The Co-evolution Of Ocean Redox Chemistry And Biota During The Early Cambrian,South China And Its Influence On Organic Matter Accmulation

The early Cambrian is a very important time period during the earth history.It has crucial meaning for both life evolution and environmental changes.For the life evolution,magnificent Chengjiang fauna replaced the Precambrian fauna and formed more complex ecosystems.For the environment changes,During the Ediacara-Cambrian transition period,the change of redox state in the ocean and its co-evolution with organisms during the Neoproterozoic Oxygenation Event have attracted much attention.The early Cambrian strata of the Yangtze platform in South China are continuously exposed with good outcrop,which is a good material for the study of this period.A large number of paleontological studies and stratigraphic correlation studies have laid a good foundation for the study of different sedimentary environments.On this basis,this paper chooses sections under different sedimentary environments and made stratigraphic comparisons using lithologic mark beds,biological zones and zircon age data to reconstruct a strata framework?Chapter 2?,and takes framboidal pyrite as well as redox sensitive elements to reconstruct the sedimentary environment of the early Cambrian Yangtze Plate?Chapter 4?.Secondly,based on the statistics of the diameter size of framboidal pyrite in Shuijingtuo Formation and the volume ratio of framboids to euhedral crystals in the sample,the redox fluctuation with high resolution can be obtained.The contradiction between the redox sensitive elements?U,Mo?and that of framboid pyrite in the upper part of Shuijingtuo Formation can be explained.We suggest that the percentage of total pyrite in euhedral form can be used as an important auxiliary index for the use of pyrite as a redox proxy?chapter 5?.compares it with the characteristics of biota in different sedimentary environments,and obtains the relationship between redox conditions and metazoan evolution?chapter 6?.Thirdly,through the study of chemical-physical weathering characteristics and productivity in different sedimentary environments of the Yangtze platform,the evolution characteristics of paleoclimate and productivity of the early Cambrian Yangtze platform are obtained,and the relationship between weathering and productivity is discussed.Finally,the controlling factors of organic matter deposition in the Yangtze platform are judged in terms of preservation conditions?i.e.redox state?and production conditions?productivity state?.In this paper,the geochemical indices of redox sensitive elements,framboid pyrite,excess barium(Ba_xs),total organic carbon?TOC?and CIA in the different environment facies of shallow water platform,outer shelf and deep-water basin are discussed,we selected Shatan,Luojiacun and Bajiaocun sections as representatives of shallow,shelf and deep ocean settings.The paleo-oxygen facies and paleoclimatic changes of the Yangtze plate of Meishucunian-Qiongzhusian steps in the early Cambrian are reconstructed.And its relationship with biological evolution and organic matter deposition is concluded.The main conclusions are as follows:1.The change of redox environment and the evolution of U,Mo inventories and framboid pyrite indicate that in the shallow water Shatan section,the oxic-suboxic is dominant in Meishucunian and the lower part of Qiongzhusian,and the upper part of Qiongzhusian becomes suboxic-anoxic.In the Luojiacun section of the continental shelf,the Meishucunian is mainly oxic-suboxic,the lower part of the Qiongzhusi terrace is mainly sulfidic,and the upper part of the oxic-suboxic sensitive elements is in contradiction with the framboid pyrite.Bajiaocun section is dominated by ferruginous in Meishucunian,vulcanization in the lower part of Jiumenchong Formation and anoxic water in the upper part,with small-scale oxidation event.Generally speaking,in the early Cambrian Yangtze platform,although the area of anoxic water in the upper part of the Qiongzhusian increased,the degree of anoxic water decreased.At the same time,the U and Mo inventories increased significantly in the Qiongzhusian,but there was still a gap between the U and Mo inventories in modern seawater and early Cambrian ocean.2.The redox state of the upper Shuijingtuo Formation is indicated by the micro-layered framboid pyrite:the average diameter of the micro-layered framboid pyrite in the upper Shuijingtuo Formation in Luojiacun section is less than 5?m.Most of the micro-layered framboid pyrite indicates anoxic environment,and only two micro-layers indicate oxidation event,which indicates that the upper Shuijingtuo Formation is still in a wide range of anoxic.Occasionally,there is a small scale oxidation event accompanied by strong hydrodynamic action to oxidize bottom water.At the same time,TOC has a strong correlation with Mo and U?R²=0.85;0.79?,and the content of U and Mo in the lower part of Shuijingtuo formation is higher and has no strong correlation with TOC,we set the threshold between anoxic and sulfidic,and show that the environment in the upper part of Shuijingtuo Formation changes from sulfidic to non-sulfidic.3.The influence of redox state on metazoan evolution shows that the early Cambrian biota distribution in the Yangtze Plate is influenced by redox state.In the Meishucun Stage,a large number of small shell fossils and benthic algae were found in the beach section and Luojiacun section,which needed bottom water oxidation to survive.In the Bajiaocun Stage,the sponge bone needle was the main bone needle.A large number of metazoan fossils,such as brachiopods,crustaceans,trilobites and hypermyotids,have been found in the Guojiaba Formation,where the beach section is located.The Luojiacun Formation in the continental shelf is dominated by planktonic trilobites,while the fossil sponge needle remains the main fossil in the Bajiaocun Formation in the deep-water basin.In the upper part of the Qiongzhusi Stage,although anoxic phenomena occurred in all three sections,swimming trilobites were the dominant biota because of the small-scale oxidation period in the water body at that time,including benthic ancient cups,swimming shells,brachiopods and hypermyositis in the upper part of the Shuijingtuo Formation.4.Paleoclimate change and its relationship with productivity:except for the Shatan section,Al content in Meishucunian is generally low,while CIA fluctuates at a higher level.Al content in Qiongzhusian increases significantly,but CIA value decreases,indicating that physical weathering increases and chemical weathering weakens,which represents a large number of unweathered land-derived materials input.The aggregation of Gondwana resulted in a great deal of orogeny and continental rifts,which greatly increased the supply of terrigenous debris.However,the destruction during this period was mostly physical weathering,resulting in a large number of unweathered minerals being transported to the seafloor during the transgression of the Qiongzhusian,resulting in the low value of CIA and high value with Al.At the same time,the CIA values of different latitudes in the world show a decreasing trend with the increase of latitude.?Baltica's CIA average is 59,situated at⁶0 degrees S,Oman's CIA average is 74-79,situated at 5-25 degrees S?.The CIA value of the Yangtze Platform in the late Qiongzhusian is also related to its lower latitude in this period.5.Deposition of organic matter:The sedimentary model of organic matter in the early Cambrian of Yangtze Plate is quite different from that in the shallow water section.In the shallow water section,the preservation of TOC is controlled by redox state,while in the deeper water section,the preservation of TOC is controlled by productivity.