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Die-off fueled by acidifying oceans
By Alexandra Witze
The question of what killed most life on
Earth 250 million years ago is a veritable
Murder on the Orient Express, with multiple characters all dealing part of the
deathblow. Now, scientists have learned
how one of the assassins — acid — could
have performed its part of the deed.
High levels of atmospheric carbon
dioxide would have turned the oceans
more than acidic enough to kill off marine
critters, a computer simulation indicates.
“This would have been another
stressor in the system that might have
pushed things toward extinction,” says
Alvaro Montenegro, a climate modeler
at St. Francis Xavier University in Anti-
gonish, Canada. He and his colleagues
describe the finding in a paper published
online August 2 in Paleoceanography.
major marine nutrient cycles and massive volcanic eruptions.
Using a computer model developed at
the University of Victoria, Montenegro
and colleagues set up nine hypothetical
worlds — mixing and matching possible
continental arrangements, seafloor
topographies and levels of atmospheric
carbon dioxide. Then the researchers
fired up the model and watched how
carbon flowed through the ocean and
atmosphere.
At atmospheric carbon dioxide levels of 3,000 parts per million — roughly
10 times modern preindustrial levels
— much of the gas dissolved in seawa-ter, forming carbonic acid and releasing
hydrogen ions. Acidity is measured on
the pH scale; the lower the number, the
more acidic the waters. Today’s oceans
have a pH of around 8. 1; those in the
modeled end-Permian world dropped
to around 7. 3 near the equator and
7. 1 near the poles. That level would have
made it hard for many marine organisms
to use calcium carbonate to build protective shells, Montenegro says.
Today’s oceans also are growing more
acidic because of carbon dioxide belched
into the atmosphere by fossil fuel burning and other sources. Back then, most
of the gas probably came from huge
volcanic eruptions in Siberia.
How quickly carbon dioxide built up
in the atmosphere would have affected
how acidic the ocean got, says Jonathan
Payne, a paleobiologist at Stanford University. If gas concentrations increased
quickly, he says, “then this model may
be a reasonable representation of how
climate was changing at the time.” If gas
built up slowly, the oceans may have been
able to buffer the change in other ways.
But the model doesn’t include factors
such as carbon that weathered off land
surfaces and into oceans — an important
player in the carbon cycle, says modeler
Lee Kump of Pennsylvania State University in University Park. Including such
effects, he says, could better show how
life’s worst extinction came to pass. s
Julie Naylor
www.sciencenews.org
