[Sorry it took me so long to respond.]
"There has been a significant literature that has attempted to find
evidences of a reducing atmosphere in the data obtained from BIFs, from
uraninite and from various other sources. None of these are now acceptable
interpretations."
There is a difference between trying to find out if the reducing atmosphere
lasted much beyond four billion years ago and suggesting that it did. You
did not provide any examples to back up your claim, and the above
information confirms rather than refutes the claims of Mason and others.
"Because of the tremendous number of posts, I was reading selectively. I am
unable to read the excerpts from Mason - and would appreciate receiving this
again. I am sufficiently confident of what I am saying to stick with the
comment I made above."
Here it is.
==========begin excerpt
Here are some contemporary (with your Miller paper) statements regarding
this issue from the other side of the fence [Stephen F. Mason, _Chemical
Evolution_, Oxford:Clarendon Press, 1991, pg. 111-114]:
"The inner planets, formed from volatile-depleted planetesimals, lacked any
significant primitive atmosphere. Gases and volatile elements were occluded
or chemically combined in the planetesimals, as they are in recovered
meteorites, and the thermal processing of the planet released noble gas
atoms and molecular gases to form a secondary atmosphere....Soon after the
accretion of the terrestrial planets, it is probable that their surfaces
were covered by magma oceans, possibly 200 km deep in the case of the Earth,
due to the heat liberated during accretion and core formation....Intense
volcanic activity during crust formation led to the degassing of occluded
and chemically combined volatiles, forming the early secondary atmosphere.
Predominantly reducing gases are obtained by heating up to 1500 K meteoritic
material modelling the terrestrial planetesimal composition (98 per cent of
high-iron chondrite and 2 per cent of C1 chondrite). The main gases
liberated up to about 1000 K are, in order of decreasing abundance, methane,
nitrogen, hydrogen, ammonia and water vapour, with minor amounts of hydrogen
sulphide, carbon monoxide, and carbon dioxide. With full outgassing over
the 1000--1500 K range, nitrogen and ammonia drop to the level of minor
constituents and comparable amounts of methane, carbon monoxide, carbon
dioxide, water vapour, and hydrogen are released (Lewis and Prinn 1984
_Planets and their atmospheres: Origin and evolution_ Academic Press,
London).
"The lifetime of a reducing atmosphere on the terrestrial planets is
expected to be short, owing to the photolysis of the hydrides by the solar
ultraviolet radiation and the escape of hydrogen atoms from the
gravitational field of the planet in its upper atmosphere. The planetary
atmosphere then becomes neutral, composed mainly of carbon dioxide and
nitrogen, as observed for Venus and Mars at present....On the Earth
uniquely, temperature conditions over much of the surface have been
restricted to the liquid range of water over the past 4 billion years or so,
thereby facilitating the fixation of atmospheric carbon dioxide. First, in
the weathering reaction, aqueous bicarbonate attacked silicate minerals to
liberate silica and form carbonate sediments, which date back to about 3.8
million years ago. Second, photosynthetic organisms reduced dissolved
carbon dioxide with hydrogen split from water, liberating the molecular
oxygen produced into the atmosphere at significant partial pressures from
about 2 billion years ago.
"It has been argued that the atmosphere of the Earth was never reducing and
that it has evolved continuously over the whole history of the Earth by the
volcanic emission of gases from the mantle throuhgh the crust. The view
rests on the nineteenth-century uniformitarian principle that the history of
the Earth must be expained only in terms of the geological forces observed
in operation today. The principle components of present-day volcanic gases
are steam and carbon dioxide, which are partially soluble in silicate melts.
At 30 kbar pressure and 1625 C melts of diopside (CaMgSi2O6) dissolve up to
24 per cent of water and 5 per cent of carbon dioxide by weight (Holland
1984 _The chemical evolution of the atmosphere and oceans_ Princeton
University Press). The gradual release of such quantities of water and
carbon dioxide from the mantle of the Earth by volcanic action over 4
billion years suffices to account for the present volume of the oceans and
the mass of the carbonate deposits. But the argon isotope ratios indicate
that the Earth was over 75% degassed by 4 billion years ago. The
accumulation of [Ar-40] in the mantle from the radioactive decay of [K-40]
now gives a mantle ratio of [Ar-40]/[Ar-36] > [10^4], estimated from the
gases entrained in the volcanic lavas at the mid-ocean ridges. The
atmospheric ratio of [Ar-40]/[Ar-36] is only 295.5, corresponding to the
mantle value more than 4 billion years ago when the major degassing of the
Earth must have occurred (Ozima 1987 _Geohistory: Global evolution of the
earth_ Springer, Berlin)."
