December issue of Geology p. 1135, an article by H. Ohmoto titled
"Evidence in pre-2.2 Ga paleosols for the early evolution of
atmospheric oxygen and terrestrial biota":
"Minimum(his italics) concentration of O2 in the 3.0-2.2Ga
atmosphere is calculated to be about 1.5% of the present
atmospheric level"
Kinda the final blow to those who still think the early earth had a reducing
atmosphere....
ABSTRACT
The loss of Fe from some pre - 2.2 Ga paleosols has been considered by
previous investigators as the best evidence for a reduced atmosphere prior
to 2.2 Ga. I have examined the behavior of Fe in both pre - and post - 2.2
Ga paleosols from depth profiles of Fe3+Ti, Fe2+Ti, and (Sigma) Fe/Ti
ratios, and Fe3+Ti vs. Fe2+Ti plots. This new approach reveals a previously
unrecognized history of paleosols. Essentially all paleosols, regardless of
age, retain some characteristics of soils formed under an oxic atmosphere,
such as increased Fe3+Ti ratios from their parental rocks. The minimum
oxygen pressure (PO2) for the 3.0 - 2.2 Ga atmosphere is calculated to be
about 1.5% of the
present atmospheric level, which is the same as that for the post - 1.9 Ga
atmosphere. The loss of (Sigma) Fe, common in paleosol sections of all ages,
was not due to a reducing atmosphere, but to reductive dissolution of ferric
hydroxides formed under an oxic atmosphere. This reductive dissolution of
ferric hydroxides occurred either (1) after soil formation by hydrothermal
fluids or (2) during and/or after soil formation by organic acids generated
from the decay of terrestrial organic matter. Terrestrial biomass on the
early continents may have been more extensive than previously recognized.
Art
http://chadwicka.swau.edu