4 / 2025-02-22 15:43:16

Marine nitrogen cycling in the aftermath of the Marinoan Snowball Earth

Ediacaran,Nitrogen,South China,nutrients,primary productivity

Theme 3: Earth-Life Co-evolution Through Time > Session 3a: Precambrian Earth and Life Co-evolution

The widespread deposition of cap carbonates is considered a marker for the end of the Marinoan global glaciation and the beginning of the Ediacaran period (around 635 Ma). This was followed by the radiation and evolutionary development of metazoans and the second global oxygenation event. Additionally, acanthomorphic acritarchs fossils appeared in the strata directly overlying the cap carbonates and fungal-like fossils have been discovered within the cap carbonate itself. Nitrogen isotope (δ¹⁵N) studies show that Ediacaran marine deposits generally exhibit higher δ¹⁵N values, suggesting that the ocean had been oxidized after the termination of the Marinoan glaciation and maintained a stable nitrate reservoir, providing a favorable material and environmental foundation for biological evolution. However, the mechanisms by which the marine environment transitioned from widespread anoxia during the Cryogenian to one that supported biological development remain unclear. There is currently a gap in our understanding of the marine nitrogen cycling during the early stages of the breakup of the Marinoan snowball Earth, and filling this nitrogen isotope record is crucial for understanding the relationship between the deglacial ocean nitrogen cycle and biological evolution. The presence of in the strata directly above cap carbonate and the occurrence of complex multicellular life through the Ediacaran period imply a potential linkage between the Marinoan deglaciation and early animal evolution. The results show a decreasing δ¹⁵N trend in the lower section of the profile, reaching a minimum of ~−4‰. This trend indicates that physical stratification of the water column occurred during the mixing of glacial meltwater and seawater, with upwelling bringing NH₄⁺ from the deep ocean, which led to an early ocean nitrogen cycling dominated by incomplete assimilation of NH₄⁺. The relatively higher δ¹⁵N values (~0‰) in the upper section of the profile suggest a shift in the nitrogen cycling to a nitrogen-fixation-dominated process after the physical stratification of seawater. This transition is likely attributed to the restoration of oceanic thermohaline circulation. However, the presence of nitrogen fixation without δ¹⁵N values greater than +2‰ indicates that the ocean had not yet established a stable nitrate reservoir. Our data ultimately suggest that the Marinoan deglacial period was a transitional phase marked by environmental change, with the gradual oxidation of seawater and the re-establishment of aerobic nitrogen cycling, which greatly influenced marine primary productivity. Therefore, the stable nutrient reservoirs formed after this transitional period may have provided the necessary nutritional and material conditions for the early evolution of Ediacaran metazoans.