Extinction Theory Falls From Favor

Categories: Biosphere
Scientists have theorized that many mass extinctions in Earth’s history are linked to impact events, however new research is showing that other factors may be involved. This painting, "K/T Hit" by artist Donald E. Davis, depicts an impact that occurred 65 million years ago, coinciding with the dinosaur extinctions.
Credit: Don Davis

The greatest mass extinction in Earth’s history also may have been one of the slowest, according to a study that casts further doubt on the extinction-by-meteor theory. Creeping environmental stress fueled by volcanic eruptions and global warming was the likely cause of the Great Dying, or Permian-Triassic Extinction, 250 million years ago, said University of Southern California doctoral student Catherine Powers. The research sheds light on how past life interacted with our planet’s changing environmentduring one of the most important events in the evolution of life on Earth.

Writing in the November issue of the journal Geology, Powers and her adviser David Bottjer, professor of earth sciences at USC College, describe a slow decline in the diversity of some common marine organisms.

The decline began millions of years before the disappearance of 90 percent of Earth’s species at the end of the Permian era, Powers shows in her study.

More damaging to the meteor theory, the study finds that organisms in the deep ocean started dying first, followed by those on ocean shelves and reefs, and finally those living near shore.

“Something has to be coming from the deep ocean,” Powers said. “Something has to be coming up the water column and killing these organisms.”

USC doctoral student Catherine Powers.
Credit: Matthew Clapham

That something probably was hydrogen sulfide, according to Powers, who cited studies from the University of Washington, Pennsylvania State University, the University of Arizona and the Bottjer laboratory at USC.

Those studies, combined with the new data from Powers and Bottjer, support a model that attributes the extinction to enormous volcanic eruptions that released carbon dioxide and methane, triggering rapid global warming.

The warmer ocean water would have lost some of its ability to retain oxygen, allowing water rich in hydrogen sulfide to well up from the deep (the gas comes from anaerobic bacteria at the bottom of the ocean).

If large amounts of hydrogen sulfide escaped into the atmosphere, the gas would have killed most forms of life and also damaged the ozone shield, increasing the level of harmful ultraviolet radiation reaching the planet’s surface.

A plot of data on mass extinctions in Earth’s history. What is known as the ‘Great Dying’ occurred around 250 million years ago.
Credit: University of Chicago

Powers and others believe that the same deadly sequence repeated itself for another major extinction 200 million years ago, at the end of the Triassic era.

“There are very few people that hang on to the idea that it was a meteorite impact,” she said. Even if an impact did occur, she added, it could not have been the primary cause of an extinction already in progress.

In her study, Powers analyzed the distribution and diversity of bryozoans, a family of marine invertebrates.

Based on the types of rocks in which the fossils were found, Powers was able to classify the organisms according to age and approximate depth of their habitat.

She found that bryozoan diversity in the deep ocean started to decrease about 270 million years ago and fell sharply in the 10 million years before the mass extinction that marked the end of the Permian era.

This image shows an advancing lava flow on the seafloor at the NW Rota-1 volcano, in the Northern Mariana Islands. Bubbles of carbon dioxide are streaming upwards as gases escape from the lava. Similar undersea eruptions on the ancient Earth may have played a role in triggering rapid global warming.
Credit: NOAA

But diversity at middle depths and near shore fell off later and gradually, with shoreline bryozoans being affected last, Powers said.

She observed the same pattern before the end-Triassic extinction, 50 million years after the end-Permian.

Powers’ work was funded by the Geological Society of America, the Paleontological Society, the American Museum of Natural History and the Yale Peabody Museum, and supplemented by a grant from USC’s Women in Science and Engineering program. Geology is published by the Geological Society of America.

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