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Science Forum Index » Bio Evolution Forum » Temp of early earth...
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| Tom Hendricks... |
 Posted: Sat Aug 09, 2008 7:32 pm |
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Guest
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This suggests new thoughts on temp of earth when life
began.
The Evolution of the Primitive Atmosphere
James F. Kasting
Department of Geosciences, Penn State University,
University Park, PA 16802
kasting at (no spam) essc.psu.edu
Environmental conditions on the early Earth are
important for both the origin and the early evolution
of life. Two variables are of particular
significance: 1) the atmospheric redox state,
and 2) the mean
surface temperature. Most recent models of Earth’s
prebiotic atmosphere (Walker, 1977; Kasting,
1993) suggest that it was weakly reduced, with N2 and
CO2 dominating over NH3 and CH4 . Some
CH4 may have been present, however (Hashimoto et al.,
2007), particularly if hydrogen escape was
relatively slow (Tian et al., 2005). Ongoing work
should help to resolve the hydrogen escape question
and may shed light on whether a more highly reduced
atmosphere could have existed.
The climate of the early Earth is also controversial.
Despite the faintness of the young Sun, the early
Earth appears to have been warm, or perhaps even hot.
Taken at face value, oxygen and silicon
isotopes in ancient cherts imply a mean surface
temperature of 70(±15)o C at 3.3 Ga (Knauth and
Lowe, 2003; Robert and Chaussidon, 2006). Ancient
carbonates also yield high Precambrian surface
temperatures (Shields and Veizer, 2002), as does a
recently published analysis of the thermal stability
of proteins which are inferred to be ancient (Gaucher
et al., 2008). This evidence for hot early surface
temperatures must be weighed against the previously
mentioned dimness of the young Sun, as well
as geomorphic evidence for glaciation at 2.9 Ga, 2.4
Ga, and 0.6-0.7 Ga. Climate models with high
CO2 and CH4 concentrations can potentially explain hot
climates, but can they explain climates that
transition from hot to cold, and back again, multiple
times? Such models must also account for the well
documented correlation between the rise of O2 at 2.4
Ga and the Paleoproterozoic glaciations which
occurred at that same time. Some of the secular
variation in oxygen isotope ratios may be accounted
for by changes in seawater isotopic composition
(Kasting et al., 2006), although that interpretation
remains controversial and cannot account for the
observed variation during the Phanerozoic (Came et
al., 2007). When all the arguments are weighed, the
early Earth appears to have been warm, rather
than hot, but more work remains to reconcile the
different pieces of evidence.
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