"According to Pflug(**) there are banded iron formations in the Isua rocks
of Greenland, 3.8 billion years old."
I've been looking for a reference about that; thanks! However, it should be
pointed out that iron in the +2 state can photoxidize in the presence of
water vapor. Also, oxygen released by the photolysis of water into hydrogen
and oxygen could provide enough to augment photoxidation.
What's interesting is that if the date for these formations are accurate
(and I have no reason to doubt it), then it suggests that it only took about
200 million years after outgassing began to drop off for enough of the
reducing gases to be photolysized, photoxidized or abiogenetically converted
to biomolecules to change the atmosphere enough to permit oxygen build-up.
Of course, reaction with iron would sop up much of what was being produced,
keeping the amount in the atmosphere extremely low until something cut off
the process. Or maybe it was this process that kept the oxygen levels at a
minimum of 1.5% instead of allowing them to steadily increase, thus
preserving early methanogenic life? Ohmoto's paper would seem to support
this.
"Begging the question. Reminds me of the following excercise in logic:"
Actually, that statement was good logic. What you object to is the
assumption that the premises that the early atmosphere had no oxygen and had
reducing gases are factually true. In fact they may not be, but that only
renders the statement factually incorrect, not illogical. However, I would
concede that being factually incorrect is more important than being logical
in science.
Kevin L. O'Brien