Siderian Period

The Siderian (Greek: sideros, meaning "iron") is the first geologic period in the Paleoproterozoic Era and lasted from 2500 Ma to 2300 Ma (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically.

Abundance of banded iron formations (BIFs ?) peaked early this period. BIFs were formed as anaerobic algae produced waste oxygen that combined with iron, forming magnetite (Fe3O4 ?, an iron oxide). This process cleared iron from the oceans, presumably turning greenish seas clear. Eventually, without an oxygen sink in the oceans, the process created the oxygen-rich atmosphere of today.

By 2400 Ma, the ratio of stable isotopes of carbon, iron and sulfur shows the action of living things on inorganic minerals and sediments and molecular biomarkers indicate photosynthesis, demonstrating that life on Earth was widespread by this time.

For the primitive organisms unlucky enough to be around 2.4 billion years ago, the first global freeze was deadly, likely the worst in the history of life on Earth. Few of the organisms escaped extinction, and those that did were forced into an evolutionary bottleneck that altered the diversity of life for eons.

Huronian glaciation began in the Siderian 2400 Ma and ended in the late Rhyacian 2100 Ma.

The algae bloom during the melting period resulted in an oxygen spike, which in turn led to a "rusting" of the iron and manganese. This caused the manganese to be laid down in a huge 45-meter-thick deposit in the Kalahari to await future human mining and metallurgy. Today, about 80 percent of the entire world's known manganese reserves are found in that one field, and it is a major economic resource for the Republic of South Africa.

The "Great Oxygenation Event" began about 2.4 billion years ago. Some scientists believe that the atmosphere was thick about this time while others believe that the earlier thick atmosphere underwent an event that created a thin atmosphere that began to grow thicker again at this time. The evidence is unclear.

It was believed that this rise in oxygen changed the planet, as oxygen was almost non-existent in the atmosphere before this time. Breathable air is thought to have been created by cyanobacteria, single-celled microbes living in the sea. These bacteria harnessed the energy of the sun through photosynthesis—the biochemical process used by plants—producing oxygen as a by-product. The oxygen-rich ozone layer was also established, shielding the Earth's surface from harmful solar radiation.

Volcanic crystal records suggests that continental drift went into a 200 million-year-long recess about 2.45 billion years ago. Earlier studies had suggested a lull in volcanism on Earth that started around 2.45 billion years ago. "Oxygenation of the atmosphere at 2.4 (billion years ago) followed by widespread glaciations at 2.4–2.3 (billion years ago) also may be related to the initiation of the global magmatic lull.


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