Increases in marine environmental heterogeneity during the early animal innovations: Evidence from nitrogen isotopes in South China

Abstract

Extraordinary innovation of metazoans and their morphological diversity have been linked to a major step in the oxygenation of the Earth surface system in the late Ediacaran–early Cambrian. However, the role of oxygen in driving animal evolution has been an intense debate, as emerging geochemical evidence has implied a static state or unidirectional change in marine redox landscape. Nitrogen is an essential nutrient for living organisms, and has the great potential to establish linkages between nutrient cycle, marine redox, and biological evolution. In this study, we conducted a high-resolution analysis of nitrogen isotopes for the early Cambrian succession within a deep basinal drill-core section (Huitong) from Hunan Province, South China. Combining new and published nitrogen isotope data from the Nanhua Basin, we propose two phases of δ15N variations alongside a long-term decrease in δ15N baseline during the late Ediacaran–Cambrian Age 4. The first one was characterized by temporal δ15N swings in the late Ediacaran–Cambrian Age 2, and the second one was marked by the rise in spatial δ15N heterogeneity in the Cambrian Age 3–Age 4. This spatiotemporal pattern of δ15N variations indicates a shift from temporal redox fluctuations to spatial redox heterogeneity, along with deoxygenation in the continental basin from the late Ediacaran to Cambrian Age 4. Correspondingly, the nitrogen sources for primary producers have changed from dominantly single speciation in the late Ediacaran–Cambrian Age 2 to multiple speciation during the Cambrian Age 3–Age 4. We propose that the spatial heterogeneities in oxygen levels and nitrogen speciation along the continental shelf was probably the main controlling factor for the rapid proliferation of metazoans in the Cambrian Epoch 2.