Geology Reference
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and would have had a dramatic short-term effect on
atmosphere composition:
by-product of the photosynthesis of carbohydrate from
carbon dioxide and water (Equation 9.3). It is likely
that photosynthesis in some form began around 3.5 Ga
BP
16
- perhaps even earlier - but it is clear that
oxygenic
photosynthesis was well established in cyanobacteria
by 2.7 Ga BP. Marine biogenic oxygen could not acc-
umulate in the atmosphere immediately, however,
because the oceans had built up through weathering a
legacy of reduced solutes, notably ferrous iron, Fe
2+
.
Until completely oxidized, this 'oxygen sink' would
have mopped up free oxygen in the oceans as it was
produced, preventing its escape into the atmosphere.
In sediments of the Archaean eon, iron occurs chiefly
in
banded iron formation
(BIF), the thin iron-oxide-rich
layers of which were precipitated from seawater
through the oxidation of dissolved Fe
2+
to insoluble
Fe
3+
(see Figure 11.8b). Each layer may represent a sin-
gle short-lived blooming of oxygenic bacteria in an
otherwise anoxic ocean, repeated countless times
through cyclic fluctuation in bacterial populations or
Fe
2+
supply. Delivery of abundant dissolved Fe
2+
to the
oceans depended, of course, on an atmosphere con-
taining negligible oxygen, an inference that is sup-
ported by the MIF-S record in sulfur isotopes shown in
Figure 10.14.
In post-Archaean sediments, in contrast, iron occurs
mainly as rusty diagenetic coatings on detrital grains
in reddish sandstones or shales ('red beds'), suggest-
ing that iron released by weathering was being oxid-
ized subaerially to ferric iron even before it reached
the sea. This marked change in the predominant form
of iron-bearing sediment, which occurred between 2.4
and 1.8 Ga (Figure 11.8a), is therefore taken to mark the
first sustained appearance of free oxygen in the Earth's
atmosphere, although its abundance - even after this
'Great Oxidation Event' ('GOE' in Figure 11.8a) - still
fell well below the current level of 21 vol.%. The period
from 2.4 to 2.3 Ga brought a number of worldwide
glaciations, as the surge in oxygenic photosynthesis
drew down atmospheric CO
2
and diminished its key
contribution to greenhouse warming.
Why does Figure 11.8a show a brief reversion to
BIF deposition in mid- to late-Neoproterozoic times?
This period, from 750 to 570 Ma, was, like the early
Proterozoic, characterized by intense Snowball Earth
(global glaciation) episodes and wild fluctuations in
For a thousand years … silicate clouds defined the visible face of the
planet. The Earth might have looked something like a small star or
a fiery Jupiter wrapped in incandescent clouds.
(Zahnle
et al
.,
2007)
The outcome, as the magma ocean beneath cooled, is
thought to have been an atmosphere consisting of H
2
O,
CO
2
, CO and H
2
in that order. Again O
2
is notable by its
absence.
The subsequent profound transformation of this
post-impact
14
anoxic atmosphere into the oxygen-rich
atmosphere we depend upon today could not have
occurred without the appearance of life around 3.5 Ga
ago (possibly even earlier). There is geological evid-
ence for the existence of liquid water on the surface of
the Earth since at least 3.8 Ga ago, and some models
place its first appearance much earlier (Zahnle
et al
.,
2007). It was in Earth's oceans that life began its com-
plex journey.
Life and oxygenic photosynthesis
Life on Earth relies on a remarkable astrophysical
coincidence. The Earth orbits the Sun at a distance
lying within the Sun's
habitable zone
,
15
the range of
orbital radius within which planets with atmospheres
enjoy surface temperatures that allow water to exist in
the liquid state. Were the Earth much closer to the Sun,
its surface temperature would boil water (as on Venus),
whereas if it were further away - like present-day
Mars - any surface water could exist only as ice; either
departure would make the huge diversity of life found
on Earth today unsustainable.
The ultimate origins of life remain shrouded in mys-
tery. The earliest organisms must have thrived in the
Earth's early anoxic atmosphere, but they included
some - similar to today's
cyanobacteria
('blue-green'
algae) - that produced oxygen in the oceans as a
14
The resemblance of the early Earth's surface conditions to a
vision of hell led to the name
Hadean
(after
Hades
, the Greek
god of the underworld) being coined for this earliest, pre-
Archaean eon of Earth's history.
15
The habitable zone is sometimes dubbed the 'Goldilocks
zone', signifying that conditions are - like porridge ready for
eating! - 'neither too hot, nor too cold, but just right'.
16
BP, before present.
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