Redox Heterogeneity and Phosphorus Availability Shaped Early Mesoproterozoic Eukaryotic Habitats

The early Mesoproterozoic was a critical interval in eukaryote evolution, yet the environmental conditions that shaped early eukaryote habitats remain poorly constrained. The ca. 1.56 Ga Gaoyuzhuang Formation of the Yanliao Basin, North China Craton, preserves the earliest known in situ benthic decimetre-scale multicellular eukaryotes, providing a rare opportunity to evaluate the redox and nutrient conditions associated with fossil-bearing horizons. Here we combine iron speciation, redox-sensitive trace-metal systematics, pyrite sulphur isotopes and phosphorus phase partitioning to reconstruct redox structure and phosphorus cycling across the shelf. We compare the shallow-water Qianxi section, which hosts the fossil-bearing intervals, with the deeper-water Kuancheng section, which provides an offshore counterpart across the same shelf system. Our results show that this early Mesoproterozoic shelf was characterized by pronounced lateral redox heterogeneity rather than uniform oxygenation. At the onset of Gaoyuzhuang Member III, both sections record ferruginous conditions, with high FeHR/FeT values (>0.38). The strongest cross-shelf divergence developed during the subsequent oxygenation interval. In the shallow Qianxi section, decreasing FeHR/FeT and elevated Feox/FeHR indicate oxidation of previously ferruginous waters, followed by increases in ReEF* and Re/Mo that point to highly dysoxic bottom waters. In contrast, the deeper Kuancheng section records enhanced pyrite formation, increasing Fepy/FeHR, enrichments in Mo, U, V and Re, and decreasing δ34Spy, indicating intensified microbial sulphate reduction and sulphide generation at the sediment–water interface. These contrasting trends suggest that the same broader oxygenation perturbation drove oxidation and dysoxic stabilization in shallow settings, but promoted porewater sulfidation in deeper settings. Phosphorus cycling varied accordingly across the shelf. In the shallow Qianxi section, total phosphorus contents are generally low, organic phosphorus is scarce, and PFe becomes increasingly important during dysoxic intervals, indicating efficient trapping of regenerated phosphate by reactive Fe phases. In the deeper Kuancheng section, total phosphorus covaries with organic carbon, and the interval of enhanced sulphide generation is marked by declining Pauth and rising Corg/Preac, consistent with enhanced phosphorus recycling under sulfidic porewater conditions. Together, these data indicate spatially decoupled phosphorus cycling, with shallow-water phosphorus retention and deeper-water phosphorus recycling within a generally oligotrophic shelf. Importantly, the fossil-bearing intervals in Qianxi are consistently associated with dysoxic shallow-water settings. This relationship suggests that early benthic macroeukaryotes were capable of inhabiting persistently low-oxygen environments and did not require fully oxygenated shallow waters. Deep-water phosphorus recycling may also have provided an additional nutrient supply to shallow-water habitats. In conclusion, these results indicate that early eukaryote habitats were shaped by coupled redox and nutrient dynamics across the shelf, with localized phosphorus recycling helping to sustain macroeukaryotic communities within an otherwise nutrient-limited Mesoproterozoic ocean.