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(mostly hematite) alternating with
bands of silica, often in the form of
chert. BIFs are characteristic of the
Gondwana continents and (with only
a few exceptions) are restricted to quite
a narrow period of time within the
early Precambrian. There is a definite
peak of BIFs at about 2.5 billion years,
but there is also quite a sharp line
defining their sudden increase (at about
2.8 billion years) and dramatic demise
(a little after 2 billion years). These
peculiar formations, which are almost
never seen again in the geological record,
tell us something about the composition
of Earth's early atmosphere and oceans.
Earth's early atmosphere was almost
devoid of oxygen (well under 1%). The
first bacteria to inhabit the planet were
thus anaerobic (i.e., they were able to live
in the absence of oxygen); in fact, oxygen
would have been lethal to them. However,
some types of bacteria (including the
cyanobacteria) were photosynthesizing
and constantly producing oxygen as a
waste product. In order for them to
survive there had to be some mechanism
which would rid the system of this poison.
One direct result of the paucity of oxygen
in the atmosphere was that the Earth's
early oceans were rich in dissolved iron in
its unoxidized ferrous state. This was the
key to the disposal of the oxygen produced
by photosynthesis. Oxygen is a very
reactive element and as soon as it was
released by photosynthesis it reacted with
the ferrous iron to form iron oxide, which
being insoluble was deposited on the
ocean floor - the oceans were turning
to rust!
At first this only happened slowly - the
rate at which the photosynthesizing
bacteria could increase was limited by
the speed at which their waste oxygen
could be processed into ferric iron.
However, at some point in time the
photosynthesizing bacteria evolved to
become aerobic - suddenly they could
live happily in the presence of oxygen
and there was no longer any restriction
on their increase. A population explosion
of photosynthesizing bacteria occurred,
producing huge amounts of oxygen,
which flushed the ferrous iron out of
solution, producing vast amounts of
iron oxide deposited on the seabed;
hence the sudden increase in BIFs
around 2.8 billion years ago. BIFs
persisted until all the available ferrous
iron dissolved in the oceans had been
used up (sometime after 2 billion years
ago), explaining their equally sudden
14
14 The large, spherical
bacteria Leptoteichoswith
possible internal nucleus. PC.
Up to 31 micron diameter.
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