A re-assessment and calibration of redox thresholds in the Permian Lucaogou Formation of the Malang Sag, Santanghu Basin, Northwest China

Abstract Analysis of redox conditions of lake water is of great significance to the reconstruction of sedimentary environments. However, previously established threshold values of redox-sensitive element proxies may not be directly applicable to various study areas even with some degree of calibration. The present study of the Permian Lucaogou Formation (P2l), Santanghu Basin, re-evaluates and calibrates commonly used redox proxies, including bimetal ratios, the C–S–Fe–P system, and proxies based on U abundances. Specifically, we rely on the compound covariation theory of proxies proposed by previous researchers within the redox framework according to three key redox thresholds (T1: suboxidized/subreduced boundary, T2: middle of subreduced zone, and T3: suboxic/euxinic boundary) and organic/inorganic geochemical data. The results show that the fixed thresholds of widely used redox proxies, including bimetal ratios, are not fully applicable to the studied succession. Reassessment of the use of bimetal ratios based on covariation theory suggests that the V/Cr, V/(V + Ni), TOC/P, and Fe/Al ratios are especially useful to redox analysis of the P2l succession whereas the Ni/Co, Ni/V, (Cu + Mo)/Zn, U/Th, and Uauth proxies yield spurious results. It is important to point out, however, that the threshold values of those proxies that have yielded useful results cannot be used without calibration. Therefore, those redox proxies used in our investigation of the P2l succession have been re-calibrated based on the proxy covariation theory. For example, V/(V + Ni) values of T1, T2, and T3 thresholds of approximately 0.78 differ from the fixed threshold values established by previous investigations, suggesting that avoiding erroneous redox interpretations will require calibration of the thresholds. Other redox proxies display a similar need for calibration. Deposits of the P2l, by virtue of small variations among the three thresholds, record a complex history of rapid transitions from suboxic to euxinic conditions.