Siderian

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

Most continental activity in this period was met with the breakup of the supercontinent Kenorland. While this event mainly occurred in the Laurentian craton, volcanic intrusions and dike swarms appeared in plates pertaining to Northwestern Europe, South Africa, and Australia in connection to the 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. Additionally, traces of sulfur isotopes have been found in this sequence's Brockman Iron Formation, indicating a rise in the atmosphere's oxygen at the time. The Woongarra Supersequence followed, consisting of depositions mainly from rhyolite, but with layers of diabase and basalt present beforehand, dating back to 2449 Ma. It was then capped by the Turee Creek Supersequence, which presents itself with a layer of sandstone and limestone sequences, and lasted from 2449 to 2410 Ma before a stratigraphic hiatus occurred. With a surface area of at least , it can be classified as a large igneous province (LIP). It was 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 intruded into the Superior Craton in the area between Lake Superior and James Bay. In the region pertaining to present-day Scotland, the Scourie dike swarm penetrated the Lewisian Gneiss Complex from about 2418 to 2375 Ma, while the Widgiemooltha dike swarm intruded into the Yilgarn Craton in Australia 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.

Breakup of Kenorland

Tectonic rifting began separating the supercontinent Kenorland at around 2450 Ma, with the breakup mainly occurring in Laurentia. As a result, the Hurwitz Group in northern Canada experienced continental stretching and depression, resulting in the depositions of the Noomut, Padlei, and Kinga Formations, along with the creation of the Hurwitz Basin. Additionally, low sulfidation deposits holding copper and nickel began to form in the Nena and Kalahari cratons, while zircons formed within the Deep Lake Group in what is now the Sierra Madre Range. Despite the intrusions contributing to the rifting, Kenorland experienced little continental movement, and there are 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.

Climate

Great Oxidation Event

thumb|right|upright=1.3|alt=A graph of changing atmosphere levels. Oxygen experiences a sharp increase, carbon dioxide and methane steadily decrease, and nitrogen remains relatively constant.|[[Semi-log plot|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. Although this may present itself as a sign of inactivity during this period, it suggests that there were multiple sources causing an equal force of sinks and rises in the levels of oxygen. This includes the influx and settlement of carbon dioxide from volcanic activity which stems from tectonic processes, along with the delivery of phosphate to oceans through cycles of chemical weathering.

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. However, this process depleted the amounts of methane by a significant amount, dropping from 300 to 4 ppmv. 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.

Huronian glaciation

Due to the loss of global temperature, the Earth entered the Huronian glaciation, which lasted from about 2450 to 2200 Ma. 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. Traces of this glaciation have been found in the diamictites and sequences of six cratons, including the Wyoming, Kaapvaal, and Karelia-Kola cratons.

The oldest glaciation correlates to quartz located in the Campbell Lake and Headquarters formations, along with glacial deposits in the Polisarka Formation. It lasted from about 2440 to 2420 Ma, and is generally referred to after the diamictites found in the Duitschland Formation. 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, occurring from about 2380 to 2360 Ma. The youngest of the three glaciations occurs from about 2340 to 2310 Ma near the end of the Siderian, represented in the Gowganda Formation of the Huronian Supergroup, and referred to after the Rietfontein diamictite located in South Africa.

Life

By the beginning of the Great Oxidation Event, cyanobacteria developed intercelluar communication through molecular exchange, and began to differentiate from each other. Strands such as those in the Pseudanabaena genus began chaining themselves in a filamentous structure, and Giardia, one of the earliest eukaryotes, emerged at around 2309 Ma. Additionally, flagellated microorganisms began to develop in the ocean's crust, appearing at about 2400 Ma.

Traces of cyanobacteria have made marks in a few deposition sites. Microfossils embedded in black chert have been dated back to 2450 Ma in Australia's Turee Creek Group, while bacterial remnants from the Conophyton and Siphonophycus genus have been preserved in South Africa's Kuruman Iron Formation. In China, stromatolites have been spotted in the Dashiling and Qingshicun formations of the Hutuo Group, existing for the duration of the Siderian Period. Additionally, findings in the Fennoscandian Shield show that the taxonomy of stromatolites began to diversify at around 2330 Ma.

