Geoscience Reference
In-Depth Information
(permanganate MnO
4
-
, dichromate Cr
2
O
7
2-
) or ozone (O
3
), have a
greater oxidizing power.
Because of this property, the gaseous form of oxygen, the form that
nevertheless composes 21% of our current atmosphere, is extremely
unstable. In this form, oxygen has a tendency to react with the majority
of bodies (gas, liquids or solids) with which it comes into contact, to
form oxides while
releasing energy. Combustion is an accelerated
manifestation of oxidation, maintained
by the large heat flux it gives off
(see also Chapter 4 of this topic, and Chapter 2 of [MON 14b].
Consequently, the massive presence of O
2
in our environment can
only be explained by a shortage of potential chemical partners and/or
by a permanent production of O
2
stimulated by an energy supply. As
we will describe, these two conditions have almost always coexisted
in the history of the Earth, even if the availability and nature of the
chemical partners have varied in the course of time. If the atmosphere
and, later, the continental surfaces, have been important players in this
history, it is in the ocean that it essentially occurred.
The Earth's primitive environment was strictly anoxic, i.e. O
2
-free
[HOL 84, SCH 10]. Even supposing that some rare abiotic processes
were locally susceptible to form O
2
, this would immediately consume
the poorly oxidized (reduced) compounds that surrounded it and
would immediately disappear, in this process of chemical “predation”.
Starting from the first living cells, the most efficient metabolic
mechanisms, at first anoxic, were selected to supply to these cells the
energy and matter that they needed. Some of them were anaerobic
mechanisms for photosynthesis. They allowed the cell to use, at least
partially, solar energy to carry out the synthesis of organic molecules
necessary for maintaining cellular machinery just as for its
development and duplication. The cells in possession of this new
mechanism gained the opportunity to free themselves from the need to
capture and digest, through respiration or anaerobic fermentation,
other cells or organic molecules available in their environment
(heterotrophy). This evolutionary development was likely selected for
