Ask any student of historical geology what the Earth's atmosphere
was like in the Precambrian and you will get some fairly
predictable responses. Oxygen was "definitely" not as plentiful
as today because, it is said, the Earth's biosphere (which
maintains the oxygen cycle) was nothing like that of today. In
the Precambrian, plants which consumed carbon dioxide and
released oxygen were represented only by single celled algae.
The earlier you look in the Precambrian, the less evidence there
is for free oxygen. In the Archaean, pre about two thousand five
hundred million years ago (pre 2.5 Ga), it has been said that the
atmosphere was either neutral (a mixture of nitrogen, carbon
dioxide, water vapour and perhaps a little hydrogen) or reducing
(with ammonia, methane, carbon dioxide, water vapour and perhaps
hydrogen). The reducing atmosphere is thought to be the original
Earth's atmosphere by analogy: the outer planets (Jupiter,
Saturn, Uranus, Neptune) have reducing atmospheres and this has
long been thought also to represent the "original" state of the
Earth's atmosphere.
This "evolution of the atmosphere" scenario is of great
importance for theories of abiogenesis. Miller, in his classic
(1951) experiments, produced amino acids, "the building blocks
of life", using a reducing atmosphere to provide the raw
materials. Miller's experiments continue to be cited because
there is no other viable source of amino acids from which to
construct proteins and other complex organic chemicals. However,
a neutral atmosphere results in severely impoverished reaction
products, whereas an oxidising atmosphere has no amino acid
production of any significance.
Overviews of research on the Earth's early atmosphere have been
published at regular intervals. Direct evidences of a reducing
atmosphere have been claimed in the past, but these evidences are
no longer regarded as conclusive. There is a growing consensus
that in the earliest period for which data is available, the
atmosphere was neutral, with negligible amounts of free oxygen.
Such is the conclusion, for example, of Kasting (1993). Prior
to 2.0 Ga, although there is no valid evidence for a reducing
atmosphere, whatever oxygen was around is believed to have been
consumed in the oxidation of other materials: organic matter,
iron bearing minerals and volcanic gases. Estimates of oxygen
levels are about 10-13 of the present atmospheric levels (PAL).
One of the best evidences for the low levels of oxygen (giving
a neutral atmosphere) comes from the loss of iron from pre-2.2
Ga paleosols (fossil soils). After about 2.0 Ga, oxygen levels
increased significantly to about 1.5% of PAL oxygen. So the
consensus in the early 1990s has been:
Early Archaean (pre 3.0 Ga) Reducing atmosphere (but
unsupported by data)
Late Archaean (3.0 - 2.5 Ga) Neutral atmosphere with some free
oxygen
Proterozoic (2.5 - 0.6 Ga) Oxidising atmosphere with about
1.5% PAL oxygen
In a major reexamination of the paleosol evidence. Ohmoto (1996)
has effectively challenged the concept of a neutral atmosphere.
He argues that the minimum oxygen pressure for the 3.0 - 2.2 Ga
(for which paleosol data is available) is about 1.5% of the
present level. We now turn to look more closely at the new data
and Ohmoto's analysis.
The arguments are based on the occurrence of compounds of iron
in certain sedimentary rocks. Iron in the ferrous state (Fe2+)
can dissolve relatively easily in oxygen-free water, but is
converted to the insoluble ferric state (Fe3+) in an oxidising
environment. Previous studies of certain Precambrian rocks
identified as weathered horizons (paleosols) have suggested a
general loss of iron, which has been interpreted as evidence for
either a neutral or a reducing atmosphere.
Ohmoto's research was stimulated by some apparent anomalies in
the conventional analysis. He found that not all paleosol
sections of >2.2 Ga showed iron loss. Even in sections that did
show Fe loss, only a minority of samples were depleted in iron.
Furthermore, many of the post 2.2 Ga paleosols had lost iron (and
in such cases, an atmosphere with some free oxygen is accepted).
