Paleoproterozoic

The Paleoproterozoic Era was a massive turning point in Earth's history. It was the first era of the Proterozoic eon. This era lasted from 2.5 to 1.6 billion years ago. It is the longest era in the entire geological history of our planet. Scientists divide this era into four distinct geologic periods. These periods are the Siderian, Rhyacian, Orosirian, and Statherian. During this long stretch of time, the Earth's continents finally began to stabilize.

One of the most important changes involved the atmosphere. Originally, the air was a weakly reducing atmosphere. It was made of nitrogen, methane, ammonia, carbon dioxide, and inert gases. This mixture was similar to the atmosphere of Titan. Everything changed when cyanobacteria evolved oxygenic photosynthesis. This process released dioxygen, or free oxygen, as a byproduct. This oxygen began to use up the reductants in the ocean and on land. Eventually, these substances like ferrous iron and methane were exhausted. This led to the Great Oxidation Event during the Siderian and Rhyacian periods. Oxygen levels rose from almost nothing to 10% of modern levels.

This shift in chemistry was very difficult for life. Before this, almost all life was made of single-cell prokaryotic anaerobic organisms. These organisms did not need oxygen to live. They used a form of cellular respiration that relied on other chemicals. When free oxygen appeared, it was highly reactive and toxic to these microbes. This caused a massive die-off of the archaea-dominated microbial mats. The crisis was made worse by the Huronian glaciation. This was a global icehouse event that lasted 300 million years. It happened partly because the atmosphere lost its methane, a powerful greenhouse gas.

Life eventually adapted to these harsh new conditions. New organisms called aerobes evolved to thrive using oxygen. They developed bioactive antioxidants to protect themselves. Some anaerobic organisms survived by living in hybrid colonies with aerobes. This symbiotic relationship helped the evolution of mitochondria in eukaryotic organisms. Eukaryotes are more complex cells than prokaryotes. We see the oldest cyanobacterial fossils from this era in the Belcher Islands of Canada. By 1.75 billion years ago, thylakoid-bearing cyanobacteria appeared in Australia. By the late Statherian period, eukaryotic life in North China was already diverse.

Geological activity was also very intense during this era. The Earth saw the development of the first global-scale continent-continent collision belts. These collisions created massive mountain-building events called orogens. In South America and West Africa, these were the Trans-Amazonian and Eburnean orogens. In North America, many belts formed, such as the Trans-Hudson and Penokean orogens. Similar belts appeared in Greenland, Siberia, and Europe. These many collision belts suggest the formation of a supercontinent. This giant landmass is known as Columbia or Nuna.

Scientists have a theory about why these mountains grew so large. They believe the Great Oxidation Event helped cause this mountain building. Increased biomass and carbon burial occurred during this time. This carbon was eventually subducted into the Earth. It is hypothesized that these carbonaceous sediments lubricated the movement of the crust. This lubrication led to crustal thickening and massive mountain ranges. This connection shows how tiny microbes can change the shape of entire continents.

Even the rotation of the Earth was different back then. Paleontological evidence suggests the planet spun at a different rate. About 1.8 billion years ago, a day lasted only 20 hours. This meant there were about 450 days in a single year. This era also saw the formation of the lithospheric mantle in Patagonia. In northern Sweden, felsic volcanism created the Kiruna and Arvidsjaur porphyries. The Paleoproterozoic was a time of extreme change for every system on Earth. It linked the tiny world of microbes to the massive movements of the continents.