Late Ordovician glaciation
The Late Ordovician Glaciation is a period at the end of the Ordovician that started at the border between the Katian and Hirnantian about 440-460 Ma (million years ago). The major glaciation during this period 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 Morocco, Libya, 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. Also, glaciation of the Northern Hemisphere was minimal because a large amount of the land was in the southern hemisphere.
- Isotopic evidence points to a global Hirnantian positive shift in marine carbonate 18O, and at nearly the same time a shift in 13C in organic and inorganic carbon. 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 the 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 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.
- 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.
- 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).
Decreases in CO2
One of the factors that hindered glaciation was atmospheric CO2 concentrations, which at the time were somewhere between 8 and 20 times pre-industrial levels. During this time though, CO2 concentrations are thought to have dropped significantly, which could have led to further glaciation, but the methods for the removal of CO2 during that time are not well known. It could have been possible for glaciation to initiate with high levels of CO2, but it would have depended highly on continental configuration.
One theory is that the Katian large igneous province had basaltic flooding caused by high continental volcanic activity during that period. This would have released a large amount of CO2 into the atmosphere but would have left behind basaltic plains replacing the granitic rock. Basaltic rocks weather substantially faster than granitic rocks, which would quickly remove CO2 from the atmosphere to lower levels than pre-volcanic activity.
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.
Unfortunately due to the paleogeographic configuration of the continents, global ocean heat transport is thought to have been stronger in the Late Ordovician, but research shows that in order for glaciation to occur, poleward heat transport had to be lower, which creates a discrepancy in what is known.
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 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.
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 trust belt began to form from repeated small-scale fluctuations in the ice. The glaciotectonic fold and trust 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.
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As the Ice sheets began to increase the weathering of the basaltic and silicate rocks decreased, which caused CO2 levels to rise again, this in turned helped push deglaciation. This deglaciation cause the basaltic weathering to start back up which caused glaciation to occur again.
The Late Ordovician Glaciation coincided with the second largest of the 5 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.
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