What could have caused this change? **READ DETAiLS**?

Question

Scientists believe that, when earth first formed, its atmosphere contained no oxygen at all. Today;, oxygen makes up about 21% of the atmosphere. What could have caused this change??? PLEASE GIVE CiTE SOURCES!!!! NEEDED!!

Answer 1

Photosynthesis. Around 2 billion years ago, the rate of oxygen production by photosynthesis of the talk of the rate at which oxygen could be removed by reducing ions (principally Fe2+) in the oceans. This led to the formation around that time of massive iron ore deposits (the great Banded Iron-Formations), followed by a buildup of oxygen in the atmosphere to around its present levels.

The present level is roughly fixed. Photosynthesis is still going on, but is balanced by forest fires and other processes that remove oxygen. This gives a reasonably stable equilibrium.

Finally, speaking as a scientist, it is not strictly true to say that I "believe" this. I am convinced by the evidence that it is true, which is a little bit different.

Answer 2

Oxygen gas in the atmosphere comes chiefly from plants, through photosynthesis over the past 3 billion years. During the first billion years, most of the oxygen produced through photosynthesis was used up to oxidize iron(II) into iron(III). Then, about 2 billion years ago, with levels around 1%, the amount has increased rapidly to the current 21%.

In environments without life, i.e. other planets, gaseous oxygen is very rare, despite oxygen being the third most abudent element in the universe. Oxygen binds very strongly to many atoms, i.e. silicon, carbon, iron, etc. and thus generally exists in the form of oxide, and not oxygen gas.