SUMMARY:
Oxidation
observed in
2.7 billion
year old
micrometeorites,
suggests Earth's
early atmosphere
was very rich in
carbon dioxide.
Micrometeorites
of various
other ages,
could be a
good proxy
for the history
of Earth's early
atmospheric
composition.
DETAILS:
Earth gets
bombarded by
a large meteorite
once in a while.
Our planet
gets pelted
by "space dust"
and cosmic rays
every day.
The effect
of cosmic ray
variations
on clouds
has been
debated
for decades.
I'd say their effect
on clouds, and
climate change,
needs more study.
I don't recall
ever reading
about the effect
of space dust
on climate change.
Space dust
now seems
to be useful
as a proxy for
Earth's early
atmosphere.
Space dust
includes tiny
micrometeorites,
that collect on
Earth's surface.
Tiny iron
meteorites
used for
this study
were about
one half of
a millimeter
across.
They all fell
into the ocean,
and were collected
from the deep sea.
A University of
Washington team
looked at very old
samples of these
tiny meteorites.
Their study was
published recently
in the open-access
journal Science
Advances.
"Our finding (is)
that the atmosphere
these micrometeorites
encountered was
high in carbon dioxide,
is consistent with
what the atmosphere
was thought
to look like,
on the early Earth,"
said first author
Owen Lehmer,
a UW doctoral
student in Earth
and space sciences.
At 2.7 billion years old,
these are the oldest
known micrometeorites.
They were collected
in limestone, in the
Pilbara region of
Western Australia.
They fell
during the
Archean eon,
when the sun
was weaker
than today.
"Life formed more than
3.8 billion years ago,
and how life formed,
is a big, open question.
One of the
most important
aspects is what
the atmosphere
was made up of
- what was available,
and what the climate
was like,"
Lehmer said.
The sand-sized grains
hurtled toward Earth
at up to 20 kilometers
per second.
For an atmosphere
of similar thickness
to today,
the metal beads
would melt at about
80 kilometers
elevation,
and the molten
outer layer of iron
would then oxidize
when exposed
to the atmosphere.
A few seconds later,
the micrometeorites
would harden again,
for the rest of their fall.
The samples would then
remain intact, especially
when protected under
layers of sedimentary
limestone rock.
The new study
uses modeling
to ask whether
carbon dioxide
could have
provided
the oxygen
to produce
that result.
A computer
simulation
finds that an
atmosphere
made up of,
from 6%, to
more than 70%
carbon dioxide
could have
produced
the effect seen
in the samples.
"The amount
of oxidation
in the ancient
micrometeorites
suggests that
the early
atmosphere
was very rich
in carbon dioxide,"
said co-author
David Catling,
a UW professor
of Earth and
space sciences.
Carbon dioxide
concentrations
today are rising,
and are currently
at about 415 parts
per million,
or 0.0415% of the
atmosphere's
composition.
High levels
of carbon dioxide,
a heat-trapping
greenhouse gas,
may have offset
the sun's
weaker output
during the
Archean era.
Knowing the exact
concentration of
carbon dioxide
in the atmosphere
could help estimate
air temperature
and ocean acidity
during that time.
"Because
these iron-rich
micrometeorites
can oxidize
when they
are exposed to
carbon dioxide,
or oxygen,
and given
that these
tiny grains
presumably
are preserved
throughout
Earth's history,
they could provide
a very interesting
proxy for the history
of atmospheric
composition,"
Lehmer said.