At www.sciencedaily.com/releases/2015/06/150629162542.htm … researchers using satellite images of the ancient shores of Lake Chad as well as studying sediments from the lake to calculate its age have come up with some unexpected results. It was subject to rapid change – turning from a huge lake much bigger than the modern counterpart, over a short period of time. Most of what was a lake is now a region of sand dunes, rocks and dust.
It was at its peak as recently as 6000 years ago – and the surrounding Sahara was also green, a savannah like environment that was home to lots of humans and lots of animals. It is assumed it was fed by monsoon rains – and this is probably quite true as the modern savannah belt relies on the monsoon track (but is situated much further to the south). It had an area of 360,000km whereas the modern lake is just 355km (which is still very big). The research is published in PNAS this month and the desertification is blamed on declining monsoon rains as a result of orbital changes (Milankovitch). The idea that orbital changes as a result of a change in the tilt of the Earth is not explored.
However, and this may be the most important point found by the researchers, it shows that the southern Sahara dried out over a few hundred years (is that rapid?) and this has implications. One part of the former lake is now known as the Bedele depression and is situated in a remote spot in the north of the state of Chad. Dust with an origin in this depression blows across the Atlantic to S America and fetches up over the Amazon rainforest. The recent paradigm is that this dust (from such places as Bedele) fertilises the rainforest – which is otherwise leached of nutrients by tropical rainfall. However, prior to 6000 years ago this could not be the case – as dust would not have been available (as the Sahara was green). This seems to demand a review of the idea that Sahara dust is important to the Brazilian ecosystem. Who came up with that idea originally and what evidence is there that dust from the Sahara is that important. Is it just speculation.
Mind you, if the desert was not in the Sahara, might it have been located elsewhere – but where?