The end Permian mass extinction – death by fire

Author: Paul Wignall.

Editor: Derek Gobbett.

Paul first summarised what died out at the end of the Permian. He showed a graph of extinction rates through time which highlighted the importance of the end Permian extinction. On land the main groups of herbivorous and carnivorous reptiles, (Parieasaurs and the Gorgonopsids), the dominant plant groups (Glossopterids, Gigantopterids, and Cordaitids) and indeed coal deposits disappeared. In the seas radiolarians, rugose and tabulate corals, trilobites, goniatites, the dominent groups of brachiopods and bryozoans all became extinct. In the early Trias a pandemic biota of low diversity was characterised by the reptile Lystrosaurus, the fern Dicroidium, the bivalve Claraia and the inarticulate brachiopod Lingula.

He then outlined the evidence for major changes in the environment across the Permian Triassic boundary.

 In Anatolia a late Permian limestone has an irregular surface immediately overlain by a late Triassic microbial  limestone with stromatolites and thrombolite mounds but without evidence of higher life forms. The irregular junction could possibly be due to solution by acidified seawater or by karstic erosion after a sea level fall.

 In Japan, terrains accreted from the floor of the Panthalassic ocean consist of thinly bedded radiolarian cherts. In the late Permian these change from red to grey chert but  at the Permian/Trias boundary the radiolaria disappear and thin black siliceous mudstones with pyrite framboids are found. These are followed by black shales full of organic matter but by the Anisian radiolarian cherts reappear. The significance of the pyrite framboids is that they form at the redox (O2/H2S) boundary,   If this lies within the sediment framboids of various sizes may be formed but if it lies within the water column the framboids fall to the bottom after growing to 5-6 µm.

In Svalbard the Permotriassic boundary is superbly exposed in shallow marine facies on steep mountain slopes more easily accessed by helicopter. Here the Permian limestones are rich in productid brachiopods and bryozoa. The basal Trias has storm- deposted sandstones with lots of pyrite and “wrinkle structures” considered to be microbial mats. Claraia and Promyalina are abundant in limestone.

In summary at the beginning of Triassic time there appear to be evidence of variable but widespread marine anoxia and a biota of microbial organisms, and possibly acidification of the oceans.

The causes of these dramatic changes and also of other mass extinctions can be attributed to major basaltic eruptions. The Siberian traps (1.5x106 km3) correlate well with the Permo-Triassic boundary being 251 Ma old and lasting for less than 1 Ma. Gasses from the eruptions would have been dominated by SO2 with some C l and F giving acid rain, and CO2 giving rise to global warming and the stagnation of oceanic circulation. Each basalt flow could produce 350,000 megatonnes of CO2 and 120,000 megatonnes of SO2.  This amount of CO2 is only about 10 yrs worth of CO2 produced by modern pollution but other sources of greenhouse gases may have been methane released from gas hydrates and CO2 from sediments thermally metamorphosed by basalt magma. The H2S would be able to acidify the oceans but on land it is quickly destroyed by oxygen although it would tend to destroy the ozone layer.

As a final thought Paul pointed out that the Parana flood basalts of the Valanginian and the North Atlantic basalts of the late Palaeocene do not correlate with mass extinctions.