Evaluating nitrogen isotopes as proxies for depositional redox conditions in shales: A case study from the Chang 7 Shale in the Ordos basin, North China.

Bulk sedimentary nitrogen isotopes (δ¹⁵N) have been used as an accurate redox proxy in well-preserved sedimentary systems, however, fewer studies of N-isotope have been performed in lacustrine shales.  In this paper, we report the first δ¹⁵N data from the Chang 7 Shale from a core drilled in the Ordos Basin. Bulk δ¹⁵N values are significantly higher in Zone A (the Chang 7₃ and the lower part of the Chang 7₂ submembers, average = 9.4 ± 1.3‰) than in Zone B (the upper part of the Chang 7₂ and the Chang 7₁ submembers, average = 5.4 ± 1.5‰). Given the lithological characteristics and previous geochemical measurements, we suggest that sediments within Zone A of the Chang 7 Shale were mainly deposited under suboxic bottom water conditions, whereas Zone B sediments show evidence of deposition under oxic deep water regimes. Additionally, organic carbon isotopes (δ¹³C_org) and total nitrogen (TN) values were measured to characterize any processes that might control alteration of the bulk δ¹⁵N signal, including changes in organic matter source and post-depositional processes. Our results show that there is no significant difference in the organic carbon isotopes (δ¹³C_org) and total nitrogen (TN) values between the two zones. In conclusion, we suggest that the difference in δ¹⁵N values through the Chang 7 Shale primarily reflects differences in the depositional redox conditions and δ¹⁵N values of shale can provide important details regarding the depositional history of unconventional resource plays.