Two oxygenation events in ancient oceans sparked spread of complex life

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 February 25 - 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 Ph.D. candidate in geobiology at Virginia Tech and
the lead author of the PNAS 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 CO2
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.
"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 URL for the paper is
http://www.pnas.org/cgi/content/abstract/0708336105v1 .

Source: Virginia Tech
http://www.physorg.com/news123180801.html

Posted by
Robert Karl Stonjek