What geological evidence is there to suggest that the Earth originally
may have had a reducing atmosphere?
The elemental composition of the universe, given in Chapter 8,
supports this idea with an overwhelming predominance of hydrogen.
The atmospheres of other planets, especially the larger ones whose
gravitational fields would have prevented loss of their early atmospheres,
are composed primarily of H2, He,
CH, CO, CO2, N2,
NH3, and H20,
with no free oxygen.
The Earth as a whole is built mainly from metal silicates. These
silicates are 90 % oxygen by volume, but this oxygen is locked up
in the mineral framework.
Iron in minerals can serve as a barometer of the state of oxidation
of its surroundings. The familiar red and orange of oxidized Fe(III)
compounds from sands are only skin deep.
A short distance below the surface, these colors give way to the
green and black of reduced Fe (II) compounds. The oxidized minerals
are a thin surface layer that is exposed to an O2-containing
atmosphere that is anomalous among the planets.
Life has "rusted" the surface of our planet, but has
had little effect on its interior.
We can learn much from old sedimentary rocks about the conditions
under which they were deposited.
Rocks that crystallized in the interior and then were thrust to
the surface have little to tell us about the atmosphere of the time.
In contrast, sedimentary rocks deposited by the weathering away
of older minerals during a long contact with the atmosphere preserve
a record of that atmosphere.
If the atmosphere were oxidizing, then the sediments would be at
least partially oxidized; if reducing, then the sediments would
remain reduced. Present-day sands are mainly quartz and other forms
of SiO2. Most other minerals in the
rocks that weathered to make sand have been oxidized. Their oxidized
metal cations have been leached out, ultimately to be redeposited
elsewhere as clay minerals.
The result is that sedimentary rocks deposited during oxidizing
conditions are of three main types: silicate sands, clay minerals,
and carbonate deposits of biological origin (from shells of marine
life). The sedimentary rocks laid down during the past 500 million
years and more are of this type. All indicate that oxidizing conditions
existed during their original weathering period.