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Nitrogen isotopes, Redox conditions, Nitrogen cycle, Early Cambrian, South China
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 (<−2‰) 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 δ15N values (0 to +3‰) are recorded in shelf sections, with lower values (−2 to +2‰) recorded in slope-basin sections. The positive δ15N values observed in shelf sections are likely a consequence of partial denitrification in the water column, whereas coeval deeper water δ15N values of ~0‰ may reflect the dominance of N2 fixation. The distribution of δ15N values, combined with a gradient in δ13Corg values, is consistent with a stratified ocean model. The δ15N 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 δ15N 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.
