Heterogenous oceanic redox conditions through the Ediacaran-Cambrian boundary limited the metazoan zonation

Abstract

Abstract Recent studies have enhanced our understanding of the linkage of oxygenation and metazoan evolution in Early Cambrian time. However, little of this work has addressed the apparent lag of animal diversification and atmospheric oxygenation during this critical period of Earth history. This study utilizes the geochemical proxy and N isotope record of the Ediacaran–Cambrian boundary preserved in intra-shelf basin, slope, and slope basin deposits of the Yangtze Sea to assess the ocean redox state during the Early Cambrian metazoan radiation. Though ferruginous conditions appear to have prevailed through the water column during this time, episodes of local bottom-water anoxia extending into the photic-zone impacted the slope belt of the basin. Heterogenous oceanic redox conditions are expressed by trace element concentrations and Fe speciation, and spatial variation of N isotopes. We propose that the coupling of ocean chemistry and Early Cambrian animal diversification was not a simple cause-and-effect relationship, but rather a complex interaction. Specifically, it is likely that animal diversification expanded not only temporally but also spatially from the shallow shelf to deep-water environments in tandem with progressive oxygenation of the extensive continental margin.