Reef development at the Frasnian/Famennian mass extinction boundary

A newly compiled global reef database indicates that the 5–6 Myr long Frasnian (Late Devonian) metazoan reef episode had relatively low diversity compared to Middle Devonian highs (with over 200 genera of calcitic rugose and tabulate corals). Following an initial early rise after Late Givetian coral and stromatoporoid extinctions, reefs expanded for the last time during mid-Frasnian sealevel highstands, but declined markedly in the Late Frasnian (rhenana-linguiformis conodont zones), below the Frasnian/Famennian (F/F) boundary. Globally, metazoan reefs were wiped out by the end Frasnian: some Famennian reefs, while partly retaining the structure of the underlying carbonate platform, were built by cyanobacterial consortia such as Renalcis, Rothpletzella, Girvanella and Epiphyton. During the Famennian, foraminiferans with calcite walls became abundant for the first time in the Phanerozoic, adding a new dimension to carbonate platforms. Colonial rugose corals (phaceloid, cerioid and thamnasterioid modules) were absent in the early post-extinction phases up into the mid-Famennian, and very rare and non-reef-building later, but solitary deep-water Lazarus corals survived locally. Coral–sponge reefs are unknown from the 21 Myr long Famennian, also a time of very low platform carbonate production. Rare, small, isolated stromatoporoid sponge, and lithistid sponge patch reefs returned episodically during the Famennian in North America, western Europe, Australia and China: the aragonitic stromatoporoids became extinct at the end of the Famennian. During a Late Devonian tectonically very active, collisional Caledonian mountain-building phase, oceanic and atmospheric cooling, accompanied by sealevel lowstand systems, exposed most carbonate platforms, accelerating coastal erosion and karsting. This increased the amount of clastics in the shelf-slope setting, in the last 1–3 Myr prior to the F/F boundary, often burying reefs. Immediately following, there were protracted losses in nearly all major tropical shelf, benthic marine invertebrates, exceeded only by the end Permian extinctions in severity. There is no apparent link between black, organic-rich horizons and reef demise at or close to the F/F boundary. The F/F boundary not also marks the largest change from widespread flooded Early and Mid-Paleozoic continental cratons to narrow, distal shelves, but also spikes the largest known global Phanerozoic shift in atmospheric O2 enrichment, and CO2 drawdown. This threshold matched the rise of the first tropical rainforests, and expansion of terrestrial biomes on the tropical coastal lowlands formerly occupied by carbonate platforms. [Copyright &y& Elsevier]