Microbial proliferation coinciding with volcanism during the Permian–Triassic transition: New, direct evidence from volcanic ashes, South China

Abstract

Coupling with major biotic extinction, both widespread microbially mediated deposits and ash beds characterize the Permian–Triassic boundary (PTB) successions worldwide. Thus, active volcanism through the PTB interval has been proposed to account for contemporary microbial bloom. Direct evidence, however, has long been scarce. Here we present several lines of evidence for microbial proliferation in the PTB ashes from South China. Petrographic analysis reveals annular, granular and tubular bioalteration textures that formed during microbial dissolution of glass and subsequent precipitation. Annular textures appear as fresh glass shards connected with bands in different thicknesses, showing rough and sharp interfaces, and are bordered with fibrous textures. Granular textures are composed of clusters of near-spherical bodies, ranging in size from 0.05 to 2μm. Tubular textures occur as straight, curved or spiral tunnels excavating into fresh glass. Their distinctive morphologies, microbe-analogous sizes, and log-normal size distributions all indicate a biogenic origin. Element mapping along alteration fronts shows enrichment of organic carbon and depletion in metabolically significant elements, providing further support for their biogenicity. Besides, textural relationships, low N, K and high C contents, as well as pronounced microstructure density variations between different ash beds in same section all suggest that the bioalteration textures are syngenetic with volcanic glass, relevant to microfossils rather than recent microbes. We interpret the fossilized organic remains (e.g. small coccoid-like spheroids, rods, filaments, and thin-films) as extracellular polymeric substances (EPS) probably produced by microorganisms, which might have served as templates for nucleation, and played a crucial role in bio-mineralization process. The diversity of microstructure sizes, shapes and organic remains may reflect a complex community of microbes in the PTB ashes. We infer that microbial proliferation may contribute to negative C-isotope excursions in the PTB ashes. Microbial blooms in ash beds directly triggered by large-scale volcanism may reveal that extreme environmental stresses prevailed during the PTB crisis.