Nitrogen isotopes from the Neoproterozoic Liulaobei Formation, North China: implications for nitrogen cycling and eukaryotic evolution

The nitrogen isotope compositions (δ15N) of sedimentary rocks can provide information about the nutrient N cycling and redox conditions that may have played important roles in biological evolution in Earth’s history. Although considerable δ15N data for the Precambrian have been published, there is a large gap during the early Neoproterozoic that restrains our understanding of the linkages among N cycling, ocean redox changes and biological evolution during this key period. Here, we report bulk δ¹⁵N and organic carbon isotope (δ¹³Corg) compositions as well as the total nitrogen (TN) and total organic carbon (TOC) contents from the Tonian fossiliferous Liulaobei Formation in the southern part of the North China Platform. The δ¹⁵N in the study section is dominated by very stable values centering around +4.3‰, which is moderately lower than in modern sediments (~ +6‰). These positive δ¹⁵N values were attributed to partial denitrification under low primary productivity (scenario 1) and/or denitrification coupled with dissimilatory nitrate reduction to ammonium (DNRA) (scenario 2). In either case, the availability of fixed nitrogen may have provided the nutrient N required to facilitate facilitated eukaryotic growth. Our study highlights the pivotal role of nutrient N in the evolution of eukaryotes.