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)

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 (delta 15NTN) and pyrite sulfur isotopes (delta 34Spy). Our study focuses on the Triassic Chang 7 Member from the Yanchang Formation, Ordos Basin, North China. Through coupling the delta 15NTN and delta 34Spy 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 delta 15NTN and delta 34Spy in reconstructing the local benthic redox landscape of benthic environments, and enrich our understanding of biogeochemical processes. Understanding past oxygen levels in Earth's waters is crucial for tracing its evolution and habitability. Euxinia, marked by severe deoxygenation, is a key indicator of environmental conditions. While a series of proxies have been used, only a handful of them are applicable to local euxinia identification, especially for lake systems that may possess unique thresholds, complicating the investigation of their biogeochemical processes. The coupling of nitrogen-sulfur isotopes is innovatively raised to distinguish local bottom euxinia from more oxic (i.e., non-euxinic) settings. The reconstructed redox landscape of the Triassic Ordos Basin exhibits strong spatiotemporal diversity. The temporal evolution is mainly controlled by water depth, while the spatial heterogeneous euxinia distribution is probably dominated by water circulation, mainly influenced by paleowinds at the surface of lake water. These deductions align with geochemical, tectonic, and sedimentological observations, such as ancient topography of the basin and thickness distribution of tuff layers. These findings put forward a unique window for identifying local euxinia and investigating paleowind orientation. Systematic analyses of C-N-S-Fe geochemistry of the Triassic Ordos Basin were conducted The spatiotemporal paleoredox landscape was reconstructed by the coupling of nitrogen-sulfur isotopes The temporal and spatial heterogeneity of euxinia was controlled by water depth and water circulation/ventilation, respectively