Isotopic and REE evidence for the paleoenvironmental evolution of the late Ediacaran Dengying Section, Ningqiang of Shaanxi Province, China

Abstract

Weakly mineralized taxa are most likely the ancestors of higher organisms that existed during the Cambrian explosion, thereby representing an important yet least understood evolutionary link between simple and complex life. An investigation of the seawater temperature and the carbon cycle during the late Ediacaran, in the time span of the emergence of weakly mineralized taxa, contributes to the deeper understanding of the environmental conditions existing during the early stages of animal skeleton mineralization. Towards this end, we conducted the analyses on the carbon and oxygen isotope composition, petrography, major elements, and trace elements, involving the rare earth elements (REEs) variations in the carbonate rocks of Ningqiang County in Shaanxi Province of China. The samples from the Dengying Formation (Fm.) include the Gaojiashan and Beiwan Members, in ascending order. In addition, the Gaojiashan Member hosts one of the most diverse collections of biota with weakly mineralized skeletons discovered to date. Our data shows that a positive carbon excursion, high δ18O values, positive correlation between δ13C and δ18O values, high REE concentrations, a distinctive MREE-enriched pattern, high Fe concentrations and slightly negative Ce anomalies in the Gaojiashan Member. These evidences indicate a restricted warm sea environment with an influx of river water or groundwater. Conversely, the low carbon isotope values, high δ18O values and its heterogeneity, the absence of correlation between carbon and oxygen, combined with a seawater-like REE distribution pattern in the Beiwan Member suggested that a restrict, shallow, slightly cool and/or arid sea environment condition that was subject to periodic perturbations in the hydrologic balance. Therefore, we surmised that the environmental differences between the Gaojiashan and the Beiwan Members probably resulted in the preservation of a number of animal skeleton fossils in the middle section of the Dengying Fm.