There is substantive evidence that the earth never did have a reducing or
neutral atmosphere."
Thanks for the reference; I've downloaded and read the abstract, and I will
obtain a copy of the paper as soon as possible.
However, in my opinion the abstract does not support your claim that there
has never been a reducing or neutral atmosphere. The data reported only
goes back to 3 billion years, whereas the information from Mason that I
reported in that very same post, in the section of the post you cut out,
establishes that the earth had a strongly reducing atmosphere between 4.5
and 4 billion years ago, followed by a weakly reducing atmosphere
progressing to a neutral atmosphere between 4 and 3 billions years ago.
Abiogenesis would have been completed by 3.5 billion years ago (according to
the fossil record), with methanogen bacteria predominating up until roughly
2 billion years ago, when photosynthesis kicked in. So in fact this
information poses no real threat to abiogenesis, not even the Miller-Urey
portion of it.
I would also object to calling an atmosphere with a minimum 1.5% oxygen an
oxidizing atmosphere, considering that the other 95% (at least) was still
carbon dioxide; at best it should be called a very weakly oxidizing
atmosphere.
"This conclusion delivers the final blow to those wishing the Precambrian
atmosphere to be reducing or neutral throughout most or all of the
Precambrian."
Yes, well, I'm not aware that anyone seriously stated that the earth had to
have a reducing atmosphere as late as 3 billion years, and I would not call
the atmosphere oxidizing until the percentage of oxygen had reached at least
10%. Still, who am I to object to a little hyperbole.
"Miller-Urey experiments have at least 2 orders of magnitude smaller yields
of amino acids in neutral and mildly reducing atmospheres,..."
Brian Harper made much the same claim, and in my reply I demonstrated he was
incorrect by giving a reference for and reproducing the abstract of a paper
co-authored by Miller. The data he presents in the paper shows that his
method can create roughly equal masses (2% by weight) of amino acids under
mildly reducing conditions (H to C ratio of 2 to 4, with CO or CO2 as the
predominant C source) as was produced (2-4%) under strongly reducing
conditions (H to C ratio of 4, with CH4 as the predominant C source), though
the variety is restricted to glycine. Under neutral conditions (H to C
ratio of 0, with CO2 as the predominant C source) production drops off
dramatically (0.001%), but as I have already pointed out, by this time
abiogenesis is virtually complete.
Good data, Art, but it occurs too late to have any significant affect on
abiogenesis.
Kevin L. O'Brien