112 / 2025-04-01 02:52:10

Constraining the Ediacaran-Cambrian boundary in South China using microfossil assemblages

Cambrian,Acritarchs,Biostratigraphy

Theme 3: Earth-Life Co-evolution Through Time > Session 3b: Evolution of the Earth-Life System During the Neoproterozoic-Paleozoic Transition

The Ediacaran–Cambrian boundary is arguably the most critical transition in Earth history. This boundary is currently defined by the Fortune Head (Newfoundland, Canada) GSSP at a point once regarded as the first appearance of the branching trace fossil Treptichnus pedum. However, T. pedum has subsequently been found below the GSSP, and its distribution is largely restricted to sandstone facies where crucial chemostratigraphic correlation tools are difficult to apply. Investigations of microfossils in latest Ediacaran and basal Cambrian strata of South China suggest that these microfossils can provide an alternative biostratigraphic marker for the Ediacaran–Cambrian boundary. Cambrian acanthomorphic acritarchs are widely distributed in chert-phosphorite layers intercalated with carbonates and shales, thus allowing their biostratigraphic occurrences to be calibrated with small shelly fossils and carbon isotope data. Here, the earliest Cambrian Asteridium-Heliosphaeridium-Comasphaeridium (AHC) assemblage zone is identified at numerous localities in chert-phosphorite layers in the basal Cambrian Yanjiahe, Liuchapo, and Niutitang formations, representing diverse depositional environments. The late Ediacaran fossils, Horodyskia minor and Palaeopascichnus jiumenensis (HmPj) are identified in the lower Liuchapo Formation but do not overlap with Cambrian acritarchs. The acritarch assemblage zone is in close stratigraphic proximity with basal Cambrian small shelly fossils and a negative δ¹³C excursion (BACE) known to occur at the Ediacaran–Cambrian boundary. Thus, in South China, the first occurrences of AHC assemblage microfossils and last occurrences of the HmPj fossil assemblage can be used to constrain the boundary to a 0.5 m resolution.