User:Mhaueter/sandbox

Overview
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. Tropical ocean temperatures were about 5°C cooler during the late Ordovician.

It is the only glacial episode that appears to 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, but there was some evidence of sea ice in the northernmost parts.

Isotopic
Isotopic evidence points to a global Hirnantian increase shift in marine carbonate 18O, and at nearly the same time a shift in 13C in Organic and Skeletal carbon. This evidence is based on 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. The magnitude of the this shift (+4%% ) is extraordinary and 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.

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.

Four facies dominate the Bighorn Dolomite (which represents end of the Ordovician period), and they are interpreted to represent deep subtidal, open shallow subtidal, restricted shallow subtidal, and peritidal environments. These types facies are associated heavily with glacial maximums. 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
1.	One of the factors at the time that hindered glaciation was that atmospheric CO2 concentrations were somewhere between 8 and 20 times pre-industrial levels. Durring this time though, C02 concentrations are thought to have dropped significantly, so this may have been a trigger leading to glaciation. 2.	The Katian large igneous province had basaltic flooding caused by high continental volcanic activity. This released a large amount of CO2 into the atmosphere as well as leaving behind basaltic plains replacing granitic rock. The basaltic rocks weather substantially faster than granitic rocks, quickly removing 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 an effect. A drop in sea level allows more land to become available for ice sheet growth. Sea level change alone though is not sufficient for ice sheet formation, and must be accompanied by other factors.

Poleward Ocean Heat Transport
Ocean heat transport is a major driver in the warming of the poles. 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 would have been stronger in the Late Ordovician which would prohibit ice sheets from forming.

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.

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.

Causes
The cause for the end of the Late Ordovician Glaciation is a matter of great research, but evidence shows that it may have occurred abruptly (in a lithologic sense), as Silurian strata marks a significant change from the glacial deposits left durring the Late Ordovician.

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 and started to allow water flow across the ice, which lead to further retreat and further collapse of glacial conditions. This recursion allowed the melting of the ice sheet, and rising sea level.

CO2
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.

Significance
A.	The Late Ordovician Glaciation is also the second largest of the 5 major extinction events. The Extinction even 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. The second phase of the extinction is thought to be a result of anoxia that took place because of the massive reduction of sulfate in the water. This would cause a stress on all of the oxic organisms thus causing a larger extinction.