Late Ordovician glaciation

The Andean-Saharan glaciation, also known as the Early Palaeozoic Icehouse, the Early Palaeozoic Ice Age, the Late Ordovician glaciation, the end-Ordovician glaciation, or the Hirnantian glaciation, occurred during the Paleozoic from approximately 460 Ma to around 420 Ma, during the Late Ordovician and the Silurian period. The major glaciation during this period, which was formerly thought only to consist of the Hirnantian glaciation itself, but has now been recognized as a longer, more gradual event that began as early as the Darriwilian, is widely considered to be the leading cause of the Ordovician-Silurian extinction event. Evidence of this glaciation can be seen in places such as Arabia, North Africa, South Africa, Brazil, Peru, Bolivia, Chile, Argentina, and Wyoming. More evidence derived from isotopic data is that during the Late Ordovician, tropical ocean temperatures were about 5 °C cooler than present day; this would have been a major factor that aided in the glaciation process.

The Late Ordovician is the only glacial episode that appears to have coincided with a major mass extinction of nearly 61% of marine life. Estimates of peak ice sheet volume range from 50 to 250 million cubic kilometers, and its duration from 35 million to less than 1 million years. There were also two peaks of glaciation. Glaciation of the Northern Hemisphere was minimal because a large amount of the land was in the Southern Hemisphere.

Description

Over an interval of 30 million years, seven glacial maxima were recorded in the sedimentary record:

# ''Guttenberg Regression''
# ''Early Rakvere Regression''
# ''Early Ashgill Regression''
# ''Hirnantian Glaciation''
# ''Early Ashgill Regression''
# ''Early Aeronian Glaciation''
# ''Late Telychian Glaciation''

According to Eyles and Young, "A major glacial episode at c. 440 Ma, is recorded in Late Ordovician strata (predominantly Ashgillian) in West Africa (Tamadjert Formation of the Sahara), in Morocco ( Tindouf Basin) and in west-central Saudi Arabia, all areas at polar latitudes at the time. From the Late Ordovician to the Early Silurian the centre of glaciation moved from northern Africa to southwestern South America."

During this period glaciation is known from Arabia, Sahara, West Africa, the south Amazon, and the Andes. The center of glaciation migrated from Sahara in the Ordovician (450–440 Ma) to South America in the Silurian (440–420 Ma). The maximum extent of glaciation developed in Africa and eastern Brazil.

In western South America (Peru, Bolivia and northern Argentina) were found glacio-marine diamictites interbedded with turbidites, shales, mud flows and debris flows, dated as early Silurian (Llandonvery), with a southward extension into northern Argentina and western Paraguay, and with a probably northern extension into Peru, Ecuador and Colombia.

A major ice age, the Andean-Saharan was preceded by the Cryogenian ice ages (720–630 Ma, the Sturtian and Marinoan glaciations), often referred to as Snowball Earth, and followed by the Karoo Ice Age (350–260 Ma).

Evidence

Isotopic

* Isotopic evidence points to a global Hirnantian positive shift in marine carbonate ¹⁸O, and at nearly the same time a positive shift in ¹³C known as the Hirnantian Isotopic Carbon Excursion. This evidence is further aided by the observation that both ¹⁸O and ¹³C fall sharply at the beginning of the Silurian.
* The direction of the ¹⁸O shift can imply glacial-cooling and possibly increases in ice-volume, and the magnitude of this shift (+4‰) was extraordinary. The direction and magnitude of the ¹⁸O isotopic indicator would require a sea-level fall of 100 meters and a drop of 10 °C in tropical ocean temperatures.
* The positive shift in ¹³C implies a change in the carbon cycle leading to more burial of carbon, or at the very least production of more carbon with the removal of ¹²C in surface waters. This decrease points toward a decrease in the atmospheric CO₂ levels which would have an inverse greenhouse effect, which would allow glaciation to occur more readily.

Lithologic

* Sedimentological data shows that Late Ordovician ice sheets glacierized the Al Kufrah Basin. Ice sheets also probably formed continuous ice cover over North African and the Arabian Peninsula. In all areas of North African where Early Silurian shale occurs, Late Ordovician glaciogenic deposits occur beneath, likely due to the anoxia promoted in these basins.
* From what we know about tectonic movement, the time span required to allow the southward movement of Gondwana toward the South Pole would have been too long to trigger this glaciation. Tectonic movement tends to take several million years, but the scale of the glaciation seems to have occurred in less than 1 million years, but the exact time frame of glaciation ranges from less than 1 million years to 35 million years, so it could still be possible for tectonic movement to have triggered this glacial period.
* The sequence of the stratigraphic architecture of the Bighorn Dolomite (which represents end of the Ordovician period), is consistent with the gradual buildup of glacial ice. The sequences of the Bighorn Dolomite display systematic changes in their component cycles, and the changes in these cycles are interpreted as being a change from a greenhouse climate to a transitional ice house climate.

