Gary sent in this link. At www.nature.com/ncomms/2015/151110/ncomms9751/full/ncomms9751.html … and for double the effect see www.dailymail.co.uk/sciencetech/article-3312097/Lost-sands-time-Hidden-a… … we learn that not so long ago, in the grand scheme of geological history, in the Late Quaternary, the Sahara had a network of freshwater lakes, rivers and streams, clearly fed by water falling out of the sky. A buried palaeo drainage network has been found on the arid coast of Mauritania, one of the dryest places on Earth – and it was activated on multiple occasions, delivering layers of sediment off the Atlantic coastal margins.
The article begins by saying that astronomical forced insolation changes have driven monsoon dynamics – presumably a reference to the orbital pattern as deduced by the Milankovitch model (which is supposed to affect the track of the monsoon belt). What they are saying is that the hydrological cycle in N Africa is caused by an intensification of the African monsoon – or the appearance of rain in regions it is nearly absent in today. However, they add the caveat that a lot of rain must also involve a migration northwards of the Inter Tropical Convergence Zone (a sort of tropical jet stream). The ITCZ is known to shift on a regular basis. When it is over certain parts of South America their is bumper harvests but when it shifts back south there is drought – and famine (and the collapse of prehistoric American civilisations). Obviously, if the ITCZ was involved we can see why it got a lot wetter in what is now an arid region – but the big problem is what causes the ITCZ to shift (northwards of southwards). This is not mentioned – merely taken as a given variance in climate with a dubious link to orbital variations as catalogued by Milankovitch. It also ignores the fact that he actually produced a tilt of the Earth, admittedly over a long period of time, but yet basically a tilt in the orbit of the Earth as it rotates around the Sun. Is that a clue?
Marine sediments from the Mediterranean and the Atlantic coastal margins have provided consistent evidence of monsoon variability, they say. This can be seen by the deposition of sediment layers in the Eastern Mediterranean which is linked to the ebb and flow of the Nile (all the way back to the Pliocene geological period). The most recent episode was in the Early Holocene, from 9700-3000BC. Some nine different layers of sediment are recorded over the last 245,000 years – and are reciprocated, it would seem, off the coast of Mauritania.
While a wet Sahara is a wonder we have another wonderful mystery in the drying up of the Mediterranean basin some 5 or 6 million years ago – and the dry period is estimated to have lasted for 270,000 years. It left behind a layer of salt 1km deep as a result of evaporation (unless the salt had a different origin) but how could such a thing happen? See http://phys.org/print366356449.html
Perhaps the dry Sahara moved northwards as the ITCZ moved northwards and the Mediterranean sea became a desert. However, the consensus theory was that tectonic movement of the plates around the Mediterranean caused it to become landlocked. Some scientists considered a global fall in sea level might have been to blame as a result of the Ice Ages and the ability of ice sheets to lock up large amounts of sea water (or that is the assumption). A computer simulation of ice sheet growth came up with the claim that overloading one region (with ice) caused it to sink while at the same time ice free areas of ocean bottom rose up (balancing each other) and one place this may have occurred was around the Straits of Gibraltar – thereby stopping the Atlantic feeding the Mediterranean which effectively became a land locked basin. As a result of no ocean water feeding this basin it gradually shrank – evaporating (but it must have been extraordinarily hot). Later, as the ice sheets collapsed in the endless cycle of glaciations the ocean bottoms sank back down around Gibraltar and the ocean rushed in – a catastrophic flooding event.
Meanwhile, at http://westerndigs.org/nearly-9000-artifacts-uncovered-in-california-des… … we learn that the Mojave desert, in the same early Holocene period as the wetter era of the Sahara, was likewise attractive to human habitation and settlement. It seems a global phenomenon involving a contraction of the desert zone – or a movement of the desert as far as latitude was concerned.