The Siderian Period ( /saɪˈdɪəri.ən, sɪ-/; Ancient Greek: σίδηρος, romanized: sídēros, meaning "iron") is the first geologic period in the Paleoproterozoic Era and lasted from 2500 Ma to 2300 Ma. Instead of being based on stratigraphy, these dates are defined chronometrically.

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Siderian
2500 – 2300 Ma Pha. Proterozoic Archean Had.
A reconstruction of the Earth's continents during the middle Siderian, circa 2.4 Ga.
A Siderian banded iron formation in Dales Gorge, Western Australia
Artist's impression of the Earth during the Huronian glaciation, starting from mid-Siderian
Chronology
−2500 — – −2480 — – −2460 — – −2440 — – −2420 — – −2400 — – −2380 — – −2360 — – −2340 — – −2320 — – −2300 — – P a l e o p r o t e r o z o i c Neoarchean Siderian Rhyacian         ← Huronian glaciation ← Beginning of Great Oxidation Event Events of the Siderian Period. Axis scale: millions of years ago.
Proposed redefinition(s)	2630–2420 Ma Gradstein et al.
Proposed container	Neoarchean Gradstein et al.
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	1991[1]
Upper boundary definition	Defined Chronometrically
Upper GSSA ratified	1991[1]

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The deposition of banded iron formations peaked early in this period. These iron rich formations were formed as anaerobic cyanobacteria produced waste oxygen that combined with iron, forming magnetite (Fe₃O₄, an iron oxide). This process removed iron from the Earth's oceans, presumably turning greenish seas clear. Eventually, with no remaining iron in the oceans to serve as an oxygen sink, the process allowed the buildup of an oxygen-rich atmosphere. This second, follow-on event is known as the oxygen catastrophe, which, some geologists believe triggered the Huronian glaciation.^[2][3]

Since the time period from 2420 Ma to 2250 Ma is well-defined by the lower edge of iron-deposition layers, an alternative period named the Oxygenian, based on stratigraphy instead of chronometry, was suggested in 2012 in a geological timescale review.^[4]