Nitrogen Isotopes from the Neoproterozoic Liulaobei Formation, North China: Implications for Nitrogen Cycling and Eukaryotic Evolution

The nitrogen isotope compositions (delta N-15) 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 delta N-15 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 delta N-15 and organic carbon isotope (delta,C-13(org)) 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 delta N-15 in the study section is dominated by very stable values centering around +4.3 parts per thousand, which is moderately lower than that in modern sediments (similar to+6 parts per thousand). These positive delta N-15 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.