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Feedbacks between Multiple Nutrient Cycles Facilitated the Expansion of Eukaryotic Life at ca. 1.56 Ga

Recent studies have revealed a significant ocean oxygenation episode and an evolutionary leap of eukaryotes at early Mesoproterozoic. However, the potential role of nitrogen availability in these environmental and biological events has not been investigated. Here we present an integrated study of nitrogen isotopes ( δ 15 N), organic carbon isotopes ( δ 13 C org ), and major and trace element concentrations from the Member III of the Gaoyuzhuang Formation in the central North China Craton where the earliest macroscopic multicellular eukaryotic fossils were reported. The enrichments of redox-sensitive elements (Mo, U, V), coupled with Mo-U covariations, δ 13 C org and I/(Ca+Mg), indicate that the Gaoyuzhuang Member III in the study area was deposited in largely suboxic-anoxic environments with ephemeral occurrences of euxinia. These data reinforce previous inferences of a strongly redox stratified ocean during the early Mesoproterozoic, but pulse of oxygenation event may result in the deepening of chemocline. The persistently high δ 15 N values from the study section likely