Geoscience Reference
In-Depth Information
domain in which this principle is worth questioning, it is for the study of the distant past
for which observations bear testimony that many geological phenomena were different in
the Archean and the Proterozoic. Probably one of the changes that so uniquely affected the
surface of the Earth and allowed terrestrial life to thrive was the increase of atmospheric
oxygen pressure. Overall, the modern chain of events is as follows: (i) photosynthesis
dissociates carbon dioxide to produce reduced biological carbon, (ii) burial and subduction
of biological matter remove reduced carbon to the mantle, and (iii) the residual oxygen
builds up in the atmosphere. To this should be added the dissociation of atmospheric water
by solar ultraviolet radiation in the upper atmosphere: loss of hydrogen, too light to be held
back by terrestrial gravitation, contributes to the increase of atmospheric oxygen. We will
return to this process in
Chapter 12
.
The modern biological processes by which oxygen
is released into the atmosphere probably appeared with cyanobacteria (now fossilized as
stromatolites) before 3.0 Ga ago. The atmosphere before that time was characterized by a
low level of oxygen as attested to by three observations:
1. Banded iron formations (BIF) are known from Archean terranes of all continents. The
archetypal 2.5 Ga old Hamersley formation in Australia is one of the largest iron
deposits in the world. These are sediments made of mm-thick laminae of quartz and
magnetite Fe
3
O
4
, which can be traced over very long distances and record some dis-
tinctive astronomical cycles. Most of these rocks are devoid of detrital minerals, which
also hints at a pelagic depositional environment. As Fe(OH)
3
is essentially insoluble in
oxic waters, it has been suggested that the deep ocean was rich in soluble Fe
2
+
and sil-
ica, most likely introduced by submarine geothermal activity, and that locally oxidizing
conditions, possibly biologically mediated, would lead to massive precipitation of iron
hydroxides and silica (Holland,
1973
). Banded iron formations are rare after 1.8 Ga but
make a remarkable comeback in the late Proterozoic (
600-800 Ma).
2. Paleosols, 2.2 to 2.4 Ga old, which are rich in chlorite, sericite (a mica similar to mus-
covite produced by weathering), and quartz are known in Canada and South Africa.
They show a characteristic loss of iron unknown in modern soils, which demonstrates
that groundwater was oxygen depleted and that Fe was in its soluble ferrous form.
3. Minerals prone to atmospheric oxidation and subsequent dissolution, most conspicu-
ously uraninite UO
2
, are essentially absent from detrital rocks. Large uraninite deposits,
however, are common in Archean rocks such as the 3.0-Ga-old Witwatersrand conglom-
erates (South Africa). They demonstrate that the atmospheric oxygen level must have
been low.
≈
For reasons which still remain to be explained, these features vanished by 2.1-1.9 Ga; this
is broadly interpreted as indicating an abrupt rise in atmospheric oxygen pressure. This
idea recently received strong support from sulfur isotope analyses in Archean sediments.
Mass fractionation of the 32, 33, 34, and 36 isotopes of sulfur in recent rocks back to 2.1
Ga is perfectly mass-dependent, which means that, for example, variations of the
34
S/
32
S
ratio are twice the variations of
33
S/
32
S. In other words, the quantity
33
S calculated as
33
S
33
S
34
S
=
δ
−
0.5
δ
(9.1)
