| The biosphere of Earth during the Ediacaran-Cambrian（E-C）transition has witnessed the extraordinary radiations of multicellular eukaryotic algae and origin of metazoan.However,the causes induced evolution events were still debated.Previous studies have shown that these events during this interval are closely related to the evolution of marine nitrogen cycle.On the one hand,the redox state of the ocean and atmosphere is characterized by significant revolutions and perturbations,which had a major effect on N2O emission and finally contributed to dramatic environment.On the other hand,nitrogen isotopic evidence indicated that the expansion of marine nitrate pool could induced the origin and radiation of metazoan.However,the environment effect induced by the fluctuations of marine nitrogen cycle during this interval is still poorly understood,and there are few studies about the marine nitrogen cycle of other blocks except for Yangtze Block.Therefore,this study focused on the nitrogen evolution of Yangtze Block and Tarim Block during late Ediacaran and early Cambrian.（1）To better understand the relationship between the possible variation of N2O emission and its related marine redox fluctuation,a multi geochemical proxy approach was conducted on the upper Ediacaran to lower Cambrian marine successions spanning from platform to slope facies of the Yangtze Block.The results show thatδ15N features during the late Ediacaran to Fortunian interval from platform facies displayed high values（up to 8.8‰）,indicating a higher degree of denitrification.The Fe speciation data along with elevated Cu/Al ratios in contemporaneous sediments suggests that the gradual depletion of dissolved Cu in water column due to expanded euxinic conditions.Because Cu is the essential element in the terminal step of denitrification pathway,the Cu deficiency in water column could result in incomplete denitrification.We infer that the possible increase of N2O discharge in this interval could contribute to a greenhouse climate and intense the anoxic degree of water column.However,during the Cambrian Stage 2 continuous consumption of NO3–and NO2–in seawater by denitrification would limit denitrification rate and reduce N2O production,and finally N2 fixation dominated the nitrogen cycle in photic zone as evidenced by lowδ15N value.Both progressive oxygenation of deep water during the Cambrian Stage 3,and habitable environment temperature caused by less N2O emission could stimulate subsequent biosphere revolution.（2）To recover the marine nitrogen cycle of Tarim Block during the early Cambrian,organic carbon and nitrogen isotopes for two sections of the Lower Cambrian Yuertusi Formation from the Tarim Block were also measured in this study.The results indicate that oxidative nitrogen metabolism dominated the nitrogen cycle in photic zone.The stage characteristics of nitrogen isotopic results suggested that the degree of water denitrification dynamic changes.However,the higherδ15N values indicated that the nitrate supply in the photic zone remains sufficient even the anoxic water expanding.Our study highlights that N2O emission due to marine redox fluctuations possibly played an important role in regulating environmental and metazoan diversification during the E-C transition.In addition,compared with the Nanhua Basin,the ocean nitrogen cycle in the Tarim Block during early Cambrian has evolved to more closely resemble the modern ocean nitrogen cycle.The nitrate pool was more stable and provided enough nitrate supply for early animal metabolism. |
