"Jim Menegay"  wrote in message
news:c7jqhv$19cc$1{at}darwin.ediacara.org...
> "irr"  wrote in message
news:...
> >  wrote in message
> > news:c75ot3$2q6a$1{at}darwin.ediacara.org...
> > >
http://www.google.com/groups?selm=bvksnf%241pli%241%40darwin.ediacara.org
>
> > > > From: jamenegay{at}ra.rockwell.com (Jim Menegay)
> > > > We know that Fe was the main final electron donor
around 2.3Gy ago -
> > > > hence the red beds.
> > >
> > > Different from banded red iron layers, or basically part of same?
> >
> > Same idea (oxidizing iron), but red beds are oxidized, iron rich ancient
> > soils and are different from banded iron formations.
> >
> > >
> > > > Perhaps the Fe was reacting with biogenic O2 or
whatever.  But I'm
> > > > guessing that it was oxidized directly by organisms.
> > >
> > > Well the banded layers we caused by alternating surplus and lack of
> > > free O2 to convert Fe++ to Fe+++, the latter which precipates out. I'm
> > > not sure whether the theory is that the oxygen from photosynthesis
> > > dissolved in the ocean water and freely reacted with Fe++, or whether
> > > Fe++ was used within the cells as a detoxifying mechamism for the O2
> > > being produced by photosynthesis. But even the latter seems different
> > > from what you are suggesting.
> >
> > Free O2 is certainly the dominant idea; precipitating an excess of
reduced
> > iron (Fe++) inside the cell during oxidation is not a good thing.
> > Detoxifying O2 (particularly superoxide) became a formidable problem
during
> > this period, and you're absolutely right that Fe++ was a primitive
solution.
> > Interestingly, Fe++ by itself does a reasonably poor job of detoxifying
> > superoxides, Fe complexed inside a heme moiety is about 1000-fold more
> > effective than that, and Fe complexed inside heme within the enzyme
> > superoxide dismutase is yet another 1000-fold better.
> >
> > > > Otherwise, I doubt that we would have so much use of Fe
in various
> > > > stages of modern electron transport chains.
> > >
> > > Fe++ was so common in the ancient ocean, and so useful complexed with
> > > organic molecules such as in heme, that I see no reason why there
> > > wouldn't have been several independent uses of Fe in early life, so
> > > that the two you mention could be independent rather than related as
> > > you suggest. Dumping O2 to Fe++ could be almost an inorganic process
> > > completely unrelated to Fe in various electron transport chains. Do we
> > > have an expert who actually knows?
> >
> > Not claiming to be an expert by any means, but certainly the widescale
> > oxidation of iron around 2.4-2.2 billion years ago was inorganic as much
as
> > anything (due to increasing atmospheric O2).  This is also the reason
for
> > extensive red bed deposits at the same time.  There are different ideas
on
> > the mechanism for banded iron formation (BIF) depending on when in
history
> > you're talking about.  Early Archean BIFs (pre-O2) are thought to have
been
> > formed by anoxygenic photosynthetic (and possibly other) organisms able
to
> > use reduced iron as an electron donor.  Extensive BIFs are also seen
around
> > 2.2 billion years ago, when oxygenic photosynthesis "turned
on", biomass
> > significantly expanded, and the Huronian glaciations occurred.  Yet
another
> > series of BIFs are associated with the more recent snowball Earth
episodes
> > around 800 million years ago.
>
> I should clarify that I am very much NOT an expert on the geological
> history of the earth.  At the time of my posting, I was unaware of
> the difference between "red beds" and BIFs, and also unaware of the
> existence of large Fe+++ deposits at times other than 2.3Gya.
>
> My speculation (that early major Fe+++ deposits were directly biogenic,
> rather than indirect via biogenic O2) is based on two observations:
>
> 1.  The biochemistry of oxidizing H2O to O2 is very complex.  The
> biochemistry of oxidizing Fe++ to Fe+++ is much simpler, and seems
> to be integral to all electron transport chains.  While Fe++ was widely
> available, there seems to be little incentive for life to develop the
> complex machinery for oxidizing H2O.
>
> 2.  A small amount of atmospheric O2 can be generated by photolysis
> of H2O at high altitudes.  Thus, evidence for low levels of O2 in the
> atmosphere before 2.3Gya are not necessarily evidence for biogenesis
> of O2 that early.
>
> But, to repeat, I am very much not an expert in this area.
>

Thanks Jim, I should have pointed out that I was very much in agreement with
your post, and saw a few points that I hoped to espouse upon in some kind of
(hopefully) insightful way.  If the earliest (pre-O2) banded iron formations
were due to microbial Fe++ oxidation, there's no reason to think this
mechanism wouldn't have contributed to later BIF deposits as well -- it just
becomes tougher to tease out this versus O2-driven oxidation after 2.3
billion years ago.  I wonder if these competing processes would fractionate
iron isotopes in a measurably different way?
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