Most continential activity in this period was met with the breakup of the supercontinentKenorland. While this event mainly occurred in the cratonLaurentia, volcanic intrusions anddike swarms have appeared in plates pertaining toNorthwestern Europe,South Africa, andAustralia in connection to the rifting. In the ocean, emissions fromhydrothermal vents contributed to the production andcrystallization of minerals, along with varying concentrations ofsulfur andiron. While this sedimentation circulated in the ocean, the amounts deposited on the ocean floor contributed to the development ofbanded iron formations, along with a diverse range of ores such aspyrite andmagnetite.
Cyanobacteria continued to develop their molecular structure, witheukaryotes beginning to appear near the end of the period, and they contributed to the ocean'soxidation. Their presence eventually became the partial cause for the build-up ofoxygen in Earth's atmosphere, becoming known as theGreat Oxidation Event. This led to a decrease inmethane andcarbon dioxide, which were two majorgreenhouse gases at the time, dropping the overall global temperature below 0 °C. As a result of this, the Earth experienced threesnowball events which have been collectively known as theHuronian glaciation.
The nameSiderian is derived from theGreek wordsideros, meaning "iron", and refers to thebanded iron formations formed during this period.[7] Before its use, the period was suggested to be named as theHuronian Era[b] with a boundary from 2450 to 2200 Ma, in correlation to the sedimentary record of Canada'sHuronian Supergroup. Despite the stratigraphic use of the term "Huronian" since the nineteenth century,[9][10] 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 theInternational Union of Geological Sciences as a subdivision of theProterozoic Eon.[4]
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, Kranendonket al. proposed to set the Siderian Period to an earlier date range, due to the slow expansion of the period's continential plates, spanning from 2630 to 2420 Ma as the final subdivision of theNeoarchean Era.[11] They also suggested adjusting the upper half of the Siderian's preceding definition to occur from 2420 to 2250 Ma as theOxygenian Period, in response to the change in Earth's atmosphere during this time.[12] In 2021, Shieldset al. presented a similar alteration, but with the Siderian ending at about 2450 Ma, and the first period of thePaleoproterozoic Era termed theSkourian 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.[3] As of December 2024[update], the Siderian retains its 1989 definition, and is the earliest internationally recognized period on Earth'sgeologic time scale.[13]
Some depositional activity in what would become present-dayAustralia involved a selection ofsupersequences, 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 ofmudrock and sediments from banded iron formations, which have been deposited during rising sea levels and times of volcanic activity.[16] Additionally, there have been traces ofsulfur isotopes found in this sequence's Brockman Iron Formation, indicating a rise in the atmosphere'soxygen at the time.[17] The Woongarra Supersequence followed, consisting of depositions mainly fromrhyolite, but with layers ofdiabase andbasalt present beforehand, dating back to 2449 Ma.[18] It was then capped by the Turee Creek Supersequence, which presents itself with a layer ofsandstone andlimestone sequences, and lasted from 2449 to 2410 Ma before a stratigraphic hiatus occurred.[16]: 982
In northwestern Europe, evidence of deposition has been found in theBaltic Shield, within the lower portion of the Karelian Supergroup. Volcanic sediments frommafic complexes have been layered withporphyry containing bluequartz within the Sumian Group, being deposited during the first half of the Sideran.[28] In the following sequence,saprolite andpyroclastics have been found in the Sarolian Group, with traces spanning from 2400 to 2300 Ma.[29]Diamictites have also been identified in this sequence, providing evidence of glacial deposits occurring in the area.[30]
The lower half of theTransvaal Supergroup inSouth Africa has been deposited for the duration of the Siderian Period.[31] It mainly consists of a patterned sequence ofshale, sandstone, andsiltstone, and is embedded with an additional diversity of sediments.[32] Traces ofhematite,magnetite, andmarl can be found in the Eastern Transvaal Basin,[33] while the Griqualand West Basin holds instances ofgraywacke, ironlutite, andhyaloclastite deposits.[34] Some sequences have been deposited as banded iron formations, such as those presented through the Penge and Doradale formations.[35] However, different types of BIFs are also present in this supergroup, with granular,silicate, and orthochemical[c] iron formations existing in the Koegas Subgroup.[37]