Biostratigraphic

* Graptolite distribution during the time interval delineated by the ''Nemacanthus gracilis'' graptolite biozone indicates a latitudinal extent of the subtropics and tropics similar to that of today, as evidenced by a steep faunal gradient that is uncharacteristic of greenhouse periods, suggesting that Earth was in a mild icehouse state by around 460 Ma.
* Although biostratigraphy dating the glacial deposits in Gondwana has been problematic, some evidence suggested an onset of glaciation as early as the Sandbian Stage (approximately 451–461 Ma).

Possible causes

CO₂ depletion

One of the factors that hindered glaciation during the early Paleozoic was atmospheric CO₂ concentrations, which at the time were somewhere between 8 and 20 times pre-industrial levels. However, CO₂ concentrations are thought to have dropped significantly in the Hirnantian, which could have induced widespread glaciation during an overall cooling trend. Methods for the removal of CO₂ during this time were not well known, and are still hotly debated, with the radiation of terrestrial plants, a reduction in volcanic outgassing of carbon dioxide, and astronomically-induced increased marine absorption of carbon dioxide having been proposed. It could have been possible for glaciation to initiate with high levels of CO₂, but it would have depended highly on continental configuration.

Long-term silicate weathering is a major mechanism through which CO₂ is removed from the atmosphere, converting it into bicarbonate which is stored in marine sediments. This has often been linked to the Taconic Orogeny, a mountain-building event on the east coast of Laurentia (present-day North America). In more recent prehistory, the collision of India with Asia, and the subsequent creation of the Himalayas, has been proposed as a driver of late Cenozoic cooling. Another hypothesis is that a hypothetical large igneous province in the Katian led to basaltic flooding caused by high continental volcanic activity during that period. In the short term, this would have released a large amount of CO₂ into the atmosphere, which may explain a warming pulse in the Katian. However, in the long term flood basalts would have left behind plains of basaltic rock, replacing exposures of granitic rock. Basaltic rocks weather substantially faster than granitic rocks, which would quickly remove CO₂ from the atmosphere at a much faster rate than before the volcanic activity. CO₂ levels could also have decreased due to accelerated silicate weathering caused by the expansion of terrestrial non-vascular plants. Vascular plants only appeared 15 Ma after the glaciation.

Ordovician meteor event

The breakup of the L-chondrite parent body caused a rain of extraterrestrial material onto the Earth called the Ordovician meteor event. This event increased stratospheric dust by 3 or 4 orders of magnitude and may have triggered the ice age by reflecting sunlight back into space.

Volcanic aerosols

Although volcanic activity often leads to warming through the release of greenhouse gasses, it may also lead to cooling via the production of aerosols, light-blocking particles. There is good evidence for elevated volcanic activity through the Hirnantian, based on anomalously high concentrations of mercury (Hg) in many areas. Sulphur dioxide (SO₂) and other sulphurous volcanic gasses are converted into sulphate aerosols in the stratosphere, and short, periodic large igneous province eruptions may be able to account for cooling in this way. Although there is no direct evidence for a large igneous province during the Hirnantian, volcanism could still be a major factor. Explosive volcanic eruptions, which regularly send debris and volatiles into the stratosphere, would be even more effective at producing sulfate aerosols. Ash beds are common in the Late Ordovician, and Hirnantian pyrite records sulphur isotope anomalies consistent with stratospheric eruptions. The enormous megaeruption that formed the Deicke bentonite layer in particular has been linked to global cooling due to it coinciding with a major positive oxygen isotope excursion and the high sulphur concentration observed in its bentonite layer.

Sea level change

One of the possible causes for the temperature drop during this period is a drop in sea level. Sea level must drop prior to the initiation of extensive ice sheets in order for it to be a possible trigger. A drop in sea level allows more land to become available for ice sheet growth. There is wide debate on the timing of sea level change, but there is some evidence that a sea level drop started before the Ashgillian, which would have made it a contributing factor to glaciation.

Poleward ocean heat transport

Ocean heat transport is a major driver in the warming of the poles, taking warm water from the equator and distributing it to higher latitudes. A weakening of this heat transport may have allowed the poles to cool enough to form ice under high CO₂ conditions. Due to the paleogeographic configuration of the continents, global ocean heat transport is thought to have been stronger in the Late Ordovician. However, research shows that in order for glaciation to occur, poleward heat transport had to be lower, which creates a discrepancy in what is known.

