End-Permian devastation of land plants

The mass extinction that marks the boundary between the Palaeozoic and Mesozoic Eras snuffed out more than 90% of marine animal species and about 70% of terrestrial vertebrates.  The most complete record of the Permian-Triassic boundary is in marine sediments atop an obducted ophiolite in Japan (Isozaki, Y., 1997.  Permo-Triassic boundary superanoxia and stratified superocean: records from lost deep sea.  Science, v. 276, p. 235-238).  These record a 20 million-year period when deep ocean water was lacking in oxygen, and the anoxia reached extreme conditions for about 4 million years across the boundary.  All the palaeontological signs are that shallow marine faunas dwindled slowly in the 10 million years before the P-T event.  Carbon isotopes from hydrocarbon-rich boundary strata in Canada suggest that over a period of  only 1000 years the oceans were almost devoid of life.  The open oceans had become dead from top to bottom; a scenario graphically expressed by Ken Hsu as a “Strangelove” ocean.  Whatever the pace of preceding extinctions the boundary event was a catastrophe, and the Japanese and Canadian sections suggest that maybe a half-million years passed before surviving organisms began to recover and diverge.

The much-studied K-T boundary’s association with abundant evidence for an associated giant impact, prompted geologists to look for a similar story for the near end of Earth’s life 190 million years earlier.  Supporting evidence has yet to emerge, although the boundary includes the period when huge volumes of continental flood basalts poured over what is now Siberia.

Terrestrial records are far less easy to divide into fine time divisions, partly because they record both deposition and erosion, and partly because fossils are less well-preserved than in marine sediments.  Continental sediments spanning the P-T boundary are particularly frustrating, because of the wide extent of arid to semi-arid conditions then.  The Karoo Basin of South Africa does record wonderfully the fate of vertebrates (only 6 out of 44 genera survived the  boundary event), but less so that of plants.  Abrupt changes in plant-life are equally as important as those of animals, simply because they are at the base of the terrestrial food chain.  One way of addressing vegetation shifts of the most general kind is to look for evidence of how river systems changed their patterns of deposition, and this is what a team from the University of Washington (Seattle) and the South African Museum have done in the Karoo Basin (Ward et al., 2000.  Altered river morphology in South Africa related to the Permian-Triassic extinction.  Science, v. 289 8 September 2000, p. 1740-1743).

Peter Ward, David Montgomery and Roger Smith examined sedimentary structures produced by river channels in the sandstone members of the Karoo sedimentary pile.  Permian rivers seem to have flowed in distinct, meandering channels, whereas those of Triassic age laid down sands that show consistent evidence for intricately braided  channel systems.  The shift from one to the other type falls right at the P-T boundary.  Meanders of large river channels typify land surfaces with abundant vegetation that binds alluvium.  Where vegetation cover is sparse, there is little to constrain river flow and alluvial erosion, and wide braided river courses develop.  The authors conclusion is that vegetation suffered a catastrophic die off at the P-T boundary, leaving formerly lush plains as sandy wastes.  Such a loss of plants that would previously have contributed to balancing the atmosphere’s CO2 levels and the proportions of light and heavy carbon isotopes in the global environment would have helped produce the “Strangelove” signal in the ocean sediments.  The land was seared, and evidence from similar sediments in Australia and Antarctica suggests a global loss of plant life.  Incidentally, the boundary in many places shows a leap in the abundance of fungal spores, so the Mesozoic began with decay on a grand scale.

See also: Kerr, R.A., 2000.  Biggest extinction his land and sea.  Science, v. 289 8 September 2000, p. 1666-1667.

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