Low Oxygen Levels in Early Neoproterozoic Shallow Seawater and Evolution Stasis of Crown‐Group Eukaryotes

Abstract Crown‐group eukaryotes diversified rapidly in the late Mesoproterozoic (ca. 1.1–1.0 Ga), but their evolution was slowed in the early Neoproterozoic (ca. 1.0–0.8 Ga), with a significant episode of diversification occurring again around 0.8 Ga. Previous studies suggest nutrient (nitrogen and phosphorus) scarcity in the seawater may have delayed eukaryotic evolution during this period. However, the influence of marine redox conditions on the evolution, either directly or indirectly, remains unclear. Applying integrated approaches of sedimentology, mineralogy and geochemistry, we analyze the shallow marine carbonates from the Zhaowei, Niyuan, and Jiudingshan formations of the Huaibei Group (ca. 1.0–0.95 Ga) in the southeastern North China to constrain the redox conditions of the early Neoproterozoic seawater. Microscopic observations revealed abundant water‐column carbonate mud (“whiting”) precipitates in these formations, indicating moderately oxygenated surface waters. Geochemical results show that the I/(Ca + Mg) values of the carbonates are mostly (98%) below the Precambrian background baseline (0.5 μmol/mol), without significant negative Ce anomaly (0.82 ± 0.06, n = 44). These findings support low‐oxygen conditions in the shallow waters. The combination of low oxygen conditions and nutrient scarcity likely delayed the Neoproterozoic diversification of early crown‐group eukaryotes. This study provides valuable insights into marine redox conditions in Earth's middle age and their potential impact on early eukaryotic evolution.