Anomalous hopane distributions at the Permian–Triassic boundary, Meishan, China – Evidence for the end-Permian marine ecosystem collapse

Abstract

Molecular and carbon isotopic geochemistry of hopanes in marine sediments of the Meishan section in Changxing County, Zhejiang Province of China, were studied to reveal biotic and environmental changes across the Permian–Triassic boundary (PTB) and the Wuchiapingian–Changhsingian boundary (WCB). The hopane distribution at the PTB is characterized by high ratios of moretane/hopane, Tm/Ts, Tm/C 30 hopane, C 31/C 32 hopane and hopane/sterane. This hopane distribution is anomalous for these marine sediments, but it is typical of coal measure strata and soils. Two possible genetic mechanisms for such an anomaly are suggested: (1) these hopanes were terrigenous and most probably originated from acidified soil and peat. This mechanism implies that the marine primary production and aerobic bacteria production at the PTB were extremely low. (2) This hopane distribution was possibly caused by freshening and acidification of the upper water column during the end-Permian to Early Triassic marine stagnation and stratification. This hopane anomaly, which coincides with the end-Permian mass extinction and carbon-isotope excursion, may signal the end-Permian mass extinctions and marine and terrigenous ecosystem collapse. The 13C-enriched hopanes with the fingerprint of typical anoxic marine shales, coupled with an n-C 15– n-C 17-dominated distribution of n-alkanes, strongly indicate that the organic matter around the WCB at Meishan originated mainly from cyanobacteria. The corresponding positive δ 13C carbonate, and higher total organic carbon and hydrocarbon index (HI) values, strongly suggest that high marine primary production and marine anoxia caused by intermittent cyanobacterial blooms, resulted in the local mass extinction at Meishan.