Earth's oxygen-rich atmosphere emerged in whiffs from a kind
of cyanobacteria in shallow oceans around 2.5 billion years ago, according to
new research from Canadian and US scientists.
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| The Earth by NASA/Apollo 17 crew; taken by either Harrison Schmitt or Ron Evans. |
These whiffs of oxygen likely happened in the following 100
million years, changing the levels of oxygen in Earth's atmosphere until enough
accumulated to create a permanently oxygenated atmosphere around 2.4 billion
years ago - a transition widely known as the Great Oxidation Event.
"The onset of Earth's surface oxygenation was likely a
complex process characterized by multiple whiffs of oxygen until a tipping
point was crossed," said Brian Kendall, a professor of Earth and
Environmental Sciences at the University of Waterloo. "Until now, we
haven't been able to tell whether oxygen concentrations 2.5 billion years ago
were stable or not. These new data provide a much more conclusive answer to
that question."
The findings are presented in a paper published this month
in Science Advances from researchers at Waterloo, University of Alberta,
Arizona State University, University of California Riverside, and Georgia
Institute of Technology. The team presents new isotopic data showing that a
burst of oxygen production by photosynthetic cyanobacteria temporarily
increased oxygen concentrations in Earth's atmosphere.
"One of the questions we ask is: 'did the evolution of
photosynthesis lead directly to an oxygen-rich atmosphere? Or did the
transition to today's world happen in fits and starts?" said Professor
Ariel Anbar of Arizona State University. "How and why Earth developed an
oxygenated atmosphere is one of the most profound puzzles in understanding the
history of our planet."
The new data supports a hypothesis proposed by Anbar and his
team in 2007. In Western Australia, they found preliminary evidence of these
oxygen whiffs in black shales deposited on the seafloor of an ancient ocean.
The black shales contained high concentrations of the
elements molybdenum and rhenium, long before the Great Oxidation Event.
These elements are found in land-based sulphide minerals,
which are particularly sensitive to the presence of atmospheric oxygen. Once
these minerals react with oxygen, the molybdenum and rhenium are released into
rivers and eventually end up deposited on the sea floor.
In the new paper, researchers analyzed the same black shales
for the relative abundance of an additional element: osmium. Like molybdenum
and rhenium, osmium is also present in continental sulfide minerals. The ratio
of two osmium isotopes - 187Os to 188Os - can tell us if the source of osmium
was continental sulfide minerals or underwater volcanoes in the deep ocean.
The osmium isotope evidence found in black shales correlates
with higher continental weathering as a result of oxygen in the atmosphere. By
comparison, slightly younger deposits with lower molybdenum and rhenium
concentrations had osmium isotope evidence for less continental input, indicating
the oxygen in the atmosphere had disappeared.
The paper's authors also include Professor Robert Creaser of
the University of Alberta, Professor Timothy Lyons from the University of
California Riverside and Professor Chris Reinhard from the Georgia Institute of
Technology.
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