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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. • Organismal shifts and redox condition variations during the Ediacaran-Cambrian transition • Flourishing multicellular organisms are reported from the upper Liuchapo Formation. • Geochemical proxies confirm the studied chert is of seawater origin. • Diverse organisms may have survived under oxygen-depleted condition. • Marine redox fluctuation was important for the biotic turnover. [ABSTRACT FROM AUTHOR]

• We document sponge spicules from Ediacaran-Cambrian boundary interval. • Earliest spiculites are reported here in the lower Yanjiahe Formation. • Geochemical signal confirms the studied chert is mainly of biogenic origin fixed by sponges. • Ecosystem engineering by sponges was probably an important trigger for the first establishment of modern-type ecosystems. • Taphonomy may have affected morphology of spicules and their scarcity in Precambrian. The lack of unequivocal sponge fossils before the Cambrian despite their probable deep origin during the Cryogenian period has been a conundrum to geologists. Their impact on the dramatic evolution of ecosystems and the seawater silica cycle during the Ediacaran-Cambrian (E-C) transition is also speculative. In this study, abundant sponge spicules and spicule-like structures that probably represent original sponge fossils were recovered from four sections of the E-C boundary interval in the Yangtze Gorges, South China. The paleontological and geochemical data presented herein provides evidence for a continuous distribution of relatively abundant sponge spicules, some even forming spiculites, as well as the earliest biogenic deposition of silica by metazoans in the Ediacaran-Cambrian boundary interval. These results further confirm that a biological takeover of oceanic dissolved Si co-occurred with the evolution of silica biomineralization in the E-C interval, resulting in the widespread deposition of biogenic chert. Hydrothermal input in spicule-bearing cherts is also observed. Such hydrothermal activity might have favored the blooming of sponges through significant nutrient supply. These results provide the first conjunction of geochemical and paleontological proxies in support of previous models considering that sponges, by ventilation, filtration and oxygenation of seawater, were important ecosystem engineers of the E-C bioradiation event and of the related establishment of modern-type ecosystems. [ABSTRACT FROM AUTHOR]