At 07:53 PM 1/19/2016, Keith F. Lynch wrote:
I've long been baffled by the claim that the free oxygen in the atmosphere came from photosynthesis. If the atmosphere started out neutral, then oxygen in the atmosphere must be balanced by reduced matter (biomass, peat, coal, oil, natural gas, etc.). It seems implausible that there's enough combustible stuff that burning it all would consume all the oxygen.
Note that carbon in carbonate rocks isn't reduced -- it's already combined with as much oxygen as it can be. Carbonate rocks can explain the disappearance of carbon dioxide from the atmosphere, but not the appearance of free oxygen.
The obvious alternative is that over many millions of years solar UV split water vapor into oxygen and hydrogen, and the hydrogen escaped into space. This would drop the sea level, but only by a few meters.
There's a much bigger problem: Compare the Earth & Venus. Both are approx the same size; both have approx the same total amount of nitrogen in their respective atmospheres. The pressure at the bottom of the Earth's ocean is approx the same as the pressure on the surface of Venus. So, where did all of Earth's CO2 go? Or similarly, where did all of Venus's H2O go? The Earth's atmosphere is waaay too thin, relative to what it *should* be. It's possible that it lost some amount of atmosphere when the Earth's Moon was formed, but probably not enough. There had to be other mechanisms to get rid of more atmosphere.