101-4 Depth-selective ecological release of microbial mats during recovery from the end-Ordovician mass extinction

Abstract:

The end-Ordovician mass extinction profoundly disrupted benthic marine ecosystems, yet post-extinction recovery is generally viewed as a largely temporal process rather than one structured along environmental gradients. Here we report abundant wrinkle structures from the Lower Silurian Daxichang Formation of Guizhou, South China, a siliciclastic shelf-transition succession deposited between shallow-water carbonate platforms and deeper graptolitic black shales. Graptolite biostratigraphy indicates that the wrinkle-bearing interval is latest Rhuddanian, immediately following the extinction interval. Unlike coeval shallow-water carbonate successions, where shelly benthos and the earliest coral–stromatoporoid reef communities had already begun to recover, deeper offshore black shales remained largely devoid of benthic metazoans. We propose that the Daxichang shelf-transition environment represents an ecological intermediate, where metazoan grazing and bioturbation remained suppressed, yet environmental conditions were sufficiently favorable to support extensive microbial-mat development and preservation. This pattern suggests that microbial mat expansion following the end-Ordovician mass extinction was not uniform across marine environments, but instead reflected a depth-selective ecological release. In this model, microbial mats reached their greatest ecological importance within a bathymetrically restricted transition zone situated between rapidly recovering shallow-water ecosystems and environmentally stressed offshore settings. Rather than representing ecosystem collapse alone, these matgrounds may have temporarily stabilized substrates, concentrated organic matter, and provided ecological opportunities for surviving and opportunistic benthic organisms before the re-establishment of fully bioturbated seafloors. This recovery model differs fundamentally from the widespread and prolonged microbial resurgence documented after the end-Permian mass extinction, suggesting that microbial ecological release after mass extinctions may be highly dependent on environmental gradients and the spatial heterogeneity of benthic recovery. The Daxichang Formation therefore provides a new perspective on how microbial ecosystems and metazoan communities interacted during one of the earliest Phanerozoic recovery intervals.

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