Ancient air trapped in rock salt for 813 million years is
changing the timeline of atmospheric changes and life on Earth.
Defining past atmospheric compositions is an important yet
daunting task for geologists. Most methods for determining past Earth surface conditions
rely on indirect proxies gleaned from ancient sedimentary rocks. Further
complicating matters, sedimentary rocks are notoriously difficult to date because
they contain remnants of other rocks formed at various times.
As a result, oxygenation, or the rise of oxygen in the
Earth's atmosphere, has been presumed to occur about 550 million years ago near
the boundary between the Precambrian and Paleozoic geologic periods.
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| The Earth seeen from Apollo 17. By NASA/Apollo 17 crew; taken by either Harrison Schmitt or Ron Evans [Public domain or Public domain], via Wikimedia Commons |
West Virginia University geologist Kathleen Benison is part
of a research team using new direct methods to measure the Earth's oxygenation.
The team's study identifies, for the first time, exactly how
much oxygen was in Earth's atmosphere 813 million years ago - 10.9 percent.
This finding, they say, demonstrates that oxygenation on Earth occurred 300
million years earlier than previously concluded from indirect measurements.
"Diversity of life emerges right around this time
period," Benison said. "We used to think that to have diversity of
life we needed specific things, including a certain amount of oxygen. (The
findings) show that not as much oxygen is required for organisms to
develop."
Fluid inclusions, the microscopic bubbles of liquids and
gases in rock salt, can contain trapped air. Analysis of this trapped air
allows researchers to understand past surface conditions and how oxygen has
changed over the course of geologic history.
The team used a quadrupole mass spectrometer to study the
air pockets. Carefully crushing minute rock salt crystals released water and
gases into the mass spectrometer, which then analyzed for various compounds of
oxygen and other gases.
"There are a lot of different environmental conditions
specific from the past that we can find occurring in modern samples,"
Benison said. "This tells us about the range of conditions on Earth and
also has implications for Mars."
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