Evidence of a Microbial Resurgence After the Cambrian Explosion

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

Authors:

Zhang et al

Abstract:

The traditional Lower–Middle Cambrian transition (Cambrian Series 2–Series 3 transition) is marked by the first major biotic extinction of the Phanerozoic Eon. This biotic crisis has been arguably linked to changes in ocean chemistry and/or marine environments but their casual relationships remain controversial. To better understand the microbial responses to paleoceanographic changes across this critical transition, we have studied the environmental conditions for mass–occurrence of oncoids in the western North China Platform. Oncoids at the Lower-Middle Cambrian transition form 1–3-m-thick massive beds, show spherical-subspherical morphology, and contain 8–14 light-dark cortical laminar couplets. The light laminae are thicker and contain densely intertwined filamentous cyanobacteria that have calcified sheaths and a prostrate growth pattern. The dark laminae are thinner and rich in organic matter relics, pyrite framboids, and heterotrophic bacteria. In most oncoids, cortical laminae show the same growth orientation for more than five light-dark laminar couplets, suggesting much less frequent grain overturning than generally thought. Both light and dark laminae contain well-preserved organomineralization fabrics/textures including extracellular polymeric substances (EPS), nanoglobules, polyhedrons, and micropeloids, suggesting oncoid formation in shallow-marine environments with high alkalinity and active sulfate reduction. The presence of pyrite framboids and heterotrophic bacterial relics implies anoxic/dysoxic bottom-water conditions. Stratigraphic correlation indicates that time-equivalent oncolites and other microbialites are widespread not only in North China, but also in other Early Cambrian successions globally. The mass-occurrence of oncoids coincides with the Kalkarindji large igneous province of Australia, a prominent negative δ13C excursion (ROECE or Redlichiid-Olenellid Extinction Carbon isotope Excursion), a significant increase in 87Sr/86Sr, and a large positive shift in δ34S. The coincidence of these events suggests that the Early Cambrian biotic crisis may have been caused by an ocean anoxic event resulting from enhanced volcanic release of CO2, global warming, and increased continental weathering. Preservation of massive oncolites is likely related to decreased metazoan grazing following an Early Cambrian biotic crisis, during which archaeocyathids and many Early Cambrian trilobite taxa went extinction. The oncoid mass-occurrence provides evidence for the resurgence of microbial life in anoxic/dysoxic marine shelf environments concomitant with the Early Cambrian extinction event.