African dust feeds the Amazon rain forest

The Sahara Desert is a near-uninterrupted brown band of sand and scrub across the northern part of Africa. The Amazon rain forest is a dense green mass of humid jungle that covers northeast South America. But after strong winds sweep across the Sahara, a tan cloud rises in the air, stretches between the continents, and ties together the desert and the jungle. It’s dust. And loads of it.

It is a regular phenomenon that occurs around noon each day. Countless particles of desiccated plankton dust fill the air, forming a giant dust storm many kilometers wide. More than half of this is believed to be from the Bodélé depression, an ancient lake bed in the Sahara. This huge cloud is blown across Africa and after crossing the west coast the dust is drawn upwards high into the sky. It then begins its journey across the Atlantic, propelled by the prevailing wind. It is estimated 54,000 tons of desert dust flow from Africa to South America every day. The minerals in the dust dissolve into water droplets and fall on the Amazon as rain. In this way, a staggering 40 million tons of African desert dust is delivered to the Amazon every year. Without this remarkable arrangement in nature the Amazonian rain forest – with all its plant and animal life – could not exist as we know it. The Amazon rain forest is believed to be one of the oldest living systems on the earth – about 55 million years old. Herein lies a big problem.

The vast Sahara desert was not always a vast desolate ocean of sand. At the end of the last Ice Age, the Sahara Desert was just as dry and uninviting as it is today. But sandwiched between two periods of extreme dryness were a few millennia of plentiful rainfall and lush vegetation. During these few thousand years, prehistoric humans left the congested Nile Valley and established settlements around rain pools, green valleys, and rivers.

Some 12,000 years ago, the only place to live along the eastern Sahara Desert was the Nile Valley. But around 10,500 years ago, a sudden burst of monsoon rains over the vast desert transformed the region into habitable land. This opened the door for humans to move into the area, as evidenced by new radiocarbon dates of human and animal remains from more than 150 excavation sites.

Then climate changed again. Recent studies have suggested the Sahara gradually shifted from savannah 6,000 years ago, possibly as a result of a tiny shift in its axis, to its now vast oceans of sand just 2,700 years ago[1].

This raises the question: how could the Amazon rain forest have survived if the Saharan dust was absent between 10,500 and 6,000 years ago? Science claims that the Amazon rain forest is around 55 million years old. 

[1] Claussen et al: Simulation of an abrupt change in Saharan vegetation in the mid-Holocene in Geophysical Research Letters - 1999

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