Rhyacian Period

An age of high volcanism; possible early eukaryotes

The Rhyacian (2.3-2.1 Ga) is the second period of the Paleoproterozoic Era, occurring after the Siderian, and before the Orosirian.

2,300–2,050 Ma
"Rhyacian" derives from a Greek word for "lava flow", indicating that this was a period of high volcanism

Period of intense volcanism on Earth
- Mineral-rich igneous rock deposited (Bushveld Complex in South Africa)
  - Source of rich mines, contains much of the world's andalusite, chromium, fluorspar, platinum, and vanadium.
  - Contains the richest deposits of platinum in the world.
- The volcanism of this period produced large quantities of carbon dioxide and other greenhouse gases
  - Ended the Huronian glaciation (~2.1 Ga), which was started by the Great Oxygenation Event
At the end of the Huronian glaciations, considerable rise in marine arsenic concentrations (Fru et al., 2015).
- There is a concurrent rapid decrease of atmospheric oxygen, which resulted from due to widespread ocean toxicity, which resulted from the release of toxic elements into the oceans when the ice melted.

Above: oldest evidence of a mobile organism from 2.1 Ga in Gabon

Above: Reconstruction of Diskagma

Prokaryotic cyanobacteria dominate along coastlines and in marine waters
Possible evidence for life on land (~2.2 Ga)
- Diskagma buttonii: controversial fungus-like fossil from South Africa
  - Fossils the size of match heads and connected into bunches by threads in the surface of an ancient soil (Retallack et al., 2013)
  - Its size and hollow shape are similar to the living fungus Geosiphon
  - If interpreted correctly, this would be one of the oldest known eukaryotes
Other evidence of early eukaryotes present (Brocks et al., 2003, Porter, 2004)
- Steranes, chemical signatures of eukaryotes, found in Rhyacian rocks
Molecular clock evidence indicates green algae are possibly present
Evidence of a mobile organism in an oxygenated shallow-marine environment (Albani et al., 2019)

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