Origin And Type Of Organic Matter,Microbial Assemblages And Their Influence On Organic Matter Preservation Of Early Cambrian In The Yangtze Plate

Late Precambrian to early Cambrian was a period of major changes in the physical and chemical environment of paleo-ocean.Eukaryotes truly started to dominate marine ecosystems,while animals had their imprint on the global biosphere.The interaction between biomass and paleo-marine environment fundamentally changed the ecosystem and the mode of accumulation and preservation of sedimentary organic matter.The origin of organic matter in early Cambrian was much richer than that in Precambrian.It is increasingly realized that the early microphytoplankton record are essential for comprehensive paleoecological interpretations,and organic matter preservation evaluation.However,the diversity of Cambrian microphytoplankton in China has not been fully realized.In this study,the information of Cambrian organic-walled microfossils in China are compiled.Pertinent literatures published in the past 30 years have been reviewed,the stratigraphical range of 79 genera and 138 species of organic-walled microfossils published from the Cambrian of China is illustrated.In order to further reveal the appearance of early Cambrian microorganisms,three sections(namely,Shatan Section,Gunshi'ao Section and Luojiacun Section)in the northern margin of the Yangtze plate have been studied.Palynological experiment,thin section study,microfossil extraction and SEM(scanning electron microscope)in situ observation have been applied,64 genera of microphytoplankton were identified under transmission light of the biomicroscope,large numbers of organic-walled macroalgae fragment and small carbonaceous fossils(SCFs)have been recovered as well.SCFs include the organic linings of spongy spicules and Small Shelly Fossils(SSFs),animal fragments,fecal structured fossils.The great deal of microfossils obtained in this study have opened a new window for exploring the mechanisms of organic matter accumulation and preservation.The palynological study shows that the sedimentary organic matter in the early Cambrian are mainly preserved in the form of amorphous organic matter(AOM),which represents the widespread organic phase in the Yangtze plate in the early Cambrian.The correlation between AOM and microphytoplankton fossils proves that AOM were derived from algae,acritarchs,fungi,cyanobacteria and macroalgae in the paleo-oceans in early Cambrian.AOM in Qiongzhusi Formation of Shatan Section and Yanjiahe Formation and Shuijingtuo Formation of Luojiacun Section were well preserved in the oxic-suboxic environment,because of the selective preservation of algaenans and other polymerizate produced in the process of the formation of organic matter.This indicates that the preservation of organic matter is not only affected by the marine environment which has been widely studied,but also by the degradation-resistant components of the organic matter.The ‘Cambrian explosion' event played a crucial role in driving and forcing the changes of physicochemical environment,and the changes of the organic matter preservation mode.The large amount of animal SCFs and coprolites fossils are the principal character of the microfossil assemblages of the study area.The feeding,defecation and a series of life activities of animals played an important role in accelerating the sinking rate of organic matter between marine transparent zones and sediments,thus promoting the organic matter preservation.Animal invoked an important feature of organic matter preservation in the early Cambrian.Obvious imprint of contraction and extrusion of digestive tract or gut system are revealed on the fossil coprolites,and some organic-walled fecal structured fossils were wrapped with acritarchs,which indicate the grazing characteristics and digestive tract structure of animals indirectly.Previous reports show that SSFs,spongy spicules,radiolarians,trilobites,brachiopods and other metazoans may be associated with this process.Neoproterozoic to Cambrian is an important period of animal evolution.In recent years,siliceous microorganisms(radiolarian and hexactinellida sponges)in early Cambrian have been brought into sharp focus.Results show that the silicon of Qiongzhusi Formation in Shatan Section and Yanjiahe Formation and Shuijingtuo Formation in Luojiacun Section were from bio-opal of siliceous organisms,despite that precious few layers were affected by terrestrial input.The content of biogenic silicon(Bio-Si)is coupled with the content of palynological organic matter and TOC.Moreover,the in situ observation and energy spectrum analysis(EDS)of the SEM exam indicate a close combination of AOM and Bio-Si.Therefore,Bio-Si could be used as a good index of organic matter enrichment in early Cambrian.The life activities of siliceous microorganisms in early Cambrian were favorable to the preservation of sedimentary organic matter.Therefore,this study indicates that,in addition to the application of geochemical indices,the qualitative and quantitative study of palynological organic matter provides more detailed information on the occurrence state of sedimentary organic matter,it complements the application of TOC on tracing organic matter distribution,and is helpful to the evaluation of sedimentary organic matter preservation.Moreover,the type and origin of organic matter and the distribution of Bio-Si can be used as important indices for organic matter preservation evaluation.The periodical cycle of paleoenvironment forced by the early Cambrian microbial assemblage and its feedback with the paleo-marine environment greatly affected of organic matter preservation.