Iron-depleted pelagic water at the end-Permian mass extinction inferred from chemical species of iron and molybdenum in deep-sea sedimentary rocks

Abstract

This study determined chemical species of iron (Fe) and molybdenum (Mo) in a pelagic deep-sea Permian–Triassic boundary section, that records redox variations across the severe mass extinction event. We applied X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) at the Fe K-edge and X-ray absorption near-edge structure (XANES) at the Mo K-edge to silicic sedimentary rock samples for analysis of the studied section. According to the EXAFS and XRD analyses, pyrite was the dominant Fe-bearing mineral in most samples, whereas ferric minerals such as hematite were absent throughout the Permian–Triassic transition. This result supports those of previous studies of other deep-sea Japanese Permian-Triassic boundary (PTB) sections. The Fe in pyrite mineral quantified by EXAFS spectra displayed synchronous variation with sulphide-sulphur (Ssulphide) but with total organic carbon content, which revealed the Fe-limited conditions for pyrite formation in the pelagic Panthalassa region. The XANES analysis indicated the presence of tetrahedral and octahedral configurations of Mo bonded to oxygen (O) and sulphur (S), which were referred to as MoO42−, MoO3 (as a possible alternative reference material of Mo in organic ligands), MoO2 and MoS2. The MoS2 species dominated in the end-Permian horizons, coinciding with high abundances of total Mo, which suggests a sulphidic depositional environment. The MoS2 and MoO3 species were the main contributors to these high Mo abundances in the end-Permian strata. The presence of the molybdenite (MoS2) species in the studied section indicates that the samples were subjected to alteration under high temperatures, with sufficiently sulphidic and limited reactive Fe availability conditions for MoS2 formation during late diagenesis. The Fe in pyrite decreased across the mass extinction boundary, which coincided with the highest total Mo amount with the MoS2 and MoO3 species. These trends reflected a decrease in reactive Fe in the sediments and contemporaneous seawaters under sulphidic conditions. The low reactive Fe condition could be due to massive pyrite formation under prolonged sulphidic water conditions in the pelagic and ambient continental margin regions in the end-Permian.