Weathering proceeds differently under a reducing atmosphere.
Quartz is still a major component of sedimentary material. The other
minerals that contain metals in lower oxidation states are less
soluble and do not leach out completely.
Among the reduced iron minerals in such sands one
finds pyrite (Fe(II)S), siderite (Fe(II)CO3),
and magnetite (Fe3O4,
Other metal oxides and sulfides in low oxidation states are common.
Ancient Precambrian sediments containing sands with
such reduced minerals have been found in Canada, Brazil, and South
Africa. These deposits have been studied intensively because the
reduced materials often include elemental gold and uranium ore.
Geologists have concluded that these are the remains
of ancient sedimentary beds laid down under reducing atmospheric
conditions. Radioisotopic methods have dated these various beds
from 1.8 billion to 3.0 billion years old. The Earth is 4.5 billion
years old, so for the first 2.5 billion years of its existence,
it had a reducing atmosphere like the other planets.
Additional evidence comes from banded iron ore deposits with mixed
oxidation states, found in Minnesota, Finland, Russia, South Africa,
India, and Australia.
These deposits are thought to have been laid down under reducing
conditions, although the evidence is not as conclusive. These banded
iron beds are 1.8 to 2.5 billion years old. In contrast, "red
beds" of fully oxidized hematite ore (Fe203)
are never dated earlier than 1.4 billion years ago.
The conclusion from these iron deposits is that the atmosphere was
predominately reducing prior to 1.8 billion years ago, has been
oxidizing for the past 1.4 billion years, and underwent a period
of gradual transition in the time between.
This is not to imply that oxygen suddenly appeared in the atmosphere
1.8 billion years ago, or that water-using photosynthesis was invented
Photosynthesis on a small scale probably appeared nearly a billion
It is difficult to calculate how fast O2
would accumulate in the atmosphere, or how great the O2
concentration would have to be before it could begin to influence
the oxidation state of iron minerals in sediments. All we can say
is that by the time this had begun, the O2
concentration must have been appreciable.
So the stage was set for an Oparin-Haldane type of evolution of
life in a reducing atmosphere.
But did the actors really appear on cue?
For this, we must turn to the fossil evidence.