There are signs of fungus-like organisms appearing at about 2400 Ma within the cracks and vesicles of filamentous structures. Open spaces and cavities below the seafloor led to the development of root-like structures such as hyphae and mycelia, which have been preserved in basalt and clay within the Ongeluk Formation in South Africa. This has raised questions as to the preexistence of a stable environment for fungal development, as evidenced by the fossil's similarities with volcanic pillows from the Devonian Period.

Marine geochemistry

Isotopic composition of iron

The fluctuation of iron in seawater was met with an increase in the creation and deposition of iron oxides and ferrous minerals. Hydrothermal vents served as the ocean's primary source of iron, increasing its isotopic 56Fe/54Fe ratio (δ56Fe) by up to 3‰, compared to values in the Neoarchean Era. Some of the iron present was oxidized into iron(II) oxide and iron(III) oxide, either through the bacterial process of disimilatory iron reduction, or by the presence of oxygen in its aqueous form. Isotopes with a particularly heavy δ56Fe value, however, deposited in iron reservoirs before 2400 Ma, which would develop into banded iron formations holding traces of ores such as siderite, magnetite, and greenalite.

Isotopic composition of sulfur

Before the Great Oxidation Event, sulfur was mainly supplied as sulfide through the volcanic outgassing of hydrogen sulfide and sulfur dioxide. These molecules were then deposited into the anoxic seawater at concentrations of 1–2 mM, with sulfide minerals such as pyrite being created as a result, and sulfate being oxidized from the aqueous solution. Due to the lack of oxygen, however, there was a very minimal amount of sulfate in circulation, falling within a concentration of 5–200 μM before 2400 Ma. At the time, most of the sulfate available converted into sulfide through processes of sulfate reduction, such as being recycled back into the mantle, or by conversion via microbial activity.

As oxygen began to rapidly accumulate in the atmosphere, sulfate levels began to increase in the seawater and sedimentary reservoirs, while the circulation of sulfide decreased as a result. Between 2500 and 2300 Ma, the isotopic ratio of sulfate (δ34S) increased from 10 to 12‰ as a result of aerobic weathering and precipitation, entering the sedimentary record as gypsum and anhydrite. At the same time, the levels of sulfide experienced decreases as a result of the spike in oxygen, with its δ34S value reaching as low as −30‰.

Isotopic composition of nitrogen

The isotopic ratio of nitrogen (δ15N) was relatively constant during the Siderian, ranging from 1.1 to 7.7‰ between 2450 and 2300 Ma. Some concentrations formed as kerogen in South Africa's Timeball Hill Formation, while traces existed as shale in Australia's Turee Creek Group. Despite the stability of nitrogen carried out through its circulation, the oxidation of the ocean's surface water slowly increased the size of nitrate reservoirs, with seawater concentrations ranging from 0.35 to 3.5 μM.

Composition of strontium and rubidium

There were fluctuations in the ocean's level of strontium. At the time, its 87Sr/86Sr isotopic ratio was relatively balanced; while its sources involved periods of high weathering rates, its sinks were due to the input of strontium from hydrothermal ventilation, along with the recrystallization of calcite and dolomite in the ocean's crust. Nonetheless, the ratio's value began a trend of increase up until the Orosirian Period, beginning with a value of 0.7022 in 2500 Ma. Traces of this strontium have been identified within the Polisarka Formation's bedding. The concentrations in carbonate rocks ranged between 560 and 1030 ppm, dating between 2441 and 2434.8 Ma, while calcites and inorganic aragonites hold values of 1000 and 9000 ppm respectively.

Strontium has also been detected through the beta decay and radiometric dating of rubidium (Rb–Sr), and is mainly connected to the deposition of volcanic rocks. At the Fennoscandian Shield, this isotopic presence has been found in dacite and basaltic andesite within the Pechenga–Varzuga Belt, dating back to 2324 Ma, and indicates the creation of paleosols from an intense weathering period. Similar Rb–Sr datings have been found in the Superior and Kaapvaal cratons; the 2330 Ma dating of volcanic tuffs in North America's McKam Formation serves as one of the marks of the beginning of the Huronian glaciation, while a 2300 Ma dating in South Africa represents a unconformity between the Transvaal and Ventersdrop supergroups. Additionally, rubidium and strontium have been detected in migmatite found in eastern Hebei's Qianxi Group, and are dated back to 2480 Ma.

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