March 28. Late Ordovician Glaciation

I’ve mentioned the late Ordovician glaciation several times in recent posts, suggesting things that might have been factors causing it. One possible factor is the fact that the largest continent, Gondwana, which included most of Africa, South America, India, Australia, and Antarctica, was located over the south pole. That alone wouldn’t do it unless the climate was cold enough.

Upsala Glacier, Argentina

During most of the Ordovician, planet Earth was in greenhouse conditions, with high concentrations of carbon dioxide in the atmosphere. We mentioned two things that might have affected that – the presence of life on land, taking more and more CO2 out of the atmosphere, and the tectonic activity that made a high mountain range in what is now eastern North America. Erosion of that mountain range, to create the immense Queenston Delta, might have produced enough sediment to have an effect on the atmosphere so that CO2 was reduced. Both of those ideas are in the “might have” category, but something certainly did affect CO2 concentrations.

And then we talked about the humongous volcanic eruptions, the Deicke and Millbrig and others near the beginning of the Late Ordovician epoch, around 455 to 457 million years ago. Cooling because of high concentrations of dust in the atmosphere was certainly likely, and the timing is good with respect to the glaciation.

The glaciation seems to have been at its height about 440-455 million years ago, which includes the first part of the Silurian – the boundary with the Ordovician is put at 443 million years ago. There’s some evidence that the glaciation might have started as long ago as 460 million years.

The evidence about CO2 values isn’t speculative. There is abundant evidence in carbon isotopes to indicate significant changes in water temperature – a pretty significant disruption of the carbon cycle, enough to imply cooling and a reduction of the greenhouse effect. In addition to the ideas that life and erosion helped remove CO2 from the atmosphere, another idea is that volcanism helped. But wait, you say – I thought volcanoes added CO2 to the atmosphere. Well, they do. But earlier in the Ordovician, extensive basaltic eruptions – when they were finished – created large expanses of solid basalt, which reacts and erodes relatively quickly, and might have been a factor in CO2 removal. That’s another pretty speculative possibility.

Glacially-deposited rocks of Late Ordovician age are common across what is now the Sahara Desert, from Morocco to Ghana to Libya, and they’ve also been found in South Africa, Brazil, Arabia, Germany, Nova Scotia, and Newfoundland. The ages of all those deposits are not absolutely determined, but they are all of Late Ordovician or Early Silurian age, coinciding nicely with and defining the glacial period.

There’s evidence that this glacial event ended quite abruptly, for reasons that seem to me highly speculative and hardly able to explain it. One suggestion is that once the ice sheets reached their limit, they essentially collapsed – which to me doesn’t adequately explain why they then retreated to practically nothing. CO2 levels increased in the Silurian, but I haven’t seen a good explanation for why.

The Ordovician-Silurian glaciation is the only one associated with a mass extinction event – one of the largest in earth’s history. We’ll talk more about that at the end of the Ordovician, in a few days.

—Richard I. Gibson

Further reading:
The carbon cycle
Photo by longhorndave via Wikipedia, under cc-by-2.0