Early Cambrian Nitrogen Isotope Characteristics and Its Significance for Paleomarine Environment in The Southeastern Margin of Yangtze

Key words: 
• Early Cambrian /
• redox /
• nitrogen cycle /
• biological explosion

Abstract: The Early Cambrian Biological Explosion is usually thought to be related to the increase of nitrate concentration, however, the latest study shows that there is no significant change of nitrate concentration in this period. In order to find out the influence of nitrate on the biological explosion, this paper takes the borehole core (well ZK0202) in the slope area of the southeastern margin of Yangtze as an anatomical research object, and reconstructs the characteristics of the paleomarine environment and nitrogen cycle in this period through the analyses of the indexes of major elements, trace elements, rare earth elements, and carbon-nitrogen isotopes. The results show that the Early Cambrian Fortunian-Middle Age 2 (>526 Ma) paleomarine was characterized by a stratified ocean with deepened chemocline, and both nitrogen fixation and denitrification developed during the same period, with nitrogen fixation being the dominant effect. Late Age 2-Early Age 3 (~526-518 Ma), the paleomarine was characterized by strong anoxic-euxinic, and nitrogen fixation was the most important nitrogen cycle pathway. Middle and late Age 3 (<518 Ma) paleomarine was dominated by suboxic-oxic, and nitrogen fixation was still developed. The evolution of the redox state may be related to the high primary productivity induced by hydrothermal action, while the persistent nitrogen fixation shows that the nitrate concentration was maintained at a low level during the same period. Therefore, the abundance of nitrate may not be the main controlling factor for the biological explosion, and ocean oxidation and the abundance of organisms at the base of the food chain may be important triggers for the biological explosion. The above research results further enhance the degree of research on the paleomarine environment and nitrogen cycle in the Early Cambrian slope area of the southeast margin of the South China Yangtze, and provide new references for the correct understanding of the mechanism of the environment-biological co-evolution in this period.