A major transgression in the Middle Ordovician created widespread shallow, warm epicontinental seas. Thus, most of the Ordovician was favorable for marine life, particularly around the well-studied European and North American cratons. However, the Ordovician ended in a brief (300-500 ky), but severe, ice age. Gondwana, particularly Africa, straddled the South Pole and became extensively glaciated. There were even glaciers in what is now the Sahara. Metazoans were severely effected. About 60% of animal genera became extinct, making this the second or third most deadly mass extinction of the Phanerozoic .
Ashgill GCM Hermann et al. (2004)As a natural consequence, a good deal of attention has been focused on the causes of the Ordovician Ice Age. In fact, it is not easy to see how an ice age could have occurred. Atmospheric carbon dioxide levels are believed to have been 8 to 20 times their current values. This ought to have prevented anything approaching an ice age. Sea levels were high through most of the Ordovician. They dropped, dramatically (about 50 m), in connection with the ice age, but it is hard to tell whether this was cause, effect, or both. One independent factor which would affect both pCO2 and sea level is the rate of sea floor spreading along mid-ocean ridges. As we might expect, the length of well-established mid-ocean ridges, i.e., the ridge between Gondwana, to the south, and Baltica plus Laurentia, to the north, was unusually short during the Late Ordovician. A former ridge between the two northern continents became inactive about this time. However, there may have been a very long ridge to the Northwest of Laurentia. The information is too sparse to be certain. In any event, the absence of active ocean crust formation would only affect the rate of CO2 outgassing, not the rate at which it was locked away in sediments.
Ashgill GCM Hermann et al. (2004)As a natural consequence, a good deal of attention has been focused on the causes of the Ordovician Ice Age. In fact, it is not easy to see how an ice age could have occurred. Atmospheric carbon dioxide levels are believed to have been 8 to 20 times their current values. This ought to have prevented anything approaching an ice age. Sea levels were high through most of the Ordovician. They dropped, dramatically (about 50 m), in connection with the ice age, but it is hard to tell whether this was cause, effect, or both. One independent factor which would affect both pCO2 and sea level is the rate of sea floor spreading along mid-ocean ridges. As we might expect, the length of well-established mid-ocean ridges, i.e., the ridge between Gondwana, to the south, and Baltica plus Laurentia, to the north, was unusually short during the Late Ordovician. A former ridge between the two northern continents became inactive about this time. However, there may have been a very long ridge to the Northwest of Laurentia. The information is too sparse to be certain. In any event, the absence of active ocean crust formation would only affect the rate of CO2 outgassing, not the rate at which it was locked away in sediments.
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