While
not always recognized as such, Banded Iron Formations (BIFs)
are another form of stromatolites. BIFs are massive, laterally
extensive and globally distributed chemical sediment deposits
that consist primarily of Fe-bearing minerals (iron oxides)
and silica. Iron can occur naturally in two states. Reduced,
or ferric iron is soluble in water. Archaean oceans were rich
in ferric iron that was released from Earth's interior. In
the presence of oxygen, however, the iron becomes oxidized
and precipitates out as a solid. Earth's BIFs are the result
of oxygen released by photosynthetic organisms combining with
dissolved iron in Earth's oceans to form insoluble iron oxides
-- this is known as "the great rusting of the earth".
Banding results from cyclicality in oxygen production. Earth
started out with vast amounts of iron dissolved in the world's
seas. BIFs in the geologic record date from about 3.8 billion
years ago in Greenland to about 1.8 billion years ago with
a maximum abundance at some 2.5 billion years ago, with another
large reoccurrence in Neoproterozoic time (from about 0.8 and
0.6 billion years ago. Interestingly, it has been estimated
that the amount of oxygen locked in earth's BIFs is some 10
times the amount contained in the atmosphere.
One
of the most famous banded iron comes from the Western Australia.
With fine
polishing, the beauty of stunning testament to one
of the most important events in geological history -- the Earth
needed to first rust, so that the atmosphere could then be
oxygenated, setting the stage for the appearance and evolution
of all eukaryotic
life. |