Mass-occurrence of oncoids at the Cambrian Series 2–Series 3 transition: Implications for microbial resurgence following an Early Cambrian extinction

摘要: Abstract The traditional Lower–Middle Cambrian transition (Cambrian Series 2–Series 3 transition) is marked by the first major biotic extinction of Phanerozoic Eon. This crisis has been arguably linked to changes in ocean chemistry and/or marine environments but their casual relationships remain controversial. To better understand microbial responses paleoceanographic across this critical transition, we have studied environmental conditions for mass–occurrence oncoids western North China Platform. Oncoids at form 1 3-m-thick massive beds, show spherical–subspherical morphology, and contain 8–14 light–dark cortical laminar couplets. light laminae are thicker densely intertwined filamentous cyanobacteria that calcified sheaths a prostrate growth pattern. dark thinner rich organic matter relics, pyrite framboids, heterotrophic bacteria. In most oncoids, same orientation more than five couplets, suggesting much less frequent grain overturning generally thought. Both well-preserved organomineralization fabrics/textures including extracellular polymeric substances (EPSs), nanoglobules, polyhedrons, micropeloids, oncoid formation shallow-marine with high alkalinity active sulfate reduction. presence framboids bacterial relics implies anoxic/dysoxic bottom-water conditions. Stratigraphic correlation indicates time-equivalent oncolites other microbialites widespread not only China, also Early successions globally. mass-occurrence coincides Kalkarindji large igneous province Australia, prominent negative δ 13 C excursion (ROECE or Redlichiid–Olenellid Extinction Carbon isotope Excursion), significant increase 87 Sr/ 86 Sr, positive shift 34 S. coincidence these events suggests may caused an anoxic event resulting from enhanced volcanic release CO 2 , global warming, increased continental weathering. Preservation likely related decreased metazoan grazing following crisis, during which archaeocyathids many trilobite taxa went extinction. provides evidence resurgence life shelf concomitant event.