If you ask Earth and Environmental Science's Brian Kendall to look at the ocean and tell you what he sees, he may give you a very different response than what you might expect. While most of us might comment on the startling blue colour of the water or the impact of waves as they crash onto the shore, Brian will likely offer up something very few of us have considered: the relationship between ocean oxygen levels and the evolution of animals.
About 635 million years ago, the most severe ice age known as the “Snowball Earth” glaciation occurred. Boulders, mud and rock were buried under hundreds or even thousands of meters of meters of ice around the world, starting a chain reaction of events. As an associate professor in the Department of Earth and Environmental Sciences whose specialty includes sedimentary and metal isotope geochemistry, Brian’s interest lay in the sudden increase of metal concentrations found in the organic-rich black shale of the Doushantuo Formation in South China.
In a research paper being published today in Nature, Brian and an international team of scientists led by the University of Nevada Las Vegas, have uncovered evidence for extensive ocean oxygenation shortly after 635 million years ago. They have found the first proof of a link between one of the most dramatic glaciations in Earth’s history, the subsequent oxygenation of the Earth’s surface environments and the appearance of the earliest primitive animals.
It took about four billion years before the Earth's surface finally had enough oxygen to support primitive animal life,” explains Kendall, who was a faculty research associate at Arizona State University during the study, “It is remarkable that it took a catastrophic glaciation to get that boost in oxygen levels”.
Fossil records show a marked increase in the diversity of animal and algae fossils shortly after the end of this glaciation. Researchers have hypothesized that post-glacial oxygenation of the atmosphere and oceans was the driving factor for this, 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 previous evidence of an animal-sustaining oxygenation event by more than 50 million years.
Researchers analyzed organic-rich black shale of the Doushantuo Formation that is positioned above the glacial deposits in South China (pictured above courtesy of M. Credit). 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 lead 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, like 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. But our research will not stop there. We need more data to tell us whether or not oxygen levels varied significantly at the dawn of animal life and how this may have shaped the course of evolution.
~ Brian Kendall
Brian’s full article entitled, “Ocean oxygenation in the wake of the Marinoan glaciation” can be found in the September 27th publication of Nature.
This story has appeared in the following news outlets:
- Globe and Mail, "New analysis on oxygen makes scientist rethink history of early life"