Paleogeography

The possible setup of the paleogeography during the period from 460 Ma to 440 Ma falls in a range between the Caradocian and the Ashgillian. The choice of setup is important, because the Caradocian setup is more likely to produce glacial ice at high CO₂ concentrations, and the Ashgillian is more likely to produce glacial ice at low CO₂ concentrations.

The height of the land mass above sea level also plays an important role, especially after ice sheets have been established. A higher elevation allows ice sheets to remain with more stability, but a lower elevation allows ice sheets to develop more readily. The Caradocian is considered to have a lower surface elevation, and though it would be better for initiation during high CO₂, it would have a harder time maintaining glacial coverage.

Orbital parameters

Orbital parameters may have acted in conjunction with some of the above parameters to help start glaciation. The variation of the earth's precession, and eccentricity, could have set the off the tipping point for initiation of glaciation. The Orbit at this time is thought to have been in a cold summer orbit for the southern hemisphere. This type of orbital configuration is a change in the orbital precession such that during the summer when the hemisphere is tilted toward the sun (in this case the earth) the earth is furthest away from the sun, and orbital eccentricity such that the orbit of the earth is more elongated which would enhance the effect of precession.

Coupled models have shown that in order to maintain ice at the pole in the southern hemisphere, the earth would have to be in a cold summer configuration. The glaciation was most likely to start during a cold summer period because this configuration enhances the chance of snow and ice surviving throughout the summer.

End of the event

The cause for the end of the Late Ordovician Glaciation is a matter of intense research, but evidence shows that it may have occurred abruptly, as Silurian strata marks a significant change from the glacial deposits left during the Late Ordovician. Most evidence points to an abrupt change rather than a gradual change.

Ice collapse

One of the possible causes for the end of this glacial event is during the glacial maximum, the ice reached out too far and began collapsing on itself. The ice sheet initially stabilized once it reached as far north as Ghat, Libya and developed a large proglacial fan-delta system. A glaciotectonic fold and thrust belt began to form from repeated small-scale fluctuations in the ice. The glaciotectonic fold and thrust belt eventually led to ice sheet collapse and retreat of the ice to south of Ghat. Once stabilized south of Ghat, the ice sheet began advancing north again. This cycle slowly shrank more south each time which lead to further retreat and further collapse of glacial conditions. This recursion allowed the melting of the ice sheet, and rising sea level. This hypothesis is supported by glacial deposits and large land formations found in Ghat, Libya which is part of the Murzuq Basin.

CO₂

As the Ice sheets began to increase the weathering of silicate rocks and basaltic important to carbon sequestration (the silicates through the Carbonate–silicate cycle, the basalt through forming calcium carbonate) decreased, which caused levels to rise again, this in turned helped push deglaciation. This deglaciation cause the transformation of silicates exposed to the air (thus given the opportunity to bind to its ) and weathering of basaltic rock to start back up which caused glaciation to occur again.

Significance

Even before the mass extinction at the end of the Ordovician, the biodiversity of chitinozoans was adversely impacted by the onset of the Andean-Saharan glaciation. Following a peak in diversity in the late Darriwilian, chitinozoans declined in diversity as the Late Ordovician progressed. An exception to this declining trend of chitinozoan diversity was exhibited in Laurentia due to its low latitude position and warmer climate.

The Late Ordovician Glaciation coincided with the second largest of the five major extinction events, known as the Ordovician–Silurian extinction event. This period is the only known glaciation to occur alongside of a mass extinction event. The extinction event consisted of two discrete pulses. The first pulse of extinctions is thought to have taken place because of the rapid cooling, and increased oxygenation of the water column. This first pulse was the larger of the two and caused the extinction of most of the marine animal species that existed in the shallow and deep oceans. The second phase of extinction was associated with strong sea level rise, and due to the atmospheric conditions, namely oxygen levels being at or below 50% of present-day levels, high levels of anoxic waters would have been common. This anoxia would have killed off many of the survivors of the first extinction pulse. In all the extinction event of the Late Ordovician saw a loss of 85% of marine animal species and 26% of animal families.

See also

* Timeline of glaciation
* Ordovician–Silurian extinction events
* Late Paleozoic icehouse

References

Glaciology
Ice ages
Ordovician events
Silurian events
Ordovician Africa
Silurian Africa
Ordovician South America
Silurian South America
Aeronian
Telychian
Hirnantian
Katian
Sandbian
Darriwilian
*

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