Microbial community from the Lower Permian (Artinskian–Kungurian) paleoclimatic transition, mid-Panthalassan Akiyoshi atoll, Japan

•Oncoids/microbialites were abundant on a Panthalassan atoll in the Artinskian.•The microbial community flourished exclusively in a subtidal environment.•The microbial community represents transitional members during a biotic shift.•The biotic shift resulted from climate change related to Gondwanan deglaciation.

Large oncoids (up to 14 cm) and microbialites are abundant in the Artinskian (Lower Permian) section of mid-Panthalassan Akiyoshi atoll carbonates. The oncoids and microbialites consist mainly of a tubular microproblematicum, girvanellid cyanobacterial filaments, microbial micrite crusts, and pore-filling sparry calcite cements. They are surrounded by intraclastic–bioclastic grainstone/rudstone, indicative of moderate- to slightly high-energy subtidal conditions. The microbial community was the primary boundstone-forming organisms on the Akiyoshi atoll during this time. It represents a transitional stage in a mid-Panthalassan reef succession between a cooler-water autotrophic Palaeoaplysina–microencruster community in the Gzhelian–Asselian and a warmer-water heterotrophic calcareous sponge–microencruster community in the Middle Permian. The flourishing mid-Panthalassan microbial community during the late Early Permian is related to enhanced alkalinity, increasing nutrient levels, elevated sea-surface temperatures, and the absence of major reef-building metazoans, which resulted from Gondwanan deglaciation, climatic changes, and a pulse of active volcanism.