Jayjay

I claim:

"The only reason why we think that the early Earth had a non-oxidizing atmosphere is that molecules needed for life cannot form in an oxidizing environment."

True or false? If false, what other collaborating evidence is there?

Undercurrent

There is the fact that oceanic ridge volcanoes, which are thought to be the source of a lot of the volitile gasses in the early atomsphere, emit reducing gasses like methane. Also, the volitile gasses of the interstellar medium support would naturally form a hydrogen/methane-rich atmosphere.

The big goelogical evidence is the halt in the deposition of uraninites and the formation of the banded iron formations around 2 billion years ago. My understanding (and I'd wait for ps418 to post confirmation before trusting my understanding too much) is that the former are soluble if oxidized, while the latter is insoluble if oxidied. The fact that one is not longer coming out of solution while the other is all of the sudden coming out of solution points to an vast increase (biogenic) in the level of atmospheric oxygen, which implies that the oxygen wasn't there in the first place.

SULPHUR

I don't much about the atmosphere but there was a definite reducing environment present in the water to produce a lot of pyrites and associated minerals.

Jayjay

Thank ye.

Secular Pinoy

I've posted a similar question on this thread.I received a lot of responses, which might be of interest to you.

ps418

Completely false, as a glance at the recent literature or any historical geology textbook will quickly reveal. There are multiple lines of geochemical evidence which convergently indicate very low atmospheric oxygen levels during the Archean (i.e. prior to 2,500Ma). The evidence includes but is not necessarily limited to sedimentary detrital pyrites and uraninite, the geochemistry and weathering patterns of paleosols and their constituent minerals, and sulpher isotope fractionation. There is still some dispute as to when and how quickly pO2 rose to its present level, but there is a strong concensus --amongst geologists who are not involved in origin-of-life research-- that the very early earth had very little free O2.

Check out some of the following refs for more info:

J. Farquhar, B. A. Wing, K. D. McKeegan, J. W. Harris, P. Cartigny, and M. H. Thiemens, Mass-Independent Sulfur of Inclusions in Diamond and Sulfur Recycling on Early Earth. Science 2002 298: 2369-2372.

Kirsten S. Habicht, Michael Gade, Bo Thamdrup, Peter Berg, and Donald E. Canfield, Calibration of Sulfate Levels in the Archean Ocean. Science 2002 298: 2372-2374.

Murakami, T., Utsinomiya, S., Imazu, Y. and Prasad, N., 2001. Direct evidence of late Archean to early Proterozoic anoxic atmosphere from a product of 2.5 Ga old weathering.” Earth Planet. Sci. Lett. 184, 523-528.

Rye, R., and Holland, H.D., 1998, Paleosols and the Evolution of Atmospheric Oxygen: A Critical Review: American Journal of Science, v. 298, p. 621-672.

Uwe H. Wiechert, Earth's Early Atmosphere. Science 298:2341-2342.


Patrick

SULPHUR

ps418 could you explain the highly reducing environment at this time leading to massive amounts of sulphides combined with large scale coal deposits.

ps418

Not really. I know basically nothing about massive sulphide deposits, except that they are volcanogenic and are sources of metal ores.

You may be using a different definition of coal than me. If by coal you mean organic-rich sedimentary deposits, then there are some Archean coals. If by coal you mean an organic-rich deposit containing significant quanitites of plant matter, which is what most in the US think of as coal, of course there are no Archean coals.

Patrick

SULPHUR

I am talking about the carboniferous

ps418

There was no reducing atmosphere during the carboniferous. The highest atmospheric O2 levels of all time were reached during the Permo-Carboniferous (Berner et al, 2003). This is probably related in some way to the vast amount of carbon that was sequestered into coal deposits during this time (as well as insect gigantism), but I'm not sure how the two processes are connected.

Berner, Phanerozoic oxygen evolution. Annu. Rev. Earth Planet. Sci., January 1, 2003; 31(1): 105 - 134.

Patrick