Magma in the form ofdike swarms penetrated the surface of multiple cratons during the Siderian, taking place in some of the major continental plates such as those spanningNorth America,South Africa, andAustralia. About 2470 Ma, theMistassini dike swarm penetrated theSuperior Craton.[42] With a surface area of at least 100,000 square kilometers (39,000 sq mi), it can be classified as alarge igneous province (LIP).[43] It was followed by theMatachewan dike swarm, an LIP occurring from about 2470 to 2450 Ma, and spanning a surface area of at least 300,000 square kilometers.[44] The Mistassini and Matachewan swarms can be genetically associated with each other, as the Matachewan swarm intruded into the Superior Craton in the area betweenLake Superior andJames Bay.[45] In the region pertaining to present-dayScotland, the Scourie dike swarm penetrated theLewisian Gneiss Complex from about 2418 to 2375 Ma,[46] while the Widgiemooltha dike swarm intruded into theYilgarn Craton in Australia at around 2410 Ma.[47] The Widgiemooltha swarm occurred in close proximity to the Sebanga Poort dike's intrusion into theZimbabwe Craton, which occurred about 2408 Ma.[48]
Tectonic rifting began separating the supercontinentKenorland at around 2450 Ma, with the breakup mainly occurring inLaurentia.[49] As a result, the Hurwitz Group innorthern 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.[50] Additionally, lowsulfidation deposits holding copper and nickel began to form in the Nena andKalahari cratons,[49] whilezircons formed within the Deep Lake Group in what is now theSierra Madre Range.[51][52] Despite the intrusions contributing to the rifting, Kenorland experienced little continental 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 theKola Peninsula.[53][54]
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 theGreat 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 × 10−6bar to at least 2 × 10−3 bar between 2410 and 2320 Ma.[5][55] As a result, the rapid change came at the expense of greenhouse gases such ascarbon dioxide andmethane, indirectly leading to a series of ice ages known as theHuronian glaciation.[56]
The levels ofcarbonates and organic carbon have been relatively stagnant. The abundance of carbon-13 isotopes (δ13C), found withindolomites and formations in theMount Bruce,Transvaal, andHuronian supergroups, has maintained a steady level of 0‰ in carbonates, while organic carbon created through the activity and burial ofcyanobacteria remained stationary at approximately −28‰.[57][58] Although this may present itself as a sign of inactivity during this period, it suggests that there have been multiple sources causing an equal force of sinks and rises in the levels of oxygen.[59] This includes the influx and settlement of carbon dioxide from volcanic activity which stems from tectonic processes,[60] along with the delivery ofphosphate to oceans through cycles of chemical weathering.[61]
As a consequence of the excess oxygen, a shift began to occur in the level and activity ofgreenhouse 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.[62][63] However, this process depleted the amounts of methane by a significant amount, dropping from 300 to 4ppmv.[64] Despite the balance in carbon dioxide, the significant change in methane caused Earth to undergo asnowball event, dropping the average global temperatures below 0 °C.[65]
Due to the loss of global temperature, the Earth entered theHuronian glaciation, which lasted from about 2450 to 2200 Ma.[66] 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.[67] Traces of this glaciation have been found in thediamictites and sequences of six cratons,[68] including theWyoming,Kaapvaal, and Karelia-Kola cratons.[67]
The oldest glaciation correlates to quartz located in the Campbell Lake and Headquarters formations,[69] along with glacial deposits in the Polisarka Formation.[70] It lasted from about 2440 to 2420 Ma,[67] and is generally referred to after the diamictites found in the Duitschland Formation.[71] The second glaciation, known as the Makganyene glaciation after its eponymous formation, is marked bycap carbonate sequences found above the Bruce and Vagner formations,[72] occurring from about 2380 to 2360 Ma.[67] The youngest of the three glaciations occurs from about 2340 to 2310 Ma near the end of the Siderian,[67] represented in the Gowganda Formation of theHuronian Supergroup, and referred to after the Rietfontein diamictite located inSouth Africa.[73]
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 thePseudanabaena genus began chaining themselves in afilamentous structure,[74] andGiardia, one of the earliesteukaryotes, emerged at around 2309 Ma.[75][76] Additionally,flagellated microorganisms began to develop in the ocean's crust, appearing at about 2400 Ma.[77]
Traces of cyanobacteria have made marks in a few deposition sites.Microfossils embedded inblack chert have been dated back to 2450 Ma in Australia'sTuree Creek Group,[78] while bacterial remnants from theConophyton andSiphonophycus genus have been preserved in South Africa's Kuruman Iron Formation.[79] In China,stromatolites have been spotted in the Dashiling and Qingshicun formations of the Hutuo Group, existing for the duration of the Siderian Period.[80] Additionally, findings in theFennoscandian Shield show that the taxonomy of stromatolites began to diversify at around 2330 Ma.[81]
There have been signs offungus-like organisms appearing at about 2400 Ma within the cracks and vesicles of filamentous structures.[82] Open spaces and cavities below the seafloor have led to the development of root-like structures such ashyphae andmycelia, and have been preserved inbasalt andclay within the Ongeluk Formation in South Africa.[77][83] This has raised suggestions to the preexistence of a stable environment for fungal development, as evidenced by the fossil's similarities withvolcanic pillows from theDevonian Period.[84]
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,[85] increasing its isotopic56Fe/54Fe ratio (δ56Fe) by up to 3‰, compared to values in theNeoarchean Era.[86] Some of the iron present was oxidized intoiron(II) oxide andiron(III) oxide, either through the bacterial process ofdisimilatory iron reduction, or by the presence of oxygen in its aqueous form.[85] 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 assiderite,magnetite, andgreenalite.[87]
Before the Great Oxidation Event, sulfur was mainly supplied as sulfide through the volcanic outgassing ofhydrogen sulfide andsulfur dioxide.[88] These molecules were then deposited into the anoxic seawater at concentrations of 1–2 mM, withsulfide minerals such aspyrite being created as a result,[89][90] and sulfate being oxidized from the aqueous solution.[91] 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.[92][93] At the time, most of the sulfate available converted into sulfide through processes of sulfate reduction, such as being recycled back into the mantle,[88] or by conversion via microbial activity.[94]
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 asgypsum andanhydrite.[95] 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‰.[96][97]
The isotopic ratio of nitrogen (δ15N) was relatively constant during the Siderian, ranging from 1.1 to 7.7‰ between 2450 and 2300 Ma.[98][99] Some concentrations formed askerogen in South Africa's Timeball Hill Formation, while traces existed asshale in Australia'sTuree Creek Group.[100] Despite the stability of nitrogen carried out throughits circulation,[101] the oxidation of the ocean's surface water slowly increased the size ofnitrate reservoirs,[100] with seawater concentrations ranging from 0.35 to 3.5 μM.[102]
There have been fluctuations in the ocean's level ofstrontium. At the time, its87Sr/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,[103] along with the recrystallization ofcalcite anddolomite in the ocean's crust.[104] Nonetheless, the ratio's value began a trend of increase up until theOrosirian Period, beginning with a value of 0.7022 in 2500 Ma.[103] 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 inorganicaragonites hold values of 1000 and 9000 ppm respectively.[104]
Strontium has also been detected through thebeta decay andradiometric dating ofrubidium (Rb–Sr), and is mainly connected to the deposition of volcanic rocks. At theFennoscandian Shield, this isotopic presence has been found indacite and basalticandesite within the Pechenga–Varzuga Belt, dating back to 2324 Ma,[105][106] and indicates the creation ofpaleosols from an intense weathering period.[107] Similar Rb–Sr datings have been found in the Superior and Kaapvaal cratons; the 2330 Ma dating of volcanictuffs in North America's McKam Formation serves as one of the marks of the beginning of the Huronian glaciation,[108] while a 2300 Ma dating in South Africa represents a unconformity between the Transvaal and Ventersdrop supergroups.[109] Additionally, rubidium and strontium have been detected inmigmatite found in eastern Hebei's Qianxi Group, and are dated back to 2480 Ma.[110]
^In the context of iron formations, "orthochemical" refers to iron formations that are dominated by fine-grained iron-rich material, consisting of tiny rock grains that were formedin situ (i.e., formed where they were deposited, not transported to their place of deposition).[36]
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