Redox
conditions of lacustrine strata assessed by redox proxies
with unified thresholds were conflicted in the Ordos Basin. The internal
cross-calibration approach of multiple redox proxies, based on the
enrichment sequence model of authigenic redox-sensitive element (RSE)
governed by redox potential, has recently been proposed to calibrate
redox thresholds in marine depositional systems. However, this assessment
model often induces an overlapped threshold between anoxic–ferruginous
and anoxic–euxinic conditions. Except for the redox potential,
the enrichment of RSE is also controlled by its host phase. Thus,
we introduced the enrichment degree and occurrence state model controlled
by the host phase content into this approach to identify anoxic–euxinic
conditions by an enrichment threshold of Mo (or U). Based on this
method including double redox assessment models, we calibrated the
thresholds of various redox conditions of the Chang 8–Chang
7 Members in the southern Ordos Basin. Subsequently, the effectiveness
of previous bi-element proxies, the evolution of redox conditions,
and their influencing factors were analyzed based on the calibrated
thresholds. Results show that this method is applicable in the lacustrine
strata. The cross plot of DOPT (degree of pyritization
based on the total Fe and S content) (or SEF)–MoEF (or UEF) (enrichment factor) is suggested for
calibrating thresholds of different redox conditions. Previous bi-element
proxies are not suggested to be used again. There is a positive redox
evolution sequence from oxic–suboxic (suboxidized) to anoxic–euxinic
conditions in Chang 8–Chang 7 Members. Hydrothermal activity
and paleoproductivity play different roles in the formation of redox
conditions.
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