Siderian

The Siderian ( /saɪˈdɪəri.ən, sɪ-/) is the first geologic period in the Paleoproterozoic Era and Proterozoic Eon. It lasted from 2500 to 2300 million years ago (Ma), spanning a time of 200 million years, and is followed by the Rhyacian Period. Instead of being based on stratigraphy, these dates are defined chronometrically.

Siderian
2500 – 2300 Ma Pha. Proterozoic Archean Had.
A Siderian banded iron formation in Dales Gorge, Western Australia
Artist's impression of the Earth during the Huronian glaciation, starting from the mid-Siderian
Chronology
−2520 — – −2500 — – −2480 — – −2460 — – −2440 — – −2420 — – −2400 — – −2380 — – −2360 — – −2340 — – −2320 — – −2300 — – −2280 — Neoarchean Paleoproterozoic Siderian Rhyacian         ← Huronian glaciation[1] ← Beginning of the Great Oxidation Event ← Breakup of the supercontinent Kenorland[2]   Major glacial period Events of the Siderian Period Vertical axis scale: Millions of years ago
Proposed redefinition(s)	2630–2420 Ma Gradstein et al., 2012
Proposed container	Neoarchean Gradstein et al., 2012
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[3]
Upper boundary definition	Defined Chronometrically
Upper GSSA ratified	1990[3]
Atmospheric and climatic data
Mean atmospheric O2 content	c. 0.014007 vol % (0% of modern)
Mean atmospheric CO2 content	c. 11000 ppm (39 times pre-industrial)

The name Siderian is derived from the Greek word sideros, meaning "iron", and refers to the banded iron formations formed during this period. 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.^[3][4] Since 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 upper half of the period stratigraphically. The term Oxygenian was suggested in 2012 due to the change in Earth’s atmosphere during this time,^[5] while the name Skourian was proposed in 2021 as a rock-based alternative.^[6] As of December 2024, the Siderian is the earliest internationally recognized period on the geological timescale.^[7]

The deposition of banded iron formations (BIFs) 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.^[8][9]

Paleogeography

Paleotectonics

Tectonic activity mainly revolved around the growth of continental plates during the early Siderian. Many cratons at the time, including the Gawler, Superior, and Pilbara cratons, experienced volcanic activity through a global plume breakout, which occurred from 2500 to 2450 Ma. During that time, depositions at banded iron formations began occurring at the Kaapvaal and Pilbara cratons.^[10] Additionally, volcanic and sedimentary rocks have begun to deposit into the Transvaal Basin at 2400 Ma, lasting until 2000 Ma in the Orosirian Period.^[11]

Some depositional activity in what would become present-day Australia involved a selection of supersequences, consisting of a diverse set of densely packed sediments. The Brockman Supersequence, lasting from 2500 to 2449 Ma, has been shown to at least consist of mudrock and sediments from BIFs, which have been deposited during rising sea levels and times of volcanic activity.^[12] Additionally, there have been traces of sulfur isotopes found in this sequence's Brockman Iron Formation, indicating a rise in the atmosphere's oxygen at the time.^[13] The Woongarra Supersequence followed, consisting of depositions mainly from rhyolite, but with layers of dolerite and basalt present beforehand, occurring 2449 Ma.^[14] It was then capped by the Turee Creek Supersequence, which presents itself with a layer of rhyodacite-basalt and sandstone sequences, and lasted from 2449 to 2410 Ma before a stratigraphic hiatus occurred.^[12]

Magmatism

Grey Badcallian gneiss (c. 2500 Ma) intruded by a dark amphibolitic Scourie dyke (c. 2400 Ma), both intruded by younger granitic veins

Magma in the form of dike swarms has penetrated the surface of multiple cratons during the Siderian, taking place in some of the major continental plates such as those spanning North America, South Africa, and Australia. About 2470 Ma, the tholeiitic and komatiitic Mistassini dike swarm penetrated the Superior Craton.^[15] With a surface area of at least 100,000 square kilometers, it can be classified as a large igneous province (LIP).^[16] It is followed by the Matachewan dike swarm, an LIP occurring from about 2470 to 2450 Ma, and spanning a surface area of at least 300,000 square kilometers. The Mistassini and Matachewan swarms can be genetically associated with each other, as the Matachewan swarm has intruded into the Superior Craton in the area between Lake Superior and James Bay.^[17] The Scourie dike swarm penetrated the Lewisian Gneiss Complex from about 2418 to 2375 Ma,^[18] while the Widgiemooltha dike swarm intruded into the Yilgarn Craton at around 2410 Ma. The Widgiemooltha swarm occurred in close proximity to the Sebanga Poort dike's intrusion into the Zimbabwe Craton, which occurred about 2408 Ma.^[19]

Breakup of Kenorland

Main article: Kenorland

Tectonic rifting began separating the supercontinent Kenorland at around 2450 Ma, with the breakup mainly occuring in Laurentia.^[2] As a result, the Hurwitz Group in northern Canada experienced continential stretching and depression, resulting in the depositions of the Noomut, Padlei, and Kinga Formations, along with the creation of the Hurwitz Basin.^[20] Additionally, low sulfidation deposits holding copper and nickel began to form in the Nena and Kalahari cratons,^[2] while zircons formed within the Deep Lake Group in what is now the Sierra Madre Range.^[21][22] Despite the intrusions contributing to the rifting, Kenorland experienced little continential movement, and there have been no signs of ocean development as a result. However, sedimentation from shallow waters began to occupy the Strel'na Group, in what is now the Kola Peninsula.^[23][24]

Climate

Great Oxidation Event

Main article: Great Oxidation Event

Semi-logarithmic chart of atmospheric levels throughout Earth’s history, with the surge of oxygen occurring approximately 2.4 billion years ago

