93 / 2025-03-31 17:36:28

Visiting the Ediacaran oceanic redox states

Ediacaran,Iron speciation,Rare earth elements,Sulfur isotope,Biotas

Theme 4: Microbial and Biogeochemical Evolution through Time > Session 4c: Reconstructing Biogeochemical Cycles in Deep Time: Assumptions, Bias, Constraints, and Directions

The Ediacaran sedimentary sequences host several well-known fossil biotas and provide key evidence for understanding the coevolution of multicellular organisms and palaeoceanic environment. Here we compile the spatiotemporal distribution of iron-speciation data in Ediacaran black shales of the Yangtze block. Compilation of 14 Ediacaran sections demonstrates frequent spatiotemporal oscillations in redox conditions throughout the Yangtze block. General ferruginous states, accompanied by euxinic zones mainly concentrated in lower slope facies, are detected during the early Ediacaran period, in contrast to more widespread euxinic settings in the middle Ediacaran period. Statistical data including new high-resolution results from the Lower Yangtze block show generally low enrichment of redox-sensitive elements (RSEs, e.g., Mo, V, and U) in the Member II shales probably due to their contemporary low seawater concentrations. An increase in RSE concentrations occurs in the Member IV shales, which probably reflects a more oxidized Earth surface environment during the middle Ediacaran period. Additionally, high-resolution rare earth elements (REEs) analysis reveals a transition from oxic to anoxic conditions in Member III of the Ediacaran Doushantuo Formation. Iron speciation analysis further indicates that the upper sections of Member III carbonates were predominantly deposited under euxinic water conditions. Regardless of the general anoxic seawater states of the Ediacaran ocean reconstructed by iron-speciation and REEs, in situ framboid measurements of pyrite sulfur isotope fractionation provide a positive view for estimating the maximum seawater oxidant concentrations in the early and middle Ediacaran ocean. We used nanoscale secondary ion mass spectrometry (NanoSIMS) to probe the morphology and geochemistry of sedimentary pyrite in Ediacaran drill-core samples of South China. Pyrite occurs as both framboidal and euhedral/subhedral crystals, which show distinct differences. Framboidal pyrite exhibits consistently negative δ³⁴S_py values within a relatively narrow range, whereas euhedral/subhedral pyrite displays both negative and positive δ³⁴S_py values with a broader range of variation. The significant sulfur isotope variability within individual euhedral/subhedral pyrite grains reflects microenvironmental heterogeneity during pyrite formation. Bulk δ³⁴S_py measurements, therefore, reflect mixed signals derived from a combination of syndepositional and diagenetic processes. Integrating evidence from iron speciation, rare earth elements (REEs), and in situ measurements, our findings suggest that the early Ediacaran ocean likely contained a larger seawater sulfate reservoir than previously indicated by bulk analyses, while the middle Ediacaran ocean was predominantly euxinic rather than oxic.