When early humans began to travel out of Africa and spread into Eurasia over a hundred thousand years ago, a fertile region around the eastern Mediterranean called the Levant served as a critical gateway between northern Africa and Eurasia. A new study, published in Geology, shows that the existence of this oasis almost entirely depended on something we almost never think about: dust.
Dr. Rivka Amit at the Geological Survey of Israel and her team initially went out with a simple question: why are some soils around the Mediterranean thin and why are some thick? Their study led them not only to discover that dust deposition played a critical role in the formation of thick soils in the Levant, but also that if the dust source had not changed 200,000 years ago, early humans might have had a much harder time leave Africa, and parts of the fertile crescent would not have been so hospitable to civilization to take root.
Thick soils tend to form in areas with wet, humid climates, and thin soils form in dry environments with lower weather speeds. But in the Mediterranean, where much of the bedrock is soluble carbonate, the opposite is true: the northern regions of Wetter have thin, unproductive soils, and drier southeastern regions have thick, productive soils. Some researchers have attributed these patterns to differences in erosion rates, driven by human activity. But for Amit, who has studied the area for years, a high erosion rate did not make sense. She challenged the existing hypotheses, arguing that another factor – dust intrusion – is likely to play a critical role when weather speeds are too slow to form soil from bedrock.
To assess the impact of the dust on the Mediterranean soils, Amit and her team needed to trace the dust back to its original source. They collected dust samples from soil in the region as well as nearby and distant dust sources and compared the grain size distribution of the samples. The team identified a key difference between areas with thin and thick soils: thin soils included only the finest grain sizes originating from distant deserts such as the Sahara, while the thicker, more productive soils had coarser dust called loess, sourced from the nearby Negev desert and its massive dune fields. The thick soils of the eastern Mediterranean were formed 200,000 years ago when glaciers covered large chunks of soil, sanded the bedrock, and created an abundance of fine-grained sediments. “The whole planet was much dustier,” said Amit, who allowed extensive dune fields like those in the Negev to build up and create new sources of dust and eventually thicker soil in places like the Levant.
Amit then had its answer: Regions with thin soils had simply not gotten enough loose to form thick, agriculturally productive soils, while the southeastern Mediterranean had