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Low Oxygen Levels Contributed To Permian/Triassic Dying


Apr. 14, 2005 | Science and Tech
Low oxygen likely made 'Great Dying' worse, greatly delayed recovery

The biggest mass extinction in Earth history some 251 million years ago
was preceded by elevated extinction rates before the main event and was
followed by a delayed recovery that lasted for millions of years. New
research by two University of Washington scientists suggests that a sharp
decline in atmospheric oxygen levels was likely a major reason for both
the elevated extinction rates and the very slow recovery.

Earth's land at the time was still massed in a supercontinent called
Pangea, and most of the land above sea level became uninhabitable because
low oxygen made breathing too difficult for most organisms to survive,
said Raymond Huey, a UW biology professor.

What's more, in many cases nearby populations of the same species were cut
off from each other because even low-altitude passes had insufficient
oxygen to allow animals to cross from one valley to the next. That
population fragmentation likely increased the extinction rate and slowed
recovery following the mass extinction, Huey said.

"Biologists have previously thought about the physiological consequences
of low oxygen levels during the late Permian period, but not about these
biogeographical ones," he said.

Atmospheric oxygen content, about 21 percent today, was a very rich 30
percent in the early Permian period. However, previous carbon-cycle
modeling by Robert Berner at Yale University has calculated that
atmospheric oxygen began plummeting soon after, reaching about 16 percent
at the end of the Permian and bottoming out at less than 12 percent about
10 million years into the Triassic period.

"Oxygen dropped from its highest level to its lowest level ever in only 20
million years, which is quite rapid, and animals that once were able to
cross mountain passes quite easily suddenly had their movements severely
restricted," Huey said.

He calculated that when the oxygen level hit 16 percent, breathing at sea
level would have been like trying to breathe at the summit of a 9,200-foot
mountain today. By the early Triassic period, sea-level oxygen content of
less than 12 percent would have been the same as it is today in the thin
air at 17,400 feet, higher than any permanent human habitation. That means
even animals at sea level would have been oxygen challenged.

Huey and UW paleontologist Peter Ward are authors of a paper detailing the
work, published in the April 15 edition of the journal Science. The work
was supported by grants from the National Science Foundation and the
National Aeronautics and Space Administration's Astrobiology Institute.