Microbial and animal evolution in relation to redox fluctuations in a deep-water setting of South China during the Ediacaran-Cambrian transition (ca. 551–523 Ma)

Abstract

The Ediacaran-Cambrian (E-C) transition witnessed significant ecosystem perturbations of the oceanic environment (e.g., oxygen concentrations of seawater) and biological evolution. However, in spite of abundant Ediacara-type megafossils, trace fossils, small shelly fossils (SSFs), macroscopic algae and metazoans recovered from shallow-water E-C transition strata throughout South China, the manifestation of geological events in deep-water settings remains elusive. Here, we present new paleontological and geochemical evidence from the ~38-m-thick deep-water Liuchapo Formation (ca. 551–523 Ma) of the Yangtou section, northeastern Guizhou Province, China, which consists mainly of chert through the E-C transition. The Liuchapo Formation can be divided into two members based on biostratigraphy and redox proxies. The lower member (0–32 m) is characterized by low Mo/Al (0.15–1.82 ppm/%) and U/Al (0.47–2.51 ppm/%) values that suggest deposition under suboxic conditions. These deposits are dominated by Horodyskia minor, Paleopascichnus jiumenensis, spherical cyanobacteria (?Myxococcoides) and random acritarchs. The upper member (32–38 m) records a transition to anoxic (non-sulfidic) conditions as suggested by marked increases in Mo/Al (5–47 ppm/%) and U/Al (8–810 ppm/%). The upper member contains SSFs, sponge spicules, filamentous cyanobacteria, multicellular algae and sheet-like fossils. The survival of diverse organisms under strongly reducing conditions reflected in the upper member suggests that, marine redox fluctuations may limit the growth of Late Ediacaran organisms and provide the living space and elevated nutrients for early Cambrian biota.