Washington, Feb 26 : A new study has suggested that 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.
The study was carried out by an international team of scientists from Virginia Tech, the University of Maryland, University of Nevada at Las Vegas, and Chinese Academy of Sciences.
According to the study, 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, the research team 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.
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. he researchers collected about 200 samples, cleaned and crushed them to powder, and reacted them with acid to release carbon dioxide from carbonate minerals, and then burned the residue to get carbon dioxide from organic matter.
"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.
According to McFadden, "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."
"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 added.