Today’s
episode, number 379, is about glaciers in what is now the Sahara Desert, and we’re
going back 340 million years, to the Mississippian or Early Carboniferous
Period of the Paleozoic Era.
In the original daily episodes of this podcast back in 2014,
when we got to the Mississippian in June, I had a very brief episode about
glaciers in Australia and South America. They probably represented an early
pulse of the well-known later glacial period during the Permian, and the
glaciation provided evidence for the existence of the supercontinent of
Gondwana, which was situated more or less over the south pole at that time,
about 340 million years ago.
Gondwana consisted of the continents and smaller blocks we
know today as South America, Africa, Arabia, Antarctica, India, and Australia,
and the pole was located somewhere in south-central Africa, so it should come
as no surprise that some recent work by Daniel Le Heron at the University of
London, published in the journal Geology in November 2017, reports evidence for
an extensive ice sheet in what is now northern Chad, in the modern Sahara
Desert.
 |
| 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 red oval shows the general area where Le Heron (2017) infers a small ice sheet in Carboniferous time. |
Le Heron described belts of sinuous lineaments carved by ice
in belts five to 12 kilometers wide and spread over an area of more than 6,000
square kilometers. The features cut into an ancient surface that is interpreted
to represent the landscape over which the glaciers flowed during that Early
Carboniferous time. He suggests that the ice sheet extended to the west to
cover much of what is now Niger, and that the glaciers flowed northward into
the sea. The coastline then was also in far northern Chad, so the glaciers
probably reached the ocean.
The present-day surface of the earth in northern Chad is
actually an exhumed, an uncovered, ancient landscape that formed about 340
million years ago when those glaciers scoured the land. The surface was buried
by later sediments which have since been eroded away.
I don’t think the causes for this glacial episode are well
understood, although there’s been a lot of work done on it. One of the most
recent reports, by Yves Goddéris & Yannick Donnadieu and their colleagues
writing in Nature Geoscience in April 2017, suggests that the onset of
glaciation was the result of tectonic activity. Just before the ice age got
going, the Hercynian Mountains had been uplifted because of continental
collisions in many parts of the world. As soon as mountains are uplifted, they
begin to erode, and a lot of erosion tends to reduce atmospheric carbon dioxide
because there’s a lot more material, the sediment, to react with the CO2. Goddéris
and his co-workers think CO2 levels fell enough to trigger the formation of
glaciers in the mountains, and ultimately by Permian time, to form extensive
ice sheets that covered a huge area of Gondwana.
According to their model, the glacial period ended when the
mountains had been eroded enough to affect the carbon cycle and allow CO2
levels (and overall temperatures) to rise again. The end of glaciation was also
likely affected by the final amalgamation of the supercontinent of Pangaea,
which changed climates to more extensive arid conditions.
If you are interested in more about the Mississippian or Early Carboniferous
Period, search the archives for June 2014. All 30 episodes that
month were about the Mississippian.
—Richard I. Gibson
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