[barntz]

Is the air covering the earth constant (a closed system) or does it's oxygen remix with the oceans? What I am trying to answer is the following question: If each breath we exhale contains roughly 10^20 air molecules, and they spread out over the whole earth, are we breathing some molecules of Aristotles exhaled air? According to my very rough calculations, if all air were in a closed system, each breath we take would contain thousands of molecules of Aristotles (or whoevers) oreviously exhaled breath. Assuming he breathed for say 50 years.

[constant thinker]

I'm not 100% sure if I'm answering your question right, but oxygen and other gases are constantly entering the oceans and leaving it. Plants and algae (or any other autotroph) are taking in CO2 and H2O, and with light, they are producing sugars. This releases O2. Those sugars that are produced get broken down by the plant or whatever happens to eat it and becomes CO2 and H2O again, usually with the aid of O2.

There is a constant cycle over oxygen through different things like bodies of water, animals, etc. All of the O2 would in theory stay within the Earth's atmosphere, but be passed around in different "forms". It is though theoretically possible that some of the air that Aristotle be breathed in by you though.

The "air" on Earth has, for the most part, been around since the dinosaurs.

I hoped I helped to answer your question.

[billnotgatez]

I would wonder about nitrogen and argon. The oxygen and carbon dioxide would probably been recycled by this time.

[jdurg]

What most likely is true is that the atoms which make up your body once made up the feces of the dinosaurs.