Geoscience Reference
In-Depth Information
found in the fact that a proportion of the organic matter produced by
photosynthesis is constantly transferred into oceanic sediments and,
thus, prevented from oxidation by O
2
.
H
2
O + CO
2
+ light → sedimentary CH
2
O + O
2
in the environment [1.3]
Even if this sedimentary flux of organic matter is very small
relative to the flux exchanged during photosynthesis and the oxidation
of organic matter, its accumulation over long periods of time has
enabled the construction of important stocks. The O
2
oxygen in our
environment can thus be considered as the counterpart of the global
stock of organic sedimentary matter, the living biomass being
quantitatively negligible in relation to it.
1.3.3.
The first important accumulation of oxygen (around 2.5
billion years ago)
Although sedimentary archives stretching back several billion
years are rare and fragmented, it has been possible to show that a first
important global accumulation of oxygen was produced around 2.5
billion years ago [CAT 05, VOE 10]. This accumulation increased the
partial pressure of O
2
(
p
O
2
) at ground level from around 10
-5
bar to
10
-1.5
(= 0.03) bar [CAT 05], in equilibrium with the oxygen dissolved
in the ocean's surface. This first oxygenation only represented around
15% of its total transformation into the
p
O
2
, of which the current
atmosphere is 0.21 bar. It nevertheless considerably modified the
course of biological evolution just as that of the evolution of the Earth
system.
It is not possible to confirm if this first accumulation was
principally linked to aerobic or anaerobic photosynthesis, or both. A
local and temporary oxygenation of the medium is undoubtedly a
prerequisite produced where, in a generally anoxic world,
photosynthetic marine ecosystems were the most productive (in certain
seasons). Such fluctuations created toxic environments for the
photosynthetic cells, since they were initially anaerobic; they poisoned
themselves.
