Platinum-group elements (PGEs) and rhenium in Permian-Triassic boundary sediments from southern China and Japan linked to concurrent eruptions of the Siberian Traps

Abstract

The end-Permian mass extinction (EPME) (251.9 Ma) was the most severe recorded extinction event. A pulse of sill-complex formation and volcanism during the Siberian Traps continental flood basalt (CFB) was apparently synchronous with the EPME, and has been proposed as a major cause of the environmental perturbations that led to the mass-extinction event. Abundances of PGEs and rhenium in sediments have been used in the past as signatures of CFB volcanism. We performed analyses of PGEs and rhenium concentrations and diagnostic PGE ratios across the Permian-Triassic boundary and EPME in three localities in southern China (Meishan, Zhongzhai, Xinmin), and one locality from SW Japan (Ubara). We compared these results with similar PGE and rhenium analyses of: 1) the upper continental crust (UCC), 2) basaltic samples from the Siberian, Deccan and Paraná CFBs, 3) the Cretaceous/Paleogene (K-Pg) boundary layer, and 4) average black shales. We also compared our results with previous PGE analyses of EPME/P-T boundary sections in China and elsewhere. In the four EPME sections studied here, PGEs (especially Pt, Pd, and Ru) and rhenium are commonly enriched relative to the upper continental crust (UCC) baseline. These enrichments occur in sediments usually having low PGE background (e.g., carbonates and cherts), and are interpreted as indicative of a source in CFB volcanism. Values of Ir/Pt < 0.03 in all sections also indicate a volcanic source, and there is no evidence of extraterrestrial PGEs. Volatile rhenium with Re/Ir > 10 is highly anomalous in all of the EPME sections, with Re anomalies up to 1300 times the UCC of apparent CFB origin in the Ubara, Japan deep-sea locality. Concurrent mercury and nickel anomalies, and related isotopic shifts, further support a CFB volcanic source for the volatile rhenium at the EPME. These results suggest that the Siberian CFB eruptions, especially an early pulse of emplacement of sill complexes, coincident with the EPME, were a source of the PGEs and rhenium in the EPME/P-T boundary sediments, and potentially an important factor in the environmental crisis that led to the major mass extinction.