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

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 similar to 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 (delta N-15(sed) and delta C-13(org)) 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 (similar to 514 Ma). A compilation of delta N-15 values from multiple early Cambrian sections of the Yangtze Block indicate that N-2 fixation dominated the nitrogen cycle during late Cambrian Stage 2. Low delta N-15 values ( < - 2 parts per thousand) preserved in shelf sections can be interpreted to represent partial assimilation of NH4+, where NH4+ was not a limiting nutrient. During the early-middle Cambrian Stage 3, more positive delta N-15 values (0 to + 3 parts per thousand) are recorded in shelf sections, with lower values (- 2 to + 2 parts per thousand) recorded in slope-basin sections. The positive delta N-15 values observed in shelf sections are likely a consequence of partial denitrification in the water column, whereas coeval deeper water delta N-15 values of similar to 0 parts per thousand may reflect the dominance of N-2 fixation. The distribution of delta N-15 values, combined with a gradient in delta C-13 or values, is consistent with a stratified ocean model. The delta N-15 values of all sections are lower than those of the modern ocean, which may indicate that the nitrate concentration of the early Cambrian Yangtze ocean was generally low during Cambrian Stage 3. The observed gradient in delta N-15 values is similar to that observed in records from Mesoproterozoic oceans, suggesting that abundant nitrate availability may have been restricted to shelf environments. We propose that increased nitrogen availability in shelf settings may have contributed to the evolution of large-celled eukaryotic phytoplankton. This provided a positive feedback on ocean oxygenation, allowing for increased complexity in early animal ecosystems on the continental shelf, and ultimately deep water oxygenation.