Two Oxygenation Events in Ancient Oceans Sparked Spread of Complex Life
- From: baalke@xxxxxxxxxxxxx
- Date: Fri, 28 Mar 2008 17:35:54 -0700 (PDT)
http://www.vtnews.vt.edu/story.php?relyear=2008&itemno=134
Two oxygenation events in ancient oceans sparked spread of complex
life
By Susan Trulove
Virginia Tech
BLACKSBURG, Va., February 28, 2008 -- The rise of oxygen and the
oxidation of deep oceans between 635 and 551 million years ago may
have
had an impact on the increase and spread of the earliest complex life,
including animals, according to a study reported in the Proceedings of
the National Academy of Sciences online Early Edition during the week
of
Feb. 25 through 29.
Today, we take oxygen for granted. But the atmosphere had almost no
oxygen until 2.5 billion years ago, and it was not until about 600
million years ago when the atmospheric oxygen level rose to a fraction
of modern levels. For a long time, geologists and evolutionary
biologists have speculated that the rise of the breathing gas and
subsequent oxygenation of the deep oceans are intimately tied to the
evolution of modern biological systems.
To test the interaction between biological evolution and environmental
change, an international team of scientists from Virginia Tech, the
University of Maryland, University of Nevada at Las Vegas, and Chinese
Academy of Sciences, examined changes in the geochemistry and fossil
distribution of 635- to 551-million-year old sediments preserved in
the
Doushantuo Formation in the Yangtze Gorges area of South China.
Millions of years ago, the Yangtze Gorges area was an ancient sea,
said
Kathleen A. McFadden, a doctoral candidate in geobiology
at Virginia Tech and the lead author of
the Proceedings of the National Academy of Sciences article.
To determine when there was enough oxygen to support animal life in
the
ocean, the researchers asked, "What kind of geochemical evidence would
there be in the rock record?" said Shuhai Xiao, associate professor of
geosciences at Virginia Tech.
Scientists hypothesized that there was a lot of dissolved organic
carbon
in the ocean when oxygen levels were low. If oxygen levels rose, some
of
this organic carbon would be oxidized into inorganic forms, some of
which can be preserved as calcium carbonate in the rock record. "We
measured the carbon isotope signatures of organic and inorganic carbon
in the ancient rocks to infer oxidation events," said co-author
Ganqing
Jiang, assistant professor of geology at the University of Nevada at
Las
Vegas.
The layers of sediment exposed by the Three Gorges Dam represent
millions of years of deposits. "We went through road cuts, bed by bed,
measuring and describing the exposed rock, then took small rock
samples
every few feet or so," said McFadden. She collected about 200 samples;
hundreds of samples were taken to three labs.
The researchers cleaned and crushed the small samples to powder, which
they reacted with acid to release carbon dioxide from carbonate
minerals, and then burned the residue to get carbon dioxide from
organic
matter. "The [carbon dioxide] that is released was measured with mass
spectrometers to gives us the isotopic signature of the carbonate and
organic carbon that was present in the rock," said McFadden.
"The relative abundances of the carbon-12 and carbon-13 isotopes,
which
are stable and do not decay with time, provide a snapshot of the
environmental processes taking place in the ocean at the different
times
recorded in the layers of rock," McFadden said.
The stratigraphic pattern of carbon isotope abundances suggested to
these researchers that the ocean, which largely lacked oxygen before
animals arrived on the scene, was aerated by two discrete pulses of
oxygen.
"The first pulse apparently had little impact on a large organic
carbon
reservoir in the deep ocean, but did spark changes in microscopic life
forms," McFadden said. "The second event, which occurred around 550
million years ago, however, resulted in the reduction of the organic
carbon reservoir, indicating that the ocean became fully oxidizing
just
before the evolution and diversification of many of Earth's earliest
animals,"she said.
"The Doushantuo Formation has a wonderful fossil record," McFadden
said.
"It allows us to look at major fossil groups, when they appear and
when
they disappear, and to see a relationship between oxidation events and
biological groups."
"This study supports the growing view that life and environment
co-evolved through this tumultuous period of Earth history," said
geochemist Alan J. Kaufman, a co-author of the study from the
University
of Maryland.
The researchers analyzed the fossils in the Doushantuo Formation, from
microscopic life forms of 635 million years ago to large algae around
551 million years ago. Looking at data from four locations with very
similar isotopic records, they report that the first oxygen spike
resulted in a rise in microscopic organisms, some of which are thought
to be the earliest animal embryos. The second spike in oxygen
coincides
with a dramatic increase in species of large complex algae.
"Both oxidation events appear to coincide with increased diversity of
fossils assemblages in the Doushantuo basin, with the number of
species
nearly doubling," McFadden said.
Following this second oxidation event, between 550 and 542 million
years
ago, there was a worldwide increase of Ediacara organisms, complex
macroscopic life forms, an event recently dubbed as the Avalon
Explosion
<http://www.vtnews.vt.edu/story.php?relyear=2008&itemno=1>. "This was
when we see the first burrowing animals and biomineralizing animals in
the fossil record," McFadden said. Biomineralizing animals are the
first
animals to form external skeletons, or shells.
The triggers for the oxidation events remain elusive, however. "These
events recorded in the ocean were probably related to oxygen in the
atmosphere reacting with sediments on land," McFadden said.
"Weathering
of rocks and soils on the continents would result in the release of
certain dissolved ions, such as sulfate, into rivers. These would then
be transported to the sea where they might be used by bacteria to
oxidize the organic carbon pool in the deep oceans," she said.
The article, "Pulsed oxidation and biological evolution in the
Ediacaran
Doushantuo Formation," was written by Kathleen A. McFadden; Jing Huang
and Xuelei Chu of the Institute of Geology and Geophysics, Chinese
Academy of Sciences; Ganqing Jiang; Alan J. Kaufman; Chuanming Zhou
and
Xunlai Yuan of the Nanjing Institute of Geology and Palaeontology,
Chinese Academy of Sciences; and Shuhai Xiao. Read the paper online.
<http://www.pnas.org/cgi/content/abstract/0708336105v1> The paper will
publish in the print issue of March 4 (Issue 9, Volume 105, pp.
3197-3202).
The joint research was supported by National Science Foundation
Sedimentary Geology and Paleobiology Program, NASA Exobiology Program,
National Natural Science Foundation of China, Virginia Tech Institute
of
Critical Technology and Applied Sciences <http://www.ictas.vt.edu/>,
Evolving Earth Foundation, and several other funding agencies.
McFadden grew up in Phoenix, Ariz., and received her master's of
science
degree from Arizona State University.
Contact Susan Trulove at strulove@xxxxxx or (540) 231-5646.
.
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