University of Waterloo researcher links surge in oceanic oxygen levels following extreme glaciation to the earliest diversification of animals
WATERLOO, Ont. (Tuesday, Oct. 2, 2012) – An international team of scientists, including University of Waterloo researcher Brian Kendall, has uncovered evidence for extensive ocean oxygenation about 635 million years ago. In the most recent issue of the journal Nature, the research team led by the University of Nevada Las Vegas makes a link between one of the most dramatic glaciations in the Earth’s history, the subsequent oxygenation of surface environments, and the appearance of the earliest animals.
Fossil records show a marked increase in the diversity of animal and algae fossils shortly after the end of a global, so-called Snowball Earth glaciation. Researchers have hypothesized that post-glacial oxygenation of the atmosphere and oceans was the driving factor for this event, but until now, scientists have not been able to find evidence for this rise in oxygen levels. The new evidence from Kendall’s team pre-dates by more than 50 million years previous evidence of an animal-sustaining oxygenation event.
"It took about four billion years before the Earth's surface finally had enough oxygen to support primitive animal life. It is remarkable that it took a catastrophic glaciation to get that boost in oxygen levels," said Kendall, a professor in the Department of Earth and Environmental Sciences at Waterloo. He was a faculty research associate at Arizona State University during the study.
Researchers analyzed organic-rich black shale of the Doushantuo Formation that is positioned above the glacial deposits in South China. They discovered metal abundances that are comparable to modern-day organic-rich ocean sediments but greater than in older black shales, indicating higher levels of oxygen in seawater. These elevated levels of molybdenum, vanadium and uranium slightly predate fossils of the earliest oxygen-demanding animals, thereby supporting the link between ocean oxygenation and animal evolution.
This spike in oxygen levels was likely caused by the burial of large amounts of organic carbon. Weathering of the continents after a severe glaciation delivers abundant nutrients to the oceans, which subsequently leads to high rates of photosynthesis and oxygen production. Rapid burial of large amounts of this photosynthetic organic carbon in sediments would allow accumulation of oxygen in the ocean and atmosphere because the organic carbon is no longer available to react with the oxygen. The increased availability in seawater of some biologically important metals, such as molybdenum, may have further spurred the rise in oxygen levels.
"This is an important milestone in our understanding of animal evolution and Earth's oxygenation," said Kendall. "But our research will not stop there. We need more data to tell us whether oxygen levels varied significantly at the dawn of animal life and how this may have shaped the course of evolution."
The joint research was supported by grants from the National Science Foundation, the NASA Exobiology Program and National Natural Science Foundation of China. The research team includes Swapan K. Sahoo and Ganqing Jiang of the Department of Geoscience at University of Nevada, Las Vegas; Noah J. Planavsky and Timothy W. Lyons of University of California, Riverside; Ariel D. Anbar of Arizona State University; Xinqiang Wang and Xiaoying Shi of the China University of Geosciences (Beijing); and Clint Scott of McGill University.
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