New evidence: Vegetation loss accompanied mass extinction

By Vince Stricherz, News and Information

The greatest mass extinction in Earth history eliminated 85 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 UW 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, published 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.

Braided streams were common until the Silurian Period some 400 million years ago, but then gave way to meandering streams as plant life evolved. Today it is rare to see a braided stream unless it is in a place, like Mount St. Helens in Washington state, where the landscape has been denuded by a catastrophic event such as a volcanic eruption.

“The thing we take so for granted now - meandering rivers - is a very recent feature on Earth,” Ward said.

The sudden reappearance of braided streams, probably on a global scale, 250 million years ago is strong evidence of a major catastrophe that wiped out plant life as well as much of animal life, Ward said.

A number of potential causes for the mass extinction have been considered, including the impact of an asteroid or comet, environmental shifts, volcanism or the overturning of the oceans to release trapped gases into the atmosphere.