To summarize Mason, from 4.5 to 4 billion years ago, 75% of the gases
trapped in the crust were released, including large amounts of methane,
ammonia, hydrogen, nitrogen and water, exactly the kind of reducing
components "considerable opinion" claims were never present. (Evidence:
same gases in same percentages are obtained when planetesimal meteorites are
heated.) Then, by 4 billion years ago, the outgassing had all but been
exhausted, allowing for a switch in gas release from reducing to neutral.
This information supports both Henderson-Sellers and Chang, but does not
support your implied conclusion that the atmosphere was always neutral from
Day One. In fact, atmospheric argon ratios support the idea that by 3.8
billion years ago nearly all the gases that would ever be released had been,
which means that from then until now continued outgassing has largely been
negligable. It also means that reducing gases would have been a major
constituent of the primeval atmosphere until outgassing ceased, at which
point they would quickly disappear. In any event, this establishes that for
the first 500 to 700 million years, there was probably enough reducing gases
to make all the amino acids needed for later proteinoid formation.
==========end excerpt
"Maybe I do need such a refresher course. However, I am of the opinion that
it is not infrequent for people to confuse theory with data."
Yes; lay people often do, but not working scientists.
"Note that I did not say 'theory can exist in the absence of data', but that
this is a case where 'theory' is pretending to be data."
If you are suggesting that some scientist takes a theory that is supported
by evidence and uses it to support a theory that IS NOT supported by
evidence, then you are saying that "theory can exist in the absence of
data". Scientists do not use theory as a substitute for data, though at
times it may appear so to non-scientists. Just because one doesn't know
what the data is doesn't mean it doesn't exist.
"I suppose the test of this argument is to be explicit as to what data
supports the existence of a reducing atmosphere on earth. The existence of
reducing atmospheres in the outer planets is data,..."
Which was used to establish the POSSIBILITY that the earth had a reducing
atmosphere, not to prove that it did.
"...but it can only be regarded as data supporting this particular thesis by
the imposition of an interpretative model of planetary origins - which
itself needs to be tested and validated."
If you are referring to the planetesimal accretion theory, it has been
tested and verified, at least as far as it can be until we can directly
analyze the chemical and mineral composition of comets and asteroids.
"More importantly, where are the geological evidences for a reducing
atmosphere on Earth?"
Mason above reports one piece of information, that of meteoritic
composition. Here is a reply I made to Brian Harper that contains more:
"We know the mantle is mildly reducing (it still outgasses a ratio of
methane to carbon dioxide of about 1% [JA Welhan, Chem. Geol. 71, 183
(1988)] from hydrothermal vents) and that it was more strongly reducing in
the past based on the thermodynamic analyses of diamond inclusions [JF
Kasting, DH Eggler, SP Raeburn, J. Geol. 101, 245 (1993)] and studies of
metal-silicate partition coefficients of siderophile elements [MJ Walter and
Y Thibault, Science 270, 1186 (1995); K Righter and MJ Drake, Earth Planet
Sci. Lett. 146, 541 (1997)]. We also know that the mantle had degassed over
75% of its volatile gases by 4 billion years ago, based on the fact that the
atmospheric Ar-40/Ar-36 ratio is only 295.5, which is what the mantle ratio
would have been 4 billion years (the current mantle ratio is greater than
10,000 [M Ozima, Geohistory: Global evolution of the earth, Springer,
Berlin (1987)]). Since it is known that the earth was created by accreting
planetesimals, in order to make an earth with its known chemical composition
the planetesimals would have had to have been very similar to meteorites
composed of 98% high-iron chondrite (stone) and 2% C1 (carbonaceous)
chondrite (20% water, 3-5% C). Since these kinds of meteorites have a
direct 1:1 relation between the relative abundances of their less-volatile
elements and the corresponding abundances of these elements in the solar
atmosphere, they are believed to be the most primitive material known in the
solar system and to have condensed directly out of the solar nebula. If you
heat these meteorites up to 1500 K you get predominantly reducing gases. If
the earth was made from these planetesimals and heated until the surface was
molten, then degassed over 75% of its trapped volatiles gases by 4 billion
years from a strongly reducing mantle, then you will get a reducing
atmosphere. This is not speculation, this is extrapolation based on known
evidence using deductive reasoning."
Let me know if you have other concerns.
Kevin L. O'Brien