Early Cambrian oceanic oxygenation and evolution of early animals: A critical review from the South China Craton

Abstract

Co-evolution of marine redox conditions and early animals during the early Cambrian was fundamental to the development of modern Earth ecosystem. Numerous Burgess Shale-type fossil Lagerstätten (i.e., the Chengjiang and Qingjiang biotas) in South China offer taphonomic windows to probe the evolutionary traits of early animals and marine redox variations, and to evaluate the relationship between the two during the Cambrian Ages 2 and 3 (~529 to 514 Ma). Here, we critically review records of various geochemical proxies and fossil assemblages from 24 Lagerstätten-bearing lower Cambrian sections representing a wide variety of depositional settings from the deep basin to inner shelf across the Nanhua Basin, South China. Geochemical redox proxies: Fe speciation, and Mo and U enrichment factors were utilized to reconstruct water depth-related redox gradients within the early Cambrian Nanhua Basin. The results show that a dynamic ‘euxinic wedge’ may have existed at the mid-depth, and its expansion/contraction may have controlled redox variations in the Nanhua Basin during the early Cambrian. The dramatic contraction of the euxinic wedge in the Cambrian Age 3 suggests that local oceanic oxygenated states coincided with the emergence of the Chengjiang and Qingjiang biotas. Meanwhile, a ~ 25‰ negative shift in pyrite sulfur isotopes deciphers a major increase in seawater sulfate availability, reflecting strongly enhanced subaerial oxidative weathering and/or a lower burial flux of pyrite. Concurrent variations in U and Mo isotopes support the global extent of this ocean oxygenation event. Accordingly, the global marine oxygenation may have facilitated the emergence of both ecologic complexity and biodiversification of early animals indicated by the Chengjiang and Qingjiang Lagerstätten in the early Cambrian (mainly Cambrian Age 3).