Marine nitrogen cycling in the aftermath of the Marinoan Snowball Earth

Abstract

The termination of the Marinoan Snowball Earth (ca. 635 Ma) was marked by the global precipitation of cap carbonate, which might have coincided with global environmental and evolutionary changes, particularly the evolution of early animals in the ocean and a significant rise in atmospheric oxygen concentrations. High nitrogen isotopic compositions (δ15N) of the Ediacaran marine sediments indicate that the seawater was oxygen-rich and maintained a substantial nitrate reservoir following the Marinoan Snowball Earth. However, the lack of nitrogen isotope records in the wake of the Marinoan Snowball Earth became a stumbling block in understanding the relationships between marine nitrogen cycling and biological evolution during the deglaciation. This study reported the δ15N of the cap carbonate in the basal Doushantuo Formation from two sections spanning from shelf margin to basin in the Yangtze Block, South China. Our results show a descending pattern in δ15N values at the lower part of the sections, reaching a nadir of ∼ − 4‰. This trend suggests that upwelling brought an abundance of NH4+ during the mixing process of glacial meltwater and seawater, resulting in incomplete assimilation of NH4+. The relatively heavier δ15N values (∼0‰) in the upper part observed in cap carbonate provide evidence of the onset of nitrogen fixation after the physical stratification of seawater. This process can be attributed to restoring the thermohaline circulation in the ocean. However, the occurrence of nitrogen fixation and δ15N values greater than +2‰ had not been observed, indicating a stable reservoir of nitrate had not yet been established. Overall, our data indicate a gradual oxidation of the ocean and the recovery of aerobic nitrogen cycling during the Marinoan deglaciation, which probably influenced ocean primary productivity. A stable nutrient reservoir built after this transitional period might provide a nutrient basis for the early animal evolution in the early Ediacaran.