The Siderian ( ) is the first geologic period in the Paleoproterozoic Era and Proterozoic Eon. It lasted from 2500 to 2300 million years ago (Ma), spanning 200 million years, and is followed by the Rhyacian Period. Instead of being based on tectonic layers, these dates are defined chronometrically. Most continental activity in this period revolved around the breakup of the supercontinent Kenorland. While this event mainly occurred in the North American craton, volcanic intrusions and dike swarms appeared in plates pertaining to Northwestern Europe, South Africa, and Australia in connection to Kenorland's rifting. In the ocean, emissions from hydrothermal vents contributed to the production and crystallization of minerals, along with varying concentrations of sulfur and iron. While this sedimentation circulated in the ocean, the amounts deposited on the ocean floor contributed to the development of banded iron formations, along with a diverse range of ores such as pyrite and magnetite. Cyanobacteria continued to develop their molecular structure, with eukaryotes beginning to appear near the end of the period, and they contributed to the ocean's oxidation. Their presence eventually became the partial cause for the build-up of oxygen in Earth's atmosphere, becoming known as the Great Oxidation Event. This led to a decrease in methane and carbon dioxide, which were two major greenhouse gases at the time, dropping the overall global temperature below 0 °C. As a result of this, the Earth experienced three snowball events which are collectively known as the Huronian glaciation.
Etymology and history The name Siderian is derived from the Greek word sideros, meaning "iron", and refers to the banded iron formations formed during this period. Before its use, the period was suggested to be named as the Huronian Era with a boundary from 2450 to 2200 Ma, in correlation to the sedimentary record of Canada's Huronian Supergroup. Despite the stratigraphic use of the term "Huronian" since the nineteenth century, the Siderian period was proposed under the current nomenclature (2500 to 2300 Ma) in 1989 by the Subcommission on Precambrian Stratigraphy. It was later ratified in 1990 by the International Union of Geological Sciences as a subdivision of the Proterozoic Eon. While the Siderian is well-defined by the lower edge of iron-deposition layers and the initial appearance of glacial deposits, alternate names have been suggested to mark the period stratigraphically. In 2012, Kranendonk et al. proposed to set the Siderian Period to an earlier date range, due to the slow expansion of the period's continental plates, spanning from 2630 to 2420 Ma as the final subdivision of the Neoarchean Era. They also suggested adjusting the upper half of the Siderian's preceding definition to occur from 2420 to 2250 Ma as the Oxygenian Period, in response to the change in Earth's atmosphere during this time. In 2021, Shields et al. presented a similar alteration, but with the Siderian ending at about 2450 Ma, and the first period of the Paleoproterozoic Era termed the Skourian instead of the Oxygenian. The name "Skourian" would refer to the oxidation of the ocean's iron supply, and its period would span from about 2450 to 2300 Ma. As of June 2026, the Siderian retains its 1989 definition, and is the earliest internationally recognized period on Earth's geologic time scale.
Paleogeography
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