To resolve the numerous questions raised by this data, Ohmoto
gathered new data involving more detailed measurements of atomic
ratios. This enabled him to obtain a chemical signature which
could be associated with weathering in reducing/oxidising/other
environments.
Problems with the conventional interpretation
Theoretically, the lower the level of oxygen ions in water (and
the greater the level of hydrogen ions), the more iron can go
into solution. However, the rates of dissolution have to be
assessed experimentally. It is found that the reactions for Fe3+
compounds proceed very slowly. The prediction is that Fe2+ will
be lost more readily than Fe3+. Using Titanium as a `standard'
immobile element, the prediction is that a "reduced"-type (R-
type) paleosol will have significant reductions in the ratio
Fe2+/Ti but little or no decrease in Fe3+/Ti. According to
Ohmoto, none of the paleosol sections examined yielded this
characteristic. Thus, there are no paleosols that support the
idea that the earth's early atmosphere was reducing (or neutral,
for the same reasons).
A new interpretation of the paleosols
Ohmoto's research allowed him to classify the observed paleosols
according to their isotopic characteristics. The details of this
need not concern us in this review, but the conclusions are of
considerable interest.
(a) Oxidised (O-type) paleosols The four isotopic trends
characterising the formation of soils under oxic conditions today
are retained in the Precambrian paleosols.
(b) Hydrothermally-altered (H-type) paleosols Isotopic trends
in these cases suggest that these rock sections (and possibly
other paleosols) are not paleosols at all, but hydrothermally-
altered rocks. Fluids carrying ions have passed through these
rocks and imparted a chemical signature to them.
(c) Mixed processes (M-type) paleosols These rocks have isotopic
fingerprints which can be related to both O-type and H-type
paleosols. Ohmoto says: "These characteristics suggest that M-
type paleosols formed under an oxic atmosphere, but their
Fe[iron] chemistry was modified during and/or after soil
formation." Ohmoto also notes that M-type paleosols are the most
common of the three types.
Implications of the new interpretation
Having discussed the three types of paleosol observed and
possible modes of formation, Ohmoto concludes that the minimum
pressure of atmospheric oxygen consistent with the data is
greater than 1.5% PAL for the entire period of 3.0 - 1.8 Ga.
This new analysis puts increasing pressure on all "reducing
atmosphere" interpretations of the Earth's early atmosphere.
There is no observed trend of reducing -> neutral -> oxidising.
As far as data is concerned, the Earth's atmosphere has always
been oxidising. Theories of abiogenesis which require a reducing
atmosphere are pushed further into a realm of speculation
supported by theoretical models but not by empirical data.
An atmosphere with free oxygen points to the contemporaneity of
plants which photosynthesise. However, to date, studies of
organic life in the Archaean have suggested the existence of only
bacteria and single-celled algae. But this evidence is not
plentiful. Even the growth mounds, the Precambrian
stromatolites, now appears to be better explained as having an
abiotic origin (Grotzinger & Rothman, 1996). This meagre fossil
evidence has allowed the earlier postulate of a neutral
atmosphere during the later Archaean. With the new evidence of
a significantly oxygenated atmosphere, it may be inferred that
more plant life was around than we have direct evidence for.
Ohmoto says: "Terrestrial biomass on the early continents may
have been more extensive than previously recognised". Following
through this thought leads to a number of interesting
possibilities for further research: (i) with more rigorous
investigation, will body fossils of this more extensive biomass
be found? (ii) were post-deposition processes less conducive to
fossil formation in the PreCambrian than in the Phanerozoic?
(iii) were sedimentary processes less conducive to fossil
formation in the PreCambrian than in the Phanerozoic?
References:
Grotzinger, J.P. & Rothman, D.H. 1996. An abiotic model for
stromatolite morphogenesis, Nature, 383(3 October), 423-425.
Kasting, J.F. 1993. Earth's early atmosphere. Science, 259,
920-926.
Ohmoto, H. 1996. Evidence in pre-2.2 Ga paleosols for the early
evolution of atmospheric oxygen and terrestrial biota. Geology,
24(12), 1135-1138.
Best wishes,
David J. Tyler.