LAUSR

Abstract Nitrogen isotope (δ15N) analysis has been used to evaluate depositional redox conditions in well-preserved sedimentary systems; however, fewer N-isotope studies have been performed to reconstruct the redox conditions in lacustrine shales. In this paper, we report the δ15N data from the Upper Triassic Chang 7 Shale of the Ordos Basin, North China. Sedimentary δ15N values are significantly higher in the Chang 73 and the lower part of the Chang 72 submembers (Zone A; average = 9.4 ± 1.3‰) than in the upper part of the Chang 72 and the Chang 71 submembers (Zone B; average = 5.4 ± 1.5‰). Previous geochemical measurements (i.e., U and TOC/P) of both zones show wide redox variation, however, the average values of these redox proxies are relatively higher in Zone A than those of Zone B. Combined with lithological aspects of the Chang 7 Shale, our study suggest that Zone A was mainly deposited under suboxic bottom water conditions, whereas Zone B deposits accumulated within an oxygenated water column. Stable organic carbon isotopes (δ13Corg) and total nitrogen (TN) values of the two zones display little variation that could be attributed to changing organic matter sources and/or post-depositional alteration of δ15N. Thus, we suggest that variations in δ15N throughout the Chang 7 Shale primarily reflect differences in depositional redox conditions and δ15N values of the shale that can provide important details regarding the depositional history of unconventional resource plays.