The linkage of nitrogen isotopic composition and depositional environment of black mudstones in the Upper Triassic Yanchang Formation, Ordos Basin, northern China

Abstract

In this study, we report new analyses of stable nitrogen isotopic compositions (δ15N) of the black mudstones from the Chang-7 Member of the Upper Triassic Yanchang Formation in the Ordos Basin, China, to examine the possible linkage of such isotopic compositions with the depositional environment. These mudstones were deposited mainly under suboxic bottom water column conditions. The core descriptions and trace element concentrations differ between samples obtained from the Chang-73 and Chang-71,2 members, situated at the bottom and top of the Chang-7 Member, respectively, such that the depositional condition of the former implies a more reduced water regime. The bulk rock nitrogen isotope (δ15Nbulk) values of the samples obtained from the Chang-73 Member are higher than those of the Chang-71,2 Member. No significant differences were identified in the organic carbon isotopes (δ13Corg), total organic carbon (TOC), or total nitrogen (TN) values between the two units. The results of δ15Nbulk cross-plotting with the TOC, TN, and carbon to nitrogen atomic ratio (C/N) show an absence of significant modification of the primary nitrogen isotopic compositions from post-depositional processes. Given the depositional history of these units, we suggest that the sedimentary redox environment can have a significant influence on δ15Nbulk. Denitrification, which occurs mainly in suboxic conditions, plays an important role in such an environment, resulting in 15N enrichment during the original deposition. Thus, owing to its sensitivity to sedimentary redox environments, the δ15Nbulk index can be an effective tool for evaluating depositional redox conditions, particularly for further subdivision of suboxic environments.