system
Given the following post, respond as if you're commenting on Reddit:
human
During the carboniferous period, the oxygen level was ~35%. Now it is ~21%. There is less oxygen in atmosphere now. Where is that oxygen now?

During the carboniferous period, the oxygen level was ~35%. Now,the level is ~21%. There was a 41.5% oxygen decrease in atmosphere. Considering the fact that the level of CO2 were always negligible - at the beginning of the carboniferous period was only 7000ppm (~0,7%),now it is ~0,04% . Where is that oxygen now? I understand that breaking down lignin was difficult and it became coal but coal is mostly carbon. This explains the fact that carbon is trapped as coal  but where is that oxygen trapped now?
gpt
Oxygen gets locked up as CO2, which can be in the atmosphere (minor), or in plants, or dead plants and animals in the ground. This includes a lot in bacteria. Another major sink is capture by rocks, as in iron and silicate oxides.