Eukaryotes expansion into non-marine habitats prohibited by nitrogen limitation during the mid-Proterozoic

Persistently anoxic conditions and nutrient limitations in the mid-Proterozoic have been invoked to explain the delayed evolution and expansion of eukaryotes. However, limited information is so far available about nutrient availability in nonmarine habitats. To fill this knowledge gap, we conducted a multi-proxy study of Statherian strata from the Dagushi, Bingmagou, Baicaoping, and Puyu formations in the Xiong'er Basin, southern margin of the North China Craton (NCC). This set of rocks offers a rare opportunity to study nutrient availability along a gradient from nonmarine to marine conditions in the mid-Proterozoic. Our δ¹⁵N data suggest that nitrogen cycling was dominated by N₂ fixation under nonmarine conditions. Hence, lacustrine waters were oligotrophic with regard to nitrogen. In contrast, nitrate (NO₃ ^-) appears to have been available in shallow marine waters. These findings suggest that oxygenated shallow marine margins were suitable for eukaryotes, but their radiation into nonmarine settings may have been N-limited.