Possible response of N2O emission to marine redox fluctuation during the Ediacaran-Cambrian transition in Nanhua Basin, South China

Abstract

The redox state of the ocean and atmosphere during the Ediacaran-Cambrian (E-C) transition is characterized by significant revolutions and perturbations, which had a major effect on the biogeochemical nitrogen cycle. Nitrous oxide (N2O), a powerful greenhouse gas that drives global warming, is primarily produced in the oxygen minimum zones of modern ocean through incomplete denitrification at low oxygen concentration, and its emission to the atmosphere has been closely tied to the redox state of the ocean. 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 that δ15N 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 low δ15N 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. 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.