A new Earth bombardment model
New model applied to understand how oxygen levels in Earth's atmosphere evolved

Date:
October 21, 2021
Source:
Southwest Research Institute
Summary:
A team has updated its asteroid bombardment model of the Earth with
the latest geologic evidence of ancient, large collisions. These
models have been used to understand how impacts may have affected
oxygen levels in the Earth's atmosphere in the Archean eon, 2.5
to 4 billion years ago.



FULL STORY ==========================================================================
A team led by Southwest Research Institute has updated its asteroid
bombardment model of the Earth with the latest geologic evidence of
ancient, large collisions. These models have been used to understand
how impacts may have affected oxygen levels in the Earth's atmosphere
in the Archean eon, 2.5 to 4 billion years ago.


==========================================================================
When large asteroids or comets struck early Earth, the energy released
melted and vaporized rocky materials in the Earth's crust. The small
droplets of molten rock in the impact plume would condense, solidify
and fall back to Earth, creating round, globally distributed sand-size particles. Known as impact spherules, these glassy particles populated
multiple thin, discrete layers in the Earth's crust, ranging in age
from about 2.4 to 3.5 billion years old. These Archean spherule layers
are markers of ancient collisions. "In recent years, a number of new
spherule layers have been identified in drill cores and outcrops,
increasing the total number of known impact events during the early
Earth," said Dr. Nadja Drabon, a professor at Harvard University and a co-author of the paper.

"Current bombardment models underestimate the number of late Archean
spherule layers, suggesting that the impactor flux at that time was up to
10 times higher than previously thought," said SwRI's Dr. Simone Marchi,
lead author of a paper about this research in Nature Geoscience. "What's
more, we find that the cumulative impactor mass delivered to the
early Earth was an important 'sink' of oxygen, suggesting that early bombardment could have delayed oxidation of Earth's atmosphere."
The abundance of oxygen in Earth's atmosphere is due to a balance of
production and removal processes. These new findings correspond to the geological record, which shows that oxygen levels in the atmosphere
varied but stayed relatively low in the early Archean eon. Impacts by
bodies larger than six miles (10 km) in diameter may have contributed to
its scarcity, as limited oxygen present in the atmosphere of early Earth
would have been chemically consumed by impact vapors, further reducing
its abundance in the atmosphere.

"Late Archean bombardment by objects over six miles in diameter would
have produced enough reactive gases to completely consume low levels of atmospheric oxygen," said Dr. Laura Schaefer, a professor at Stanford University and a co- author of the paper. "This pattern was consistent
with evidence for so-called 'whiffs' of oxygen, relatively steep but
transient increases in atmospheric oxygen that occurred around 2.5 billion years ago. We think that the whiffs were broken up by impacts that removed
the oxygen from the atmosphere. This is consistent with large impacts
recorded by spherule layers in Australia's Bee Gorge and Dales Gorge."
SwRI's results indicate that the Earth was subject to substantial
numbers of large impacts throughout the late Archean era. Around 2.4
billion years ago, during the tail end of this bombardment, the Earth
went through a major shift in surface chemistry triggered by the rise
of atmospheric oxygen, dubbed the Great Oxidation Event (GOE), which
is attributed to changes in the oxygen production-sink balance. Among
the proposed scenarios are a presumed increase in oxygen production
and decrease in gases capable of removing oxygen, either from volcanic
sources or through their gradual loss to space.

"Impact vapors caused episodic low oxygen levels for large
spans of time preceding the GOE," Marchi said. "As time
went on, collisions become progressively less frequent and
too small to be able to significantly alter post-GOE oxygen
levels. The Earth was on its course to become the current planet." ========================================================================== Story Source: Materials provided by Southwest_Research_Institute. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Marchi, S., Drabon, N., Schulz, T. et al. Delayed and variable late
Archaean atmospheric oxidation due to high collision rates on
Earth. Nat.

Geosci, 2021 DOI: 10.1038/s41561-021-00835-9 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211021120905.htm

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