Meteor may not have been behind greatest mass extinction 250 mln years ago

November 14th, 2007 - 2:52 am ICT by admin  
According to the study in the forthcoming issue of the journal Geology, creeping environmental stress fuelled by volcanic eruptions and global warming was the likely cause of the Permian era mass extinction.

Lead author, doctoral student Catherine Powers, and David Bottjer, professor of earth sciences at USC, describe a slow decline in the diversity of some common marine organisms some millions of years before the disappearance of 90 percent of Earth’s species at the end of the Permian era.

According to the study, organisms in the deep ocean started dying first, followed by those on ocean shelves and reefs, and finally those living near the shore.

“Something has to be coming from the deep ocean. Something has to be coming up the water column and killing these organisms,” said Powers, adding that it was probably hydrogen sulphide that was wiping out the organisms.

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.

Diversity at middle depths and near shore fell off later and gradually, with shoreline bryozoans being affected last, she said.

She said similar studies from the University of Washington, Pennsylvania State University, the University of Arizona and the Bottjer laboratory at USC, combined with the new data from this present study, also supported a model that attributed the extinction to enormous volcanic eruptions that released carbon dioxide and methane, triggering rapid global warming.

She said the warmer ocean water probably lost some of its ability to retain oxygen, allowing water rich in hydrogen sulphide to well up from the deep (the gas comes from anaerobic bacteria at the bottom of the ocean).

If large amounts of hydrogen sulphide escaped into the atmosphere, the gas would have killed most forms of life, besides damaging the ozone shield, thereby increasing the level of harmful ultraviolet radiation reaching the planet’s surface, she said.

Powers said the same deadly sequence repeated itself for another major extinction 200 million years ago, at the end of the Triassic era, 50 million years after the end-Permian.

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

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