Marine redox evolution and organic accumulation in an intrashelf basin, NE Sichuan Basin during the Late Permian

Abstract

The Late Permian oceanic redox conditions are suggested to have had a causal relationship with the latest Permian mass extinction (LPME); however, the tempo-spatial variations of marine redox conditions which could have controlled environmental context of the LPME and organic accumulation remain loosely constrained. Here, we selected an organic-rich Lopingian succession deposited in an intrashelf basin, NE Sichuan Basin, SW China. To document the coeval oceanic changes, we present multiple geochemical proxies, including iron speciation, (major and trace) element contents, mercury (Hg) contents, total organic carbon contents (TOC) and carbonate carbon isotopic composition. Four intervals (I-IV) of discrete redox conditions were identified based on Fe–Mo–U–V datasets, revealing the evolutions from oxic to euxinic, and further to ferruginous conditions. Meanwhile, the primary productivity increased, then decreased in these intervals. The euxinic water wedge appeared in parallel with the high primary productivity and enrichment of organic matter, and advanced upslope and retreated downslope dynamically in the pace of sea-level rise and fall. In this case, marine redox variations were related to interactions of eustatic changes, organic decomposition, paleogeographic setting, upwelling currents, terrigenous input and volcanic activities. Compared with previous studies from other continents, it seems that the Paleo-Tethys was more anoxic and stagnant than Neo-Tethys and Panthalassa during the Late Permian, due to paleogeographic contexts and the more restricted oceanic circulation. In general, the prominent marine anoxia prevailed in the moderate water depth, and predated the major pulse of the LPME in several sections, which implies that anoxia may have not acted as the direct “killing mechanism” of this event.