[Ernest_at_the_Beach]

Searching on "Ordovician glaciation"...turns up this abstract:

An atmospheric pCO2 threshold for glaciation in the Late ...

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Geology; May 1997; v. 25; no. 5; p. 447-450; DOI: 10.1130/0091-7613(1997)025<0447:AAPCTF>2.3.CO;2
© 1997 Geological Society of America

An atmospheric pCO2 threshold for glaciation in the Late Ordovician

Mark T. Gibbs¹, Eric J. Barron¹ and Lee R. Kump¹

¹ Department of Geosciences and Earth System Science Center, Pennsylvania State University, University Park, Pennsylvania 16802

The Late Ordovician glaciation differs from other major Phanerozoic glaciations because of its short duration (1 m.y.). A satisfactory explanation for this glaciation must be able to account for its sudden initiation, short duration, and rapid termination; mechanisms operating on tectonic time scales appear to be precluded. Given recent studies of a major perturbation in the carbon cycle during the glaciation, a climate model investigation of the sensitivity of the Late Ordovician climate to changes in atmospheric pCO₂ was undertaken. Under the condition of a 4.5% reduction in solar luminosity, permanent snow cover (taken as a key indicator of potential for glaciation) is dramatically different between five experiments. The range of 18X present atmospheric level CO₂ (ice free) to 8X ("runaway" icehouse) lies within the uncertainty of previous geochemical estimates of Late Ordovician atmospheric pCO₂. The strong sensitivity to the modest direct forcing from pCO₂ changes is due to the ice-albedo feedback. A plausible increase in organic carbon burial could have drawn down enough atmospheric CO₂ to have briefly lowered the climate system below a critical glacial inception threshold at the end of the Ordovician. Conversely, a high pCO₂ is required for the rest of the early Paleozoic, which was essentially ice free.

[Ernest_at_the_Beach]

And from Wikipedia...which in this area may not be trust worthy:

Ordovician

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Ordovician Period

Mean atmospheric O2 content over period duration	ca. 13.5 Vol %[1] (68 % of modern level)
Mean atmospheric CO2 content over period duration	ca. 4200 ppm[2] (15 times pre-industrial level)
Mean surface temperature over period duration	ca. 16 °C[3] (2 °C above modern level)
Sea level (above present day)	180m; rising to 220m in Caradoc and falling sharply to 140m in end-Ordovician glaciations[4]

The Ordovician [/ɔɹdəˈvɪʃən/] is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago (ICS, 2004,^[5]. It follows the Cambrian Period and is followed by the Silurian Period. The Ordovician, named after the Welsh tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian periods respectively. Lapworth, recognizing that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian periods, realized that they should be placed in a period of their own.

While recognition of the distinct Ordovician Period was slow in the United Kingdom, other areas of the world accepted it quickly. It received international sanction in 1906, when it was adopted as an official period of the Paleozoic Era by the International Geological Congress.