The greatest mass extinction in Earth history eliminated 85 percent to 90 percent of all marine and land vertebrate species 250 million years ago, at the end of the Permian Period and the beginning of the Triassic. New evidence from researchers at the University of Washington and the South African Museum shows the extinction was accompanied by a massive loss of vegetation, causing major changes in river systems.
Probing sedimentary layers in the Karoo Basin of South Africa, the scientists found evidence that, with the loss of deep-rooting plants, meandering river systems changed rapidly to braided systems. Braided streams run much straighter and faster and branch out for short distances before merging back to the primary stream. They also cause much faster sediment buildup because vegetation is not holding streamside soil in place and it is easily swept away by the faster-moving water.
Using data from the Karoo and elsewhere, the scientists attribute the drastic change in river character to a catastrophic global die off of vegetation that likely resulted from the same cause as the mass extinction among marine and land animals.
Peter Ward, a UW geological sciences professor, along with David Montgomery, a UW associate geological sciences professor, and Roger Smith, the South African Museum's curator of geology, publish their findings in the Sept. 8 issue of the journal Science.
Sedimentary layers from the Permo-Triassic boundary were examined at seven different sites scattered across 250 miles of the Karoo Basin, and the researchers found striking similarities in the evidence for a rapid shift from meandering to braided streams.
Major tectonic activity could change streams from meandering to braided, Ward said. However, recent studies have shown there was no major tectonic activity at the time of the Permo-Triassic extinction, which occurred when the Earth's land was still locked in a supercontinent called Pangea.