Nitrogen isotopic compositions of organic-rich shales (∼560 Ma) in the Chengkou region, South China: Implications for a stable and relatively large nitrate reservoir of the late Ediacaran ocean

Abstract

The late Ediacaran period is a critical transitional period in Earth's history, marked by the evolution of the Ediacara biota (∼574 to 538.8 Ma). The marine redox state and nutrient level, having a close relationship with bioevolution, can be reflected by sedimentary nitrogen isotopes (δ15N). Doushantuo Formation Member IV, a widespread organic-rich interval during this critical period of South China, may provide important clues for nitrogen (N) cycling. However, high-resolution δ15N research on Member IV is still lacking. In this study, a drill core from the Chengkou sub-basin, with the well-developed Doushantuo Member IV, was carefully studied base on its N isotopic composition. The implications for the N cycling characteristics are discussed using a combination of proxies for bottom water redox conditions and basin openness. The total nitrogen (TN) concentrations (up to 0.79 wt%) of these organic-rich samples are substantially higher than those of other counterpart profiles. The TOC content shows a positive linear relationship with TN concentration, and a significant positive TN-intercept (∼0.21 wt%) represents the addition of inorganic N produced by the remineralization of sinking organic N. The high TN concentrations may also be due to the flourishing and enhanced burial of primary producers. The δ15N value in the Doushantuo Member IV progressively decreases from ∼8.2‰ to ∼3.2‰ (from bottom to top), a result of the changes in basin openness and the enhancement of N2-fixation. Compilation shows the δ15N and δ13Corg values are approximately covariant, and both have significant temporal and spatial variations at this member of South China, indicating a strong heterogeneity in the water column redox environments and changes in the type of primary producers. A mode δ15N value of 4‰ indicates a stable and relatively large nitrate reservoir in the late Ediacaran ocean.