Huronian glaciation

The Huronian glaciation (or Makganyene glaciation) was a period where several ice ages occurred during the deposition of the Huronian Supergroup, rather than a single continuous event as it is commonly misrepresented to be. The deposition of this group extended from 2.5 billion years ago (Gya) to 2.2 Gya, during the Siderian and Rhyacian periods of the Paleoproterozoic era. This led to the deposition of several diamictites. Most of the deposits of the Huronian are typical passive margin deposits in a marine setting. The diamictites within the Huronian are on par in thickness with Quaternary analogs.

Evidence comes from glacial deposits identified within the stratigraphic record of the Huronian Supergroup. Within it are three distinct formations of diamictite, from the oldest to youngest, the Ramsay, Bruce, and Gowganda Formations. Although there are other glacial deposits recognized throughout the world, the Huronian is restricted to the North American Midwest. Other similar deposits are known from South Africa.

The Huronian glaciation broadly coincides with the Great Oxygenation Event (GOE), a time when increased atmospheric oxygen decreased atmospheric methane. The oxygen reacted with the methane to form carbon dioxide and water, both much weaker greenhouse gases than methane, greatly reducing the efficacy of the greenhouse effect, especially as water vapor readily precipitated out of the air with dropping temperature. This caused an icehouse effect and, possibly compounded by the low solar irradiation at the time as well as reduced geothermal activities, led to a global glaciation that essentially created a Snowball Earth. The combination of increasing free oxygen (which causes oxidative damage to organic compounds) and climatic stresses likely caused an extinction event, the first and longest lasting in the Earth's history, which wiped out most of the anaerobe-dominated microbial mats both on the Earth's surface and in shallow seas.

Discovery and name

In 1907, Arthur Philemon Coleman first inferred a "lower Huronian ice age" from analysis of a geological formation near Lake Huron in North America. This formation consists of two non-glacial sediment deposits found between three horizons of glacial deposits of the Huronian Supergroup, deposited between 2.5 and 2.2 Gya. Despite the name, the Huronian glaciation does not in fact represent a single glaciation.

The confusion of the terms glaciation and ice age has led to the more recent impression that the entire time period represents a single glacial event. 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. Diachronic or geochronometric units should be used.

Geology and climate

The Gondwana Formation (2.3 Gya) contains "the most widespread and most convincing glaciogenic deposits of this era", according to Eyles and Young. Similar deposits are found in Michigan (2.23–2.15 Gya), the Black Hills (2.6–1.6 Gya), Chibougamau, Canadian Northern Territories (2.1 Gya) and Wyoming. Similar age deposits occur in the Griquatown Basin (2.3 Gya), India (1.8 Gya) and Australia (2.5—2.0 Gya).

The tectonic setting was one of a rifting continental margin. New continental crust would have resulted in chemical weathering. This weathering would pull CO₂ out of the atmosphere, cooling the planet through the reduction in greenhouse effect.^{[citation needed]}

One or more of the glaciations may have been snowball earth events, when all or almost all of the earth was covered in ice. Although the palaeomagnetic evidence that suggests ice sheets were present at low latitudes is contested, and the glacial sediments (diamictites) are discontinuous, alternating with carbonate rocks and other sediments indicating temperate climates, providing scant evidence for global glaciation.

Implications of the Huronian

Before the Huronian Ice Age, most organisms were anaerobic, relying on chemosynthesis and retinal-based anoxygenic photosynthesis for production of biological energy and biocompounds. But around this time, cyanobacteria evolved porphyrin-based oxygenic photosynthesis, which produced dioxygen as a waste product. At first, most of this oxygen was dissolved in the ocean, and afterwards absorbed through the reduction by surface ferrous compounds, atmospheric methane and hydrogen sulfide. However, as the cyanobacterial photosynthesis continued, the cumulative oxygen oversaturated the reductive reservoir of the Earth's surface and spilt out as free oxygen that "polluted" the atmosphere, leading to a permanent change to the atmospheric chemistry known as the Great Oxygenation Event. The once-reducing atmosphere, now an oxidizing one, was highly reactive and toxic to the then-anaerobic biosphere. Further more, atmospheric methane was depleted by oxygen and reduced to trace gas levels, and replaced by much less powerful greenhouse gases such as carbon dioxide and water 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 reduced greenhouse effect and partly because solar luminosity and/or geothermal activities were also lower at that time, leading to an icehouse Earth.

After the combined impact of oxidization and climate change devastated the anaerobic biosphere (then likely dominated by archaeal microbial mats), aerobic organisms capable of oxygen respiration were able to proliferate rapidly and exploit the ecological niches vacated by anaerobes in most environments. The surviving anaerobe colonies were forced to adapt a symbiotic 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 begin invaginating their cell membranes into endomembranes in order to shield and protect the cytoplasmic nucleic acids, allowing endosymbiosis with aerobic eubacteria (which eventually became ATP-producing mitochondria), and this symbiogenesis contributed to the evolution of eukaryotic organisms during the Proterozoic.

See also

• Timeline of glaciation