2 oxygenation events in ancient oceans sparked spread of complex life

February 25, 2008

Blacksburg, Va. -- 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 (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..
-end-
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. The URL for the paper is 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 NSF Sedimentary Geology and Paleobiology Program, NASA Exobiology Program, National Natural Science Foundation of China, Virginia Tech Institute of Critical Technology and Applied Sciences, Evolving Earth Foundation, and several other funding agencies.

McFadden grew up in Phoenix, AZ and received her master's of science degree from Arizona State University.

Virginia Tech

Related Carbon Articles from Brightsurf:

The biggest trees capture the most carbon: Large trees dominate carbon storage in forests
A recent study examining carbon storage in Pacific Northwest forests demonstrated that although large-diameter trees (21 inches) only comprised 3% of total stems, they accounted for 42% of the total aboveground carbon storage.

Carbon storage from the lab
Researchers at the University of Freiburg established the world's largest collection of moss species for the peat industry and science

Carbon-carbon covalent bonds far more flexible than presumed
A Hokkaido University research group has successfully demonstrated that carbon-carbon (C-C) covalent bonds expand and contract flexibly in response to light and heat.

Metal wires of carbon complete toolbox for carbon-based computers
Carbon-based computers have the potential to be a lot faster and much more energy efficient than silicon-based computers, but 2D graphene and carbon nanotubes have proved challenging to turn into the elements needed to construct transistor circuits.

Cascades with carbon dioxide
Carbon dioxide (CO(2)) is not just an undesirable greenhouse gas, it is also an interesting source of raw materials that are valuable and can be recycled sustainably.

Two-dimensional carbon networks
Lithium-ion batteries usually contain graphitic carbons as anode materials. Scientists have investigated the carbonic nanoweb graphdiyne as a novel two-dimensional carbon network for its suitability in battery applications.

Can wood construction transform cities from carbon source to carbon vault?
A new study by researchers and architects at Yale and the Potsdam Institute for Climate Impact Research predicts that a transition to timber-based wood products in the construction of new housing, buildings, and infrastructure would not only offset enormous amounts of carbon emissions related to concrete and steel production -- it could turn the world's cities into a vast carbon sink.

Investigation of oceanic 'black carbon' uncovers mystery in global carbon cycle
An unexpected finding published today in Nature Communications challenges a long-held assumption about the origin of oceanic black coal, and introduces a tantalizing new mystery: If oceanic black carbon is significantly different from the black carbon found in rivers, where did it come from?

First fully rechargeable carbon dioxide battery with carbon neutrality
Researchers at the University of Illinois at Chicago are the first to show that lithium-carbon dioxide batteries can be designed to operate in a fully rechargeable manner, and they have successfully tested a lithium-carbon dioxide battery prototype running up to 500 consecutive cycles of charge/recharge processes.

How and when was carbon distributed in the Earth?
A magma ocean existing during the core formation is thought to have been highly depleted in carbon due to its high-siderophile (iron loving) behavior.

Read More: Carbon News and Carbon Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.