Spatio-temporal evolution of ocean redox and nitrogen cycling in the early Cambrian Yangtze ocean

Abstract The early Cambrian was a critical interval for the Earth system, during which a rise in oceanic and atmospheric oxygen levels coincided with the rapid diversification of metazoans. A variety of contrasting models have been proposed for the spatiotemporal redox evolution of the early Cambrian ocean. These include the development of a well-oxygenated deep ocean at the base of Cambrian Stage 3 (commencing at ~521 Ma), or alternatively, persistent and widespread anoxic (ferruginous) conditions throughout the early Cambrian ocean. Here, we present redox sensitive trace element (RSTE), Fe speciation, and N and C isotope (δ15Nsed and δ13Corg) data for samples from a section (Zhongnancun) of the early Cambrian Niutitang Formation, which was deposited on the outer-shelf of the Yangtze Block, South China. The Fe speciation and RSTE data provide evidence of a transition from euxinic, through ferruginous, to oxic conditions during deposition of the Niutitang Formation. The combination of these new data with existing data from the inner-shelf to basin environment, implies regional redox stratification across the Yangtze Block during Cambrian stages 2 and 3, with oxic shallow waters above ferruginous deep waters, and spatial variability in the degree of mid-depth euxinia. Oxygenation of deeper waters may have occurred by early Cambrian Stage 4 (~514 Ma). A compilation of δ15N values from multiple early Cambrian sections of the Yangtze Block indicate that N2 fixation dominated the nitrogen cycle during late Cambrian Stage 2. Low δ15N values (