 20 ?m,and at the top of Jinogondolella xuanhanensis conodonts,there is a sample with small size and narrow range;The upper stratum of Maokou formation lock of framboidal pyrite,contains idiomorphic crystal pyrite and some mass framboidal pyrite.From the top of Maokou formation to Heshan formation,relative smaller size framboidal pyrite appears,most of the sample maximum size is less than 18 ?m,some samples with small size and narrow range appear intermittently.The characteristics of Pyrite morphology suggests that the marine water is an oxic environment during early Maokou period(Jinogondolella shannoni conodonts),and become a long-term dysoxic environment during middle period Capitanian(Jinogondolella altuaensis to Jinogondolella xuanhanensis),and at the end of the Jinogondolella xuanhanensis conodonts,there is a short-term anoxia environment.From late Maokou period to early Wujiaping period,deposition water turns into an anoxic environment,and intermittently appears euxinic or oxic-dysoxic deficient environment.The euxinic-anoxia stratum of Penglaitan section corresponds to the mass extinction at the end of Guadalupian,which possible indicating that there is causal relationship between ocean anoxia and mass extinction.Sulfuretted water invades shallow which could cause the death of benthic,the ocean anoxia is likely the main cause of the mass extinction at the end of Guadalupian.The crystallite size of framboidal pyrite is generally less than 0.68 ?m which forms in euxinic environment,and it's standard deviation is less than 0.3 ?m.And the crystallite size of framboidal pyrite is greater than 0.86 ?m which forms in oxic-dysoxic environment,and it's standard deviation is generally greater than 0.5 ?m.According to above standard,euxinic-anoxia environment,oxic-dysoxic environment and the transition zone between them can be divided.