What a 250-Million-Year-Old Extinction Event Can Tell Us about the Earth Today

Approximately 250 million years ago, vast numbers of species
disappeared from Earth. This mass-extinction event may hold clues to
current global carbon cycle changes, according to Jonathan Payne,
assistant professor of geological and environmental sciences. Payne, a
paleobiologist who joined the Stanford faculty in 2005, studies the
Permian-Triassic extinction and the following 4 million years of
instability in the global carbon cycle. In the July issue of the
Geological Society of America Bulletin, Payne presented evidence that a
massive, rapid release of carbon may have triggered this extinction.



"People point to the fossil record as a place where we can learn about
how our actions today may affect the future course of evolution," Payne
said. "That's certainly true: The deep geologic record provides context
for modern events. We may miss very important processes or
underestimate the magnitude of changes in the future by using only the
past couple thousand years as a baseline."


Great Bank of Guizhou


Payne has spent the past five years unearthing the deep geologic record
in south China. The kilometer-thick, limestone fossil beds at the Great
Bank of Guizhou formed in shallow ocean waters during the late Permian
and early Triassic periods. As the ocean floor sank, new, younger
layers of limestone formed on top of deeper, older ones. Since then,
plate tectonics have turned these rocks on their side. Now, Payne and
his colleagues can walk back in time across the formerly horizontal
layers.


Marine fossil beds such as these offer two advantages for someone
studying broad patterns in the history of life, according to Payne.
Because ocean waters cover large areas for long periods of time and
somewhat protect the underlying rocks from erosion, marine fossil beds
tend to be physically larger and cover a longer period of time with
finer temporal resolution.


More than 90 percent of all marine species disappeared from the Great
Bank of Guizhou and other end-Permian fossil formations 250 million
years ago. Land plants and animals suffered similar losses. Douglas
Erwin, curator of the Paleozoic invertebrates collection at the
Smithsonian National Museum of Natural History, has dubbed this event
"the greatest biodiversity crisis in the history of life." An unusually
long period of time passed before biological diversity began to
reappear. Scientists disagree on the causes of this extinction.
However, nearly all explanations cite the high levels of greenhouse
gases, including carbon dioxide, low levels of oxygen in the oceans and
high levels of toxic gases.


Siberian Traps


In 1991, scientists reported that the largest known volcanic event in
the past 600 million years occurred at the same time as the end-Permian
extinction. Magma extruded through coal-rich regions of the Earth's
crust and blanketed a region the size of the continental United States
with basalt to a depth of up to 6 kilometers. The eruptions that formed
the Siberian Traps not only threw ash, debris and toxic gases into the
atmosphere but also may have heated the coal and released vast
quantities of carbon dioxide and methane into the atmosphere.


Rapid release of these greenhouse gases would have caused the oceans
first to become acidic and then to become supersaturated with calcium
carbonate. In the July Bulletin, Payne presents evidence that
underwater limestone beds around the world eroded at the time of the
end-Permian extinction. This finding, coupled with geochemical evidence
for changes in the relative abundances of carbon isotopes, strongly
suggests an acidic marine environment at the time of the extinction.
The rock layers immediately covering this eroded surface include
carbonate crystal fans, which indicate oceans supersaturated with
calcium carbonate.



"This end-Permian extinction is beginning to look a whole lot like the
world we live in right now," Payne said. "The good news, if there is
good news, is that we have not yet released as much carbon into the
atmosphere as would be hypothesized for the end-Permian extinction.
Whether or not we get there depends largely on future policy decisions
and what happens over the next couple of centuries."


Coral reefs


Payne plans to learn more about the causes and consequences of this
massive extinction event this summer. Three students left Aug. 1 to
join him in southern China for four weeks of field studies.



If volcanic activity released sufficient quantities of carbon into the
air within less than 100,000 years, the Earth would have transiently
cooled and then experienced a prolonged period of global warming, Payne
said. This summer, Ellen Schaal, a graduate student in the Department
of Geological and Earth Sciences, will use one geochemical index to try
to understand how climate did change during the end-Permian period.


Two other students will examine coral reef structures. The Great Bank
of Guizhou contains the fossilized reefs from just before and just
after this extinction event. Undergraduate Mindi Summers hopes to
describe the ecological structure of coral reefs just before the
extinction, and graduate student Brian Kelley will study the
development and diversification of reefs after the global carbon cycle
began to stabilize.


Reef communities are a sort of canary in the mineshaft, Payne
explained. Today, coral reef health is considered a measure of
environmental stability. When stressed by environmental conditions, the
algae that inhabit the reef leave, and the reef loses color—and one
reason why algae might leave is temperature. For example, when ocean
temperatures rise during El Niño years, corals bleach. This type of
immediate response to environmental change is hard to track in the
geologic record.


"We hope to reconcile the short-term processes we observe operating in
the modern world with the very long time scales seen in the geologic
record," said Seth Finnegan, a postdoctoral scholar in Payne's lab.


Co-authors of the paper are Daniel Lehrmann, David Follett and Margaret
Seibel of the University of Wisconsin-Oshkosh; Lee Kump and Anthony
Riccardi of Pennsylvania State University; Demir Altiner of Middle East
Technical University; Hiroyoshi Sano of Kyushu University; and Jiayong
Wei of Guizhou Geological Survey. The study was sponsored by the
National Science Foundation, NASA's Astrobiology Institute and Sigma
Xi.


Fieldwork this summer is sponsored by the National Geographic Society
and the Petroleum Research Fund (administered by the American Chemical
Society).


From:

http://news-service.stanford.edu/news/2007/august8/payne-080807.html?view=print