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

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 Lagerstatten (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 (similar to 529 to 514 Ma). Here, we critically review records of various geochemical proxies and fossil assemblages from 24 Lagerstatten-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 similar to 25 parts per thousand 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 Lagerstatten in the early Cambrian (mainly Cambrian Age 3).

Automated summary

The co-evolution of marine redox conditions and early animals during the early Cambrian was crucial to the development of the modern Earth ecosystem. Geochemical proxies and fossil records from Burgess Shale-type fossil Lagerstatten in South China reveal a dynamic euxinic wedge controlling redox variations in the Nanhua Basin, with expansion and contraction influencing the emergence of early animals like those in the Chengjiang and Qingjiang biotas. Global marine oxygenation may have facilitated ecologic complexity and biodiversification of early animals during Cambrian Age 3.