| The Ediacaran(-635-541 Ma)of the Neoproterozoic(?1000-541 Ma)is a key period in the evolutionary history of the Earth,during which the earliest skeletal metazoans and complex ecosystems occurred.It is generally believed that the increase in the oxygen content of the Earth's surface may have resulted in a series of biological evolution events,but many existing data show that the oceans from the late Ediacaran to the early Cambrian have a general hypoxic nature,and,there are still big debates in the chemical evolution and oxidation processes underscores of the oceans.Therefore,multi-disciplinary comprehensive studies further defined Ediacaran a period of prosperous life in the history of this planet.During the Ediacaran Period,atmospheric oxidation levels increased,and the earliest multicellular organisms appeared.However,the underlying cause of biodiversity and bioradiations is still controversial.For the oceanic environment evolution and oxidation,exposing the internal relationship between changes in oxygen content and early animal evolution are of great scientific significance for a profound understanding of early life cycles.The threshold of oxygen required by early animals is still unclear.The evolution of eukaryotes and the oxidation of the environment are closely linked to the production,transportation,decomposition,and burial of organic matter through a complex biochemical feedback network.Skeletonization,increased body volume,predation,motility and the emergence of complex food webs are key stages of the evolution of complex animal systems in respect of late Ediacaran Period.The oxidation level in the late Ediacaran Period needs to meet the physiological limitations of these complex activities of macroorganisms.A large amount of geochemical evidence in the machinations of environment sediments show that the gradual increase in oxidation level coincides with biological evolution.During late Ediacaran Period,the earliest skeletal metazoans and complex ecosystems appeared and ruled the earth's biosphere.The increase of oxygen in the surface environment of the Earth may have caused these biological evolution events,but many studies have shown that the oceans in the late Ediacaran and early Cambrian have the characteristics of general hypoxia.In order to further clarify the redox conditions of the late Ediacaran ocean,the possible link between ocean oxidation,early animal biological evolution and skeletonization,Driedoornvlagte and Zebra River sections in the Nama Basin(?550-541 Ma)(Zaris sub-basin)in southern Namibia were selected as the targets in this study.This study aims at revealing the evolution of marine chemical conditions in the late Ediacaran Lower Nama Group(?550-547 Ma)by analyzing the sequence stratigraphy,sedimentary facies and sedimentary geochemistry of the carbonate stratigraphic sequence.The sections also address animal diversity and skeletonization in response to the above.Elemental geochemistry and iodine compositional studies have shown that the seawater chemistry conditions in the late Ediacaran Nama Basin have undergone a complex dynamic evolution,which generally shows a trend of gradually increasing oxidation from bottom to top,and is in harmony with early animal ecology diversification and skeletonization consistency.The iodine contents from the Lower Nama Group under the Driedoornvlagte and Zebra river sections were systematically analyzed.The results showed that the values of I/[Ca+Mg]fluctuate between 0.08 and 6.20?mol/mol,with an average of 0.94 ?mol/mol for the Driedoornvlagte section and 0.7?mol/mol for the Zebra River section.The high value of I/[Ca+Mg](>2.6 ?mol/mol)appears above the boundary between the Lower Omkyk sequence and the Upper Omkyk sequence,up to a maximum of 6.20 ?mol/mol,indicative of well oxygenated conditions in relation to modern marine surface waters.The skeletal animals Cloudina,Namacalathus and Namapoikia only appear above the high I/[Ca+Mg]value horizon.This result indicates that the bio-communities with the skeletal biomes of Nama Group mainly lived above the chemocline layer,in oxic([O2]>20-70 ?M)and dysoxic([O2]>10 ?M)environments,consistent with previous understanding in the water column.In these intervals,phosphorus(P)abundance and P/Al ratio also increased significantly,which may indicate that the increase of bio-essential nutrients in water column also plays an important role in promoting the development and diversification of early animals.The spatial and temporal changes exhibited by I/[Ca+Mg]are also consistent with the view that the water bodies in the sedimentary basin of the late Ediacaran stratification and the redox conditions are highly dynamic.Furthermore,in this study,the sequence stratigraphy and sedimentary facies analysis of the carbonate rocks of the Lower Nama Group in the northern Nama Basin are examined.The stratigraphic profiles illustrate the horizontal and vertical sedimentary distribution and characteristics of the late Ediacaran strata in the study areas.Comprehensive studies confirmed that the first-order sequence was mainly affected by the settlement caused by tectonic loads along the edge of the Kalahari craton.The carbonate rock sequence of the Lower Nama Group is mainly of carbonate ramp deposits,some of which are derived from the bioclastics of the oldest calcareous organisms on earth,indicating that the importance of early animal activities in carbonate deposition during this period had begun to appear.The thrombolite facies are well developed in the study area,and often constitute the framework of the reef complex,which is rich in early skeletal animal fossils.Their spatial distribution and symbiotic characteristics also indicate that the fossils are the most abundant,showing that the nature of the sedimentary basement has a strong control on the habitat of early animals. |
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