Since the beginning of the Siderian, there has been an irreversible rise of oxygen in the Earth’s atmosphere, which has come to be known as the Great Oxidation Event. The partial pressure of oxygen in the air (pO₂) has been shown to have increased to at least 10⁴ times its original level, rising from 2 × 10⁻⁶ bar to at least 2 × 10⁻³ bar between 2410 and 2320 Ma.^[25][26] As a result, the rapid change came at the expense of greenhouse gases such as carbon dioxide and methane, indirectly leading to a series of ice ages known as the Huronian glaciation.^[27]

The levels of carbonates and organic carbon have been relatively stagnant. The abundance of carbon-13 isotopes (δ¹³C), found within dolomites and formations in the Mount Bruce, Transvaal, and Huronian supergroups, has maintained a steady level of 0‰ in carbonates, while organic carbon created through the activity and burial of cyanobacteria remained stationary at approximately −28‰.^{[28]: 3819–3820 [29]} Although this may present itself as a sign of inactivity during this period, it suggests that there has been multiple sources causing an equal force of sinks and rises in the levels of oxygen.^[30] This includes the influx and settlement of carbon dioxide from volcanic activity which stems from tectonic processes,^[31] along with the delivery of phosphate to oceans through cycles of chemical weathering.^[32]

As a consequence of the excess oxygen, a shift began to occur in the level and activity of greenhouse gases. The carbon dioxide in the atmosphere maintained equilibrium at a partial pressure of 1.1 × 10⁻² bar, due to the oxidation of methane in the air, silicate weathering on the surface, and emissions from volcanic activity.^[9][33] However, this process depleted the amounts of methane by a significant amount, dropping from 300 to 4 ppmv.^[34] Despite the balance in carbon dioxide, the significant change in methane caused Earth to undergo a snowball event, dropping the average global temperatures below 0°C.^[9]: 11134

Huronian glaciation

Main article: Huronian glaciation

Due to the loss of global temperature, the Earth entered the Huronian glaciation, which lasted from about 2450 to 2200 Ma.^[35] While this event has been divided into four separate glaciations, only the first three occur in the Siderian Period, serving as a reaction to the oxidizing environment.^[36] Traces of the glaciation have been found in the diamictites and sequences of six cratons,^[37] including the Wyoming, Kaapvaal, and Karelia-Kola cratons.^[36]

The oldest glaciation correlates to quartz located in the Campbell Lake and Headquarters formations,^[38] along with glacial deposits in the Polisarka Formation.^[39] It lasted from about 2440 to 2420 Ma,^[36] and is generally referred to after the diamictites found in the Duitschland Formation.^[40] The second glaciation, known as the Makganyene glaciation after its eponymous formation, is marked by cap carbonate sequences found above the Bruce and Vagner formations,^[41] occuring from about 2380 to 2360 Ma.^[36] The youngest of the three glaciations occurs from about 2340 to 2310 Ma near the end of the Siderian,^[36] represented on the Gowganda Formation in the Huronian Supergroup, and referred to after the Rietfontein diamictite located in South Africa.^[42]

Life

By the beginning of the Great Oxidation Event, cyanobacteria have developed intercelluar communication through molecular exchange, and have begun to differentiate from each other. Strands such as those in the Pseudanabaena genus began chaining themselves in a filamentous structure,^[43] and Giardia, one of the earliest eukaryotes, began to appear at around 2309 Ma.^[44][45] Traces of these bacteria have made marks in a few deposition sites. Microfossils in Australia's Turee Creek Group are embedded in black chert, which dates back to 2450 Ma.^[46] In China, stromatolites have been spotted in the Dashiling and Qingshicun formations of the Hutuo Group, existing for the duration of the Siderian Period.^[47] Additionally, findings in the Fennoscandian Shield show that the taxonomy of stromatolites began to diversify at around 2330 Ma.^[48]

Marine geochemistry

In correspondence with the Great Oxidation Event, there has been a shift in the concentration levels of the Earth's oceans. At around 2300 Ma, the values of the relative abundance of iron-56 isotopes (δ⁵⁶Fe) increased in oceans by up to 3‰, compared to those in the Archean Era. This has been correlated with the oceanic deep stratification and an increase in sulfide precipitation, compared to iron oxide precipitation, as zero or slightly positive δ⁵⁶Fe values are characteristic of seawater under an oxygenated atmosphere. Today, the δ⁵⁶Fe values are no longer below –0.5‰, whereas in the Archean, they could still fall to –3.5‰.^[49][50]

The increase in δ⁵⁶Fe values has caused a reduced influence on iron cycling in open seawater. This has been interpreted as an impairment of dissimilatory iron reduction due to the titration of reactive iron. As a result, bacterial sulfate reduction increased, which consequentially led to a rise in sulfide concentrations.^[51]

With the beginning of the Siderian Period, there was an increase in the sulfate concentration of seawater, recognizable by the sulfur-34 isotope (δ³⁴S) values. In the course of bacterial sulfate reduction, this led to significant sulfur isotope fractionations, with an excess amount of sulfate.^[52] However, the sulfate concentrations remained at 1–2 millimoles per liter, and were much lower than today's concentrations of 28 millimoles per liter.^[53] The cause is now considered to be the increased oxidative weathering on the continents. Rocks spanning 2322±15 million years of age that do not exhibit sulfur-mass independent fractionation indicate that the atmosphere's oxygen concentration had already exceeded 10⁻⁵ times the present atmospheric level.^[54] At the same time, very strongly negative carbon-13 isotope (δ¹³C) values in the Lower Timeball Hill Formation of South Africa are considered a definitive indication of the presence of sulfate (anhydrite) in seawater and its bacterial reduction.^[55]