When the first humans began to travel out of Africa and spread to Eurasia more than a hundred thousand years ago, a fertile region around the eastern Mediterranean Sea called the Levant served as a crucial crossing point between North Africa and Eurasia. A new study, published in Geology, shows that the existence of that oasis depended almost entirely on something we hardly ever think about: dust.
Dr Rivka Amit, at the Israel Geological Survey, and her team initially asked a simple question: Why are some soils around the Mediterranean thin and why are some thick? Their investigation led them to discover not only that dust deposition played a pivotal role in the formation of thick soils in the Levant, but also that if the dust source hadn’t changed 200,000 years ago, early humans could have had a much harder time to leave Africa, and parts of the Fertile Crescent would not have been so hospitable to civilization as to take root.
Thick soils tend to form in areas with humid and humid climates, while thin soils form in arid environments with lower weather exposure rates. But in the Mediterranean, where much of the bedrock is dissolvable carbonate, the opposite is true: the wetter northern regions have thin, unproductive soils, and the drier regions of the southeast have thick, productive soils. Some scientists have attributed these patterns to differences in erosion rates, driven by human activity. But for Amit, who has been studying the area for years, a high rate of erosion alone made no sense. He challenged existing assumptions by reasoning that another factor – dust ingress – likely plays a critical role when erosion rates are too slow to form soil from bedrock.
To assess the dust’s influence on Mediterranean soils, Amit and his team had to trace the dust back to its original source. They collected dust samples from the region’s soils, as well as dust sources near and far, and compared the particle size distribution of the samples. The team identified a key difference between areas with thin and thick soils: thin soils comprised only the finest grain sizes from distant deserts such as the Sahara, while thicker, more productive soils had a coarser dust called loess, which came from the nearby Negev desert and its huge dune fields. Thick soils in the eastern Mediterranean formed 200,000 years ago when glaciers covered large areas of land, grinding up the bedrock and creating an abundance of fine-grained sediment. “The whole planet was much dusty,” Amit said, which allowed for the formation of extensive dune fields like those in the Negev, creating new sources of dust and ultimately thicker soils in places like the Levant.
Amit, then, had his answer: regions with thin soils simply did not receive enough loess to form thick, agriculturally productive soils, whereas the southeastern Mediterranean did. “The erosion here is less important,” he said. “The important thing is if you get an influx of coarse [dust] fractions. [Without that], you get thin and unproductive soils. “
Amit didn’t stop there. He now knew that the denser soils had received a large stream of coarse dust, leading to the area being designated the “land of milk and honey” for its agricultural productivity. His next question was: Was it always like this?
She was surprised at what they had discovered. Looking under the loess in the soil profile, they found a shortage of fine-grained sediment. “What it was [deposited] before loess was very thin ground, “he said.” It was a big surprise … The landscape was completely different so I’m not sure people [have chosen] this area to live in because it was a harsh environment and [an] almost bare landscape, without much soil. “Without the changing winds and the formation of the Negev dune field, then, the fertile area that served as a passage for early humans may have been too difficult to traverse and survive.
In the modern Mediterranean, soils no longer accumulate. “The source of dust has been cut,” explained Amit, as the glaciers retreated in the Holocene, “now we’re just reworking the old loess.” Even if there was a source of dust, it would take tens of thousands of years to rebuild land there. This leaves these mountainous terrains in a fragile state, and the people who live there have to balance conservation and agricultural use. Employing responsible farming practices in the region, as terraces have been used for thousands of years, is crucial for soil conservation if agriculture is to continue.
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Rivka Amit et al, Quaternary influx of proximal coarse-grained dust altered the productivity of circum-Mediterranean soil and influenced early human culture, Geology (2020). DOI: 10.1130 / G47708.1
Dust May Have Controlled Ancient Human Civilization (2020, September 15)
recovered on September 16, 2020
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