Oceanic redox condition and the evolution of Ediacaran life: Evidence from nitrogen isotopes and biogenic silica in the Yangtze Block, South China

Abstract

The late Ediacaran period witnessed the largest global carbon cycle disturbance in geological history, known as the “Shuram Excursion”, coinciding with the emergence of early complex multicellular life. While the rise in atmospheric and oceanic oxygen is seen as a major catalyst for this evolutionary leap, the link between oxygenation and Ediacaran diversification remains contested. To investigate this, we reported nitrogen, carbonate, and organic carbon isotopes, as well as biogenic silica (BSi) content from a drill core in the Yangtze Platform, South China. During the lower Shuram excursion, δ¹⁵N decreased from 5.9 ‰ to 2.9 ‰, suggesting a shrinking nitrate pool in the euphotic zone associated with intensified denitrification due to the expansion of the anoxic zone. During the middle Shuram excursion, a significant decline in δ¹³C_carb was accompanied by a rapid increase in δ¹⁵N (up to 5.8 ‰), which likely reflects a transitional redox state characterized by moderate denitrification in the water column, enriching the remaining nitrate in the photic zone with ¹⁵N. The subsequent decline in δ¹⁵N signatures likely reflects reduced water-column denitrification, driven by further ocean oxygenation and the deepening of the chemocline. The concurrent positive excursions in δ¹⁵N and negative δ¹³C_carb correspond to the peak in BSi contents, suggesting that the rise in seawater O₂ levels and the increase in surface nitrate concentrations provided a favorable environment for siliceous organisms. The geochemical signatures observed correspond with the diversification of ecologically important animal groups, emphasizing the accelerated evolution of the Avalon biota in oxygen and nitrate-enriched environments.