Geochronological constraints on stratigraphic correlation and oceanic oxygenation in Ediacaran-Cambrian transition in South China

摘要

The continuous late Ediacaran - early Cambrian deep-water successions of South China archive the complete evolution of seawater chemical conditions in the deep ocean during this critical time interval. However, the geochemical data from these poorly fossiliferous and condensed successions lack high-resolution stratigraphic constraints, hampering their interpretation for the spatio-temporal evolution of the sweater chemistry in this time interval. In this study, we report a new SIMS and CA-ID-TIMS zircon U-Pb age 545.76 +/- 0.66 Ma (total uncertainty) for an ash bed at the lower Liuchapo Formation in the deep-water Longbizui section in western Hunan Province. The new age suggests that the lower and the middle-upper parts of the Liuchapo Formation in the deep water facies can be correlated with the lower Dengying Formation and the upper Dengying - lower Zhujiaqing formations in the shallow water facies, respectively. This correlation implies that the correlative horizon of the Ediacaran-Cambrian boundary in the deep water facies in South China is likely located near the base of a widespread negative delta C-13(org), excursion at the upper Liuchapo Formation. Based on our new geochronological framework, the compilations of Fe-speciation, Mo, and U data indicate that the deep ocean was characterized by widespread anoxic, ferruginous water, with intermittent euxinic water impinged on the middle-lower slope in Ediacaran-Cambrian transition, and significant oxygenation events occurred in 533-520 Ma. The compilations do not support any significant oceanic oxygenation events in 551-535 Ma.