Rhyacian
Second period of the Paleoproterozoic Era
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The Rhyacian (/raɪˈeɪsiən/) is the second geologic period in the Paleoproterozoic Era. It spans 250 million years and lasted from 2300 to 2050 million years ago (Ma), following the Siderian Period and preceding the Orosirian Period. These dates are defined chronometrically rather than stratigraphically.
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| Rhyacian | ||||||||||||||||||||
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A sample of a Rhyacian banded iron formation found in North America | ||||||||||||||||||||
| Chronology | ||||||||||||||||||||
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| Etymology | ||||||||||||||||||||
| Name formality | Formal | |||||||||||||||||||
| Usage information | ||||||||||||||||||||
| Celestial body | Earth | |||||||||||||||||||
| Regional usage | Global (ICS) | |||||||||||||||||||
| Time scale(s) used | ICS Time Scale | |||||||||||||||||||
| Definition | ||||||||||||||||||||
| Chronological unit | Period | |||||||||||||||||||
| Stratigraphic unit | System | |||||||||||||||||||
| Time span formality | Formal | |||||||||||||||||||
| Lower boundary definition | Defined Chronometrically | |||||||||||||||||||
| Lower GSSA ratified | 1990[2] | |||||||||||||||||||
| Upper boundary definition | Defined Chronometrically | |||||||||||||||||||
| Upper GSSA ratified | 1990[2] | |||||||||||||||||||
Etymology and history
The name Rhyacian is derived from the Greek word rhyas, meaning "stream of lava", and refers to the layered intrusions of the Bushfeld Complex in South Africa. The term was proposed by the Subcommission on Precambrian Stratigraphy as a subdivision of the Proterozoic Eon, and was ratified by the International Union of Geological Sciences in 1990.[2][3] In 2012, there were suggestions to replace the Rhyacian with an alternate name, bearing a time interval of 2250 to 2060 Ma, and decided on the basis of its stratigraphy. The name Jatulian was proposed in reference to the Lomagundi-Jatuli excursion event, while the term Eukaryian was chosen due to the period's existing signs of the earliest eukaryotic fossils.[4] As of December 2024[update], a replacement for the Rhyacian has not been officially adopted by the IUGS.[5] The term Jatulian, however, has been used in the regional stratigraphy of Fennoscandia.[6]
Paleogeography
The Bushveld Igneous Complex and some other similar intrusions[specify] formed during this period.[7]
Climate
Life
Eukaryotes are thought to have originated during the Rhyacian, after an endosymbiotic event between asgardarchaea and alphaproteobacteria; sexual reproduction, a strategy unique to eukaryotes, is also thought to have developed during this period.[9][10][11] Possible signs of macroscopic life have been found in Rhyacian aged rocks,[12][13] although these are heavily disputed due their age and probable abiotic origins.[14][15]
