The desertification of northern Africa may have started 7 million years ago, or more than twice as long ago as earlier estimates of the Sahara's age, climate simulations suggest. The findings also hint that this shift in climate would have been triggered by the gradual shrinking of the Tethys Sea, the predecessor of today’s Mediterranean.
The sands of the Sahara may seem timeless, but most geological data suggests that the world's largest non-polar desert formed between 2 million and 3 million years ago, about the same time as cycles of ice ages began plaguing the Northern Hemisphere.
Still, scientists have unearthed tantalizing clues that the Sahara may be much older than 3 million years old, says Zhongshi Zhang, a palaeoclimatologist at the Bjerknes Center for Climate Research in Bergen, Norway and a a co-author of the latest study, published on September 17 in Nature. Almost a decade ago, he notes, one team reported evidence of widespread dune deposits estimated to be 7 million years old in northern Chad.
Other analyses, including of long-term variations in the amounts of dust and pollen in sediments drilled from seafloors off northern Africa, chronicle extended dry spells in the region starting about 8 million years ago.
But it has been unclear what might have triggered the aridification, says Zhang. There were no major episodes of mountain formation that might have influenced climate in the region, he notes. Nor do his team's climate simulations suggest that long-term changes in Earth’s orbit or atmospheric concentrations of carbon dioxide are to blame.
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