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TheHuronian glaciation (orMakganyene glaciation)[1] was a period where at least threeice ages occurred during the deposition of theHuronian Supergroup.Deposition of this largely sedimentary succession extended from approximately 2.5 to 2.2 billion years ago (Gya), during theSiderian andRhyacian periods of thePaleoproterozoic era. Evidence forglaciation is mainly based on the recognition ofdiamictite, that is interpreted to be of glacial origin. Deposition of the Huronian succession is interpreted to have occurred within arift basin that evolved into a largely marine passive margin setting.[2] The glacial diamictite deposits within the Huronian are on par in thickness with Quaternary analogs.
The three glacial diamictite-bearing units of the Huronian are, from the oldest to youngest, the Ramsay Lake, Bruce, and Gowganda formations. Although there are other glacial deposits recognized throughout the world at this time, the Huronian is restricted to the region north ofLake Huron, betweenSault Ste. Marie, Ontario, andRouyn-Noranda, Quebec. Other similar deposits are known from elsewhere in North America, as well as Australia and South Africa.[3]
The Huronian glaciation broadly coincides with theGreat Oxygenation Event, a time of increasedatmospheric oxygen and decreasedatmospheric methane. The oxygen reacted with the methane to formcarbon dioxide and water, both much weakergreenhouse gases than methane, greatly reducing the efficacy of thegreenhouse effect, especially aswater vapor readily precipitated out of the air with dropping temperature.[4] This caused anicehouse effect and, possibly compounded by the lowsolar irradiation at the time as well as reducedgeothermal activities, the combination of increasing free oxygen (which causesoxidative damage toorganic compounds) and climatic stresses likely caused anextinction event, the first and longest lasting in the Earth's history, which wiped out most of theanaerobe-dominatedmicrobial mats both on theEarth's surface and inshallow seas.[5][6]
In 1907,Arthur Philemon Coleman first inferred a "lower Huronian ice age"[7][8] from analysis of a geological formation near Lake Huron in Ontario. In his honour, the lower (glacial) member of the Gowganda Formation is referred to as the Coleman member. These rocks have been studied in detail by numerous geologists and are considered to represent the type example of a Paleoproterozoic glaciation.[9][10]
The confusion of the termsglaciation andice age has led to the more recent impression that the entire time period represents a single glacial event.[11] The term Huronian is used to describe a lithostratigraphic supergroup and should not be used to describe glacial cycles, according to The North American Stratigraphic Code, which defines the proper naming of geologic physical and chrono units.[12]Diachronic orgeochronometric units should be used.
The Gowganda Formation (2.3 Gya) contains "the most widespread and most convincing glaciogenic deposits of this era", according to Eyles and Young. In North America, similar-age deposits are exposed in Michigan, theMedicine Bow Mountains, Wyoming,Chibougamau, Quebec, and central Nunavut. Globally, they occur in theGriquatown Basin of South Africa, as well as India and Australia.[13]
Thetectonic setting was one of a riftingcontinental margin. Newcontinental crust would have resulted in chemicalweathering. This weathering would pull CO2 out of the atmosphere, cooling the planet through the reduction ingreenhouse effect.[citation needed]
Popular perception is that one or more of the glaciations may have beensnowball Earth events, when all or most of Earth's surface was covered in ice.[11][14][15] However thepalaeomagnetic evidence that suggests ice sheets were present at low latitudes is contested,[16][17] and the glacial sediments (diamictites) are discontinuous, alternating with carbonate and other sedimentary rocks, indicating temperate climates, providing scant evidence for global glaciation.
Before the Huronian Ice Age, most organisms wereanaerobic, relying onchemosynthesis andretinal-basedanoxygenic photosynthesis for production ofbiological energy andbiocompounds. But around this time,cyanobacteria evolvedporphyrin-basedoxygenic photosynthesis, which produceddioxygen as a waste product. At first, most of this oxygen was dissolved in the ocean and afterwards absorbed through thereduction by surfaceferrous compounds,atmospheric methane andhydrogen sulfide. However, as the cyanobacterial photosynthesis continued, the cumulative oxygen oversaturated the reductive reservoir of the Earth's surface[11] and spilled out as free oxygen that "polluted" the atmosphere, leading to a permanent change to theatmospheric chemistry known as theGreat Oxygenation Event.
The once-reducing atmosphere, now an oxidizing one, was highly reactive and toxic to the anaerobicbiosphere. Furthermore, atmospheric methane was depleted by oxygen and reduced totrace gas levels, and replaced by much less powerfulgreenhouse gases such ascarbon dioxide andwater vapor, the latter of which was also readily precipitated out of the air at low temperatures. Earth's surface temperature dropped significantly, partly because of the reducedgreenhouse effect and partly becausesolar luminosity and/orgeothermal activities were also lower at that time,[6] leading to anicehouse Earth.
After the combined impact of oxidization andclimate change devastated the anaerobic biosphere (then likely dominated byarchaealmicrobial mats),aerobic organisms capable ofoxygen respiration were able to proliferate rapidly and exploit theecological niches vacated by anaerobes in most environments. The surviving anaerobe colonies were forced to adapt asymbiotic living among aerobes, with the anaerobes contributing the organic materials that aerobes needed, and the aerobes consuming and "detoxing" the surrounding of oxygen molecules lethal to the anaerobes. This might have also caused some anaerobic archaea to begininvaginating theircell membranes intoendomembranes in order to shield and protect thecytoplasmicnucleic acids, allowingendosymbiosis with aerobiceubacteria (which eventually becameATP-producingmitochondria), and thissymbiogenesis contributed to the evolution ofeukaryotic organisms during theProterozoic.[citation needed]
