Microfossil Assemblages and Indication of the Source and Preservation Pattern of Organic Matter from the Early Cambrian in South China

The beginning of the Phanerozoic was marked by abundant phytoplankton,the occurrence of animals and a newly established ecosystem structure,which in turn,resulted in different modes of organic matter(OM)cycling and preservation.In this study,we present an integrated paleontological and geochemical study of the Lower Cambrian Yanjiahe and Shuijingtuo formations in the Three Gorges area,South China.Rock samples were analyzed for major and trace elements,in order to evaluate the marine productivity.Fossil samples were recovered using palynological methods,destructive analytical techniques,SEM analysis and thin section microscopy.The organic-walled microfossils in the investigated sections include marine microphytoplankton,linings of sponge spicules,small shelly fossils and some unnamed sheet-like and net-like microfossils(animal parts).Additionally,fecal structures and coprolites fossils were found.Three intervals in the Luojiacun Section were proposed to discuss the links among microfossil assemblages and their influence on OM cycling and preservation.We found that the marine microphytoplankton was the main source of OM in the investigated sections.The appearance of fecal structures groups and massive appearance of animals in the pelagic zone indicate a higher transfer efficiency of OM aggregates through the water column.Oxygenated conditions in the water-column would have led to a moderate to low preservation of OM,an effect which would have been further enhanced by the potential presence of bioturbators.Overall,our study reveals the contribution of various organisms to the sedimentary OM and highlights the links and feedbacks among microfossil assemblages,OM recycling efficiency and OM preservation in the tipping point of the Phanerozoic“new world”.