According to the Precambrian rock record, much of Earth’s first free oxygen combined with iron.
Explanation;
Oxygen originated from tiny organisms known as cyanobacteria or blue-green algae. These organisms conducted photosynthesis using sunlight, water and carbon dioxide to produce simple sugars and oxygen.
Initially, the oxygen produced, quickly combined with elements in Earth's crust, primarily Iron removing oxygen from the atmospheres.
After the supply of oxidizable surfaces ran out, oxygen began to accumulate in the atmosphere and the modern high-oxygen rich atmosphere would have developed.
