Geoscience Reference
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the ocean, even if the continents are now largely colonized by plants.
This condition is explained in the paragraph below.
The oxygen produced by photosynthesis is, as we have already
seen, highly unstable. It therefore reacts immediately with numerous
compounds or chemical elements present in the primitive anoxic
environment. It reacts, for example, with sulfurs (S 2- ) to produce
sulfates (SO 4 2- ). We note in passing that it thus recycles the oxidizing
compounds necessary for certain anaerobic (sulfate-reducing) cells to
fulfil their energy needs, since these “respire” sulfates by transforming
them into sulfurs. In consequence, without the oxygen issuing from
anaerobic photosynthesis, any aerobic respiration would not have been
able to maintain itself or to develop in the long-term in the terrestrial
environment. A massive and non-localized source of external energy
was necessary, which only solar rays could furnish via photosynthesis.
However, O 2 also reacts with CH 2 O to form the more stably
composed CO 2 , releasing a small amount of heat as it does
so. This is expressed by a formula strictly inverse to photosynthesis,
where only the form of the energy is modified but not its quantity. The
following formula summarizes the oxidation of organic matter:
CH 2 O + O 2 → H 2 O + CO 2 + heat
[1.2]
For an excess of O 2 to accumulate in the environment, it is thus
necessary that the availability of its chemical partners, (whether sulfur
or organic matter, but also oxidizable ions in solution such as Fe 2+
(which is transformed into solid iron oxide Fe 2 O 3 )), is inferior to the
flux of oxygen produced by photosynthesis.
The precipitation of solid oxides did, certainly, consume some of
the chemical prey present in the ancient oceans. We can observe traces
of it within certain sedimentary accumulations of the Archean Period,
for example in the banded iron formations at Barberton in South
Africa dated around 3.2 to 3.5 billion years ago [HOF 05].
Nevertheless, the key to the accumulation and maintenance in the
long-term of an excess of O 2 in the environment, up until to today, is
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