Hirnantian glaciation

In the last stage of the Ordovician, the Hirnantian, glaciers built up on the great southern continent Gondwana, which then lay over the South Pole. Glacial deposits from this ice age, including striated pavements and boulder clays, are found today in the Sahara of Algeria and Libya, as well as in Arabia, South Africa and South America. The ice age began around 445 million years ago and lasted about a million years. Sea level fell by tens of metres, draining shallow seas where most marine life lived.¹³

The cooling followed a long warm period, so it was a sharp change. Its cause is not settled. One idea is that spreading land plants and their roots increased rock weathering, which pulls carbon dioxide out of the air, as Tim Lenton and colleagues suggested in 2012. Volcanic weathering and shifts in ocean circulation may have contributed. Shelf animals suffered first as the sea retreated and cooled, then again as the ice melted and the sea rose into low-oxygen waters, which killed off many remaining species. This double blow is a leading explanation for the first of the Big Five mass extinctions, the end-Ordovician mass extinction. It is one of the few mass extinctions linked to cooling instead of warming.¹²³