Volcanic activity and changes in Earth's mantle were key to rise of atmospheric oxygen

Date:
June 9, 2020
Source:
University of Washington
Summary:
Evidence from rocks billions of years old suggest that volcanoes
played a key role in the rise of oxygen in the atmosphere of the
early Earth.



FULL STORY ========================================================================== Oxygen first accumulated in the Earth's atmosphere about 2.4 billion
years ago, during the Great Oxidation Event. A long-standing puzzle has
been that geologic clues suggest early bacteria were photosynthesizing
and pumping out oxygen hundreds of millions of years before then. Where
was it all going?

========================================================================== Something was holding back oxygen's rise. A new interpretation of rocks billions of years old finds volcanic gases are the likely culprits. The
study led by the University of Washington was published in June in the open-access journal Nature Communications.

"This study revives a classic hypothesis for the evolution of atmospheric oxygen," said lead author Shintaro Kadoya, a UW postdoctoral researcher
in Earth and space sciences. "The data demonstrates that an evolution
of the mantle of the Earth could control an evolution of the atmosphere
of the Earth, and possibly an evolution of life." Multicellular life
needs a concentrated supply of oxygen, so the accumulation of oxygen is
key to the evolution of oxygen-breathing life on Earth.

"If changes in the mantle controlled atmospheric oxygen, as this study suggests, the mantle might ultimately set a tempo of the evolution of
life," Kadoya said.

The new work builds on a 2019 paper that found the early Earth's mantle
was far less oxidized, or contained more substances that can react with
oxygen, than the modern mantle. That study of ancient volcanic rocks,
up to 3.55 billion years old, were collected from sites that included
South Africa and Canada.



========================================================================== Robert Nicklas at Scripps Institution of Oceanography, Igor Puchtel at
the University of Maryland, and Ariel Anbar at Arizona State University
are among the authors of the 2019 study. They are also co-authors of the
new paper, looking at how changes in the mantle influenced the volcanic
gases that escaped to the surface.

The Archean Eon, when only microbial life was widespread on Earth,
was more volcanically active than today. Volcanic eruptions are fed by
magma -- a mixture of molten and semi-molten rock -- as well as gases
that escape even when the volcano is not erupting.

Some of those gases react with oxygen, or oxidize, to form other
compounds.

This happens because oxygen tends to be hungry for electrons, so any atom
with one or two loosely held electrons reacts with it. For instance,
hydrogen released by a volcano combines with any free oxygen, removing
that oxygen from the atmosphere.

The chemical makeup of Earth's mantle, or softer layer of rock below the Earth's crust, ultimately controls the types of molten rock and gases
coming from volcanoes. A less-oxidized early mantle would produce more
of the gases like hydrogen that combine with free oxygen. The 2019 paper
shows that the mantle became gradually more oxidized from 3.5 billion
years ago to today.

The new study combines that data with evidence from ancient sedimentary
rocks to show a tipping point sometime after 2.5 billion years ago,
when oxygen produced by microbes overcame its loss to volcanic gases
and began to accumulate in the atmosphere.

"Basically, the supply of oxidizable volcanic gases was capable of
gobbling up photosynthetic oxygen for hundreds of millions of years after photosynthesis evolved," said co-author David Catling, a UW professor of
Earth and space sciences. "But as the mantle itself became more oxidized,
fewer oxidizable volcanic gases were released. Then oxygen flooded the
air when there was no longer enough volcanic gas to mop it all up."
This has implications for understanding the emergence of complex life
on Earth and the possibility of life on other planets.

"The study indicates that we cannot exclude the mantle of a planet
when considering the evolution of the surface and life of the planet,"
Kadoya said.

This research was funded by the National Science Foundation.


========================================================================== Story Source: Materials provided by University_of_Washington. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Shintaro Kadoya, David C. Catling, Robert W. Nicklas, Igor
S. Puchtel,
Ariel D. Anbar. Mantle data imply a decline of oxidizable
volcanic gases could have triggered the Great Oxidation. Nature
Communications, 2020; 11 (1) DOI: 10.1038/s41467-020-16493-1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200609144448.htm

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