[Robcat (OP)]

At some past era oxygen became a percentage of our atmosphere along with lots of nitrogen.
My first question is ...was the ratio of oxygen to nitrogen the same as of today or what was the balance then,
And the source of both??

The real question I wish to ask is this :- Our atmosphere has a percentage of Oxygen. All the time methane
And all the giga tons of fossil fuels have been converting oxygen to carbon dioxide or carbonates etc.
Is the ratio of Oxygen to Nitrogen changing and the Oxygen percentage falling.
If Yes by how much??? If not, where is the oxygen being formed from to retain the balance.

The latter question is what started my thought, IE where is any oxygen coming from in our biosphere ??
I.m a chemist!!!???

[chiralSPO]

All of the oxygen atoms were originally synthesized in stars, and most of it was incorporated into the Earth as components of silicates, borates, water, and metal oxides. There was also a substantial amount of CO₂ in our primordeal atmosphere (much of which reacted with the previously mentioned metal oxides to form metal carbonates and bicarbonates.)

Once liffe began photosynthesizing, substantial amounts of H₂O and CO₂ were converted into biomass and O₂. https://en.wikipedia.org/wiki/Great_Oxygenation_Event

Nitrogen content in the atmosphere has not been effected nearly as much.

Currently, (not counting water content, which can be anywhere between 0% and 2%) the atmosphere is about 78% N₂, 21% O₂, and the remaining 1% is mostly Ar, followed by CO₂, and then traces of nitrogen oxides, sulfur oxides, ozone, and small organic molecules. CO₂ concentrations appear to have increased from about 200 ppm to about 400 ppm since the industrial revolution--even if the formation of each mole of CO₂ is associated with the consumption of 2 moles of O₂
CH₄ + 2 O₂ → CO₂ + 2 H₂O
That still would be only a decrease from 210000 ppm to 209200 ppm (21% to 20.9%). If we continued burning fossil fuels to the extent that it would have a significant effect on the concentration of O₂ in our atmosphere, we would succumb to the effects of the CO₂ long before the O₂ shortage would be an issue. (Increased CO₂ will lead to acidified oceans, climate change, and if it gets high enough can even be directly toxic http://www.ivhhn.org/index.php?option=com_content&view=article&id=84)

[Tim the Plumber]

At some past era oxygen became a percentage of our atmosphere along with lots of nitrogen.
My first question is ...was the ratio of oxygen to nitrogen the same as of today or what was the balance then,
And the source of both??

The real question I wish to ask is this :- Our atmosphere has a percentage of Oxygen. All the time methane
And all the giga tons of fossil fuels have been converting oxygen to carbon dioxide or carbonates etc.http://www.thenakedscientists.com/forum/index.php?topic=66324.msg484388;topicseen#new
Is the ratio of Oxygen to Nitrogen changing and the Oxygen percentage falling.
If Yes by how much??? If not, where is the oxygen being formed from to retain the balance.

The latter question is what started my thought, IE where is any oxygen coming from in our biosphere ??
I.m a chemist!!!???

The amount of CO2 produced by humans and the amount by which it has risen in the air is tiny. So there is no significant change in the amount of oxygen in the air.

[evan_au]

It's counter-intuitive, but oxygen is one of the most common elements in or on the Earth, making up about 46% of the rock, and about 30% of the mass overall.

Even sand (SiO₂) is mostly oxygen; over 50% by mass and over 90% by volume.
http://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements#Abundance_of_elements_in_the_Earth