Meteor no longer prime suspect in great extinction
Oct 24, 2007 - 4:00:00 AM
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But diversity at middle depths and near shore fell off later and gradually,with shoreline bryozoans being affected last, Powers said.
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By University of Southern California,
[RxPG] The greatest mass extinction in Earth�s history also may have been one ofthe slowest, according to a study that casts further doubt on theextinction-by-meteor theory.
Creeping environmental stress fueled by volcanic eruptions and globalwarming was the likely cause of the Great Dying 250 million years ago, saidUSC doctoral student Catherine Powers.
Writing in the November issue of the journal Geology, Powers and her adviserDavid Bottjer, professor of earth sciences at USC, describe a slow declinein the diversity of some common marine organisms.
The decline began millions of years before the disappearance of 90 percentof 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 thedeep ocean started dying first, followed by those on ocean shelves andreefs, and finally those living near shore.
�Something has to be coming from the deep ocean,� Powers said. �Somethinghas to be coming up the water column and killing these organisms.�
That something probably was hydrogen sulfide, according to Powers, who citedstudies 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 amodel that attributes the extinction to enormous volcanic eruptions thatreleased 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 gascomes from anaerobic bacteria at the bottom of the ocean).
If large amounts of hydrogen sulfide escaped into the atmosphere, the gaswould have killed most forms of life and also damaged the ozone shield,increasing the level of harmful ultraviolet radiation reaching the planet�ssurface.
Powers and others believe that the same deadly sequence repeated itself foranother major extinction 200 million years ago, at the end of the Triassicera.
�There are very few people that hang on to the idea that it was a meteoriteimpact,� she said. Even if an impact did occur, she added, it could not havebeen the primary cause of an extinction already in progress.
In her study, Powers analyzed the distribution and diversity of bryozoans, afamily of marine invertebrates.
Based on the types of rocks in which the fossils were found, Powers was ableto classify the organisms according to age and approximate depth of theirhabitat.
She found that bryozoan diversity in the deep ocean started to decreaseabout 270 million years ago and fell sharply in the 10 million years beforethe mass extinction that marked the end of the Permian era.
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 millionyears after the end-Permian.
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