Answer:
oxygen became plentiful on earth
Step-by-step explanation:
This is because banded iron formation is formed as a result of oxygen present in the ocean which is produced from cyanobacteria which undergo photosynthesis to produce carbohydrates and oxygen.
The oxygen produced are plentiful and they react with dissolved iron on the Earth's ocean to produce iron oxides which precipitated out and form a thin line floor on the ocean.