August 9. Permo-Carboniferous glaciation

Glacial development in the southern continent of Gondwana began during the Carboniferous. We attributed the cyclic nature of coal cyclothems to alternating high- and low-stands of the sea related to advance and retreat of glaciers. The glacial period lasted into the Permian, so it is usually called the Permo-Carboniferous glaciation, and it lasted close to 90 million years, the longest glacial epoch in Phanerozoic history, the past 550 million years.   

Some of the best evidence for the glaciers comes from South Africa, where rocks called tillites are many hundreds of feet thick. Till is the poorly sorted deposit of material left behind by a glacier, and is has a distinctive chaotic character. You can get till-like deposits in various ways, however, but in addition the cobbles and pebbles within till are often striated – scratched by other rocks held firmly in glacial ice. Together with broader geometry of the deposits, including striated pavements over which glaciers carrying rocks flowed, we’re sure that these sediments were laid down by glaciers.

Permian Gondwana reconstruction and inferred ice cap (blue outline) from A. du Toit, South African geologist, 1937 (Our Wandering Continents). Du Toit's work was remarkably prescient. The map above differs only slightly from modern reconstructions of Gondwana.

The tillites in South Africa have been well known for more than 100 years. South Africa lay in the heart of Gondwana, well inland and probably at a relatively high elevation, and not far from the South Pole. This made for conditions favoring snow and ice accumulation. The ultimate causes of the Permo-Carboniferous glaciation aren’t completely clear, but the presence of a large polar continent is almost certainly an important factor. Other factors include the overall geometry of seaways and precipitation patterns. As Pangaea formed and equatorial seaways closed, oceanic circulation changed. Interiors of continents, distant from the sea and especially in mid-latitudes like the Sahara today would see low precipitation, and in cold climates the precipitation that did fall would likely be snow.

Details of stratigraphy in the South African tillites suggest that there may have been at least four distinct glacial periods within the overall glacial epoch, lasting 5 to 7 million years each, with intervening interglacial periods.

Water frozen in ice obviously affects sea levels, as we’ve said, but it can also affect carbon dioxide levels in the atmosphere by locking oxygen in water into the ice. Or was it the other way around? In late Carboniferous time CO2 levels were falling dramatically, perhaps partly as a result of the vast extent of plant life around the globe. Did that reduce greenhouse conditions, cooling the planet and enhancing the growth of glaciers? The glacial maximum appears to have been in early Permian time, and the glaciation seems to have pretty much ended later in the Permian, coinciding with a rebound in CO2 levels. The interplay of all these factors is still under considerable investigation.

Rocks virtually identical to the glacial deposits of South Africa were found in South America, East Africa, Antarctica, India, and Australia, and this discovery was another nail in the coffin helping to define the supercontinent of Gondwana. Together with fossil evidence like glossopteris that we talked about yesterday, the concept of a continuous sheet of ice was important to the idea that the southern continents had once been assembled into one landmass.

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On this date, August 9, 1138 a.d., an earthquake destroyed the city of Aleppo, Syria. The generally accepted death toll, 230,000, makes it one of the deadliest earthquakes in history. Aleppo sits more or less at the end of a transform fault that is the boundary between the Arabian Plate and Africa, right where that fault impinges on the Anatolian Plate in Turkey, a small block that is more or less amalgamated to the Eurasian Plate. This tectonic activity is obviously modern, but its heritage goes back to the Permian, when Africa, as part of Gondwana, was beginning to impact Eurasia. It’s still doing that, 270 million years later.

—Richard I. Gibson

Links:
Permian glaciation 
Permo-Carboniferous glaciation
Gondwana glaciation 
Crowell, 1978