What long-term environmental change on Earth led to the deposition of banded iron formations?

Banded iron formations (BIF) are sedimentary rocks that contain layers of iron-rich minerals, such as hematite and magnetite, alternated with layers of silicate minerals like chert or quartz. These structures were predominantly formed during the Precambrian era, between 3.8 billion to 1.8 billion years ago.

One of the significant environmental factors that contributed to the formation of BIF is the presence of dissolved iron (Fe2+) in ancient oceans. It is believed that volcanic activity released a large amount of iron into the oceans at that time. Additionally, early Earth's atmosphere had little to no free oxygen, which allowed iron to remain dissolved in the water without oxidizing and precipitating.

Around 2.4 billion years ago, a critical event occurred, known as the Great Oxygenation Event (GOE). This was when cyanobacteria, photosynthesizing microorganisms, began to produce a significant amount of oxygen (O2) as a byproduct of their photosynthesis process. This led to a gradual increase in the oxygen levels of the Earth's atmosphere and oceans.

As the oxygen levels rose due to the cyanobacteria's activity, the dissolved iron (Fe2+) in the oceans started to react with the oxygen, forming iron oxide minerals such as hematite (Fe2O3) and magnetite (Fe3O4). These minerals then precipitated and settled at the ocean floor, along with silicate minerals, forming alternating layers of iron-rich and silica-rich sediments. Over time, these sediments compacted and lithified, resulting in the formation of banded iron formations. In conclusion, the long-term environmental change that led to the deposition of banded iron formations was the Great Oxygenation Event, which resulted from the photosynthetic activity of cyanobacteria increasing the oxygen levels in the Earth's atmosphere and oceans. This caused the dissolved iron in the oceans to oxidize and precipitate, leading to the formation of BIFs.