Reconstructing Redox Landscape With Coupled Nitrogen-Sulfur Isotopes: A Case Study From Middle-Late Triassic Chang 7 Member of the Yanchang Formation in the Ordos Basin (North China)

Abstract

Euxinia, a crucial geological condition, usually signifies more severe extinction events attributed to deoxygenation in Earth's history. Despite extensive exploration of various proxies in paleoredox studies, most are primarily utilized to reconstruct atmospheric pO₂, the proportion of anoxic water relative to the entire basin, and broader trends in redox states. Few, however, hold the capacity to precisely delineate local euxinia within confined areas. To address this gap and gain insights into the temporal and spatial extent of benthic euxinia, we propose leveraging the synergistic analysis of total nitrogen isotopes (δ¹⁵N_TN) and pyrite sulfur isotopes (δ³⁴S_py). Our study focuses on the Triassic Chang 7 Member from the Yanchang Formation, Ordos Basin, North China. Through coupling the δ¹⁵N_TN and δ³⁴S_py systematics on 11 drill cores within the Ordos Basin, we reconstruct the temporal and spatial distribution of the benthic euxinia zone during the Chang-7 period. Our results suggest strong spatial heterogeneity of benthic redox conditions, with the euxinia boundary shifting from the central lake to the southwestern sections. Moreover, we identify redox-controlling factors, including organic carbon loading, water depth, and potential water circulation, and evaluate their interplay with benthic euxinia. Furthermore, the discernment of water circulation patterns may provide an innovative approach to restore the paleowind direction. These findings highlight the effectiveness of coupling δ¹⁵N_TN and δ³⁴S_py in reconstructing the local benthic redox landscape of benthic environments, and enrich our understanding of biogeochemical processes.