Ask AI: in a paragraph Describe the relationship between banded-iron formations and oxygen gas.

Banded-iron formations (BIFs) are sedimentary rocks composed of alternating layers of iron oxides, such as hematite, magnetite and siderite, and silica-rich chert. They were formed during the Precambrian era when free oxygen in the atmosphere was much lower than it is today. This means that the dissolved iron in the oceans was able to exist in a form that wouldn't immediately react with oxygen, allowing it to accumulate into BIFs. Today, oxygen gas is a major component of the atmosphere, but it wasn't until the Great Oxygenation Event some 2.4 billion years ago that it reached significant levels, after which the dissolved iron in the oceans started to oxidize and react with oxygen gas, which caused BIFs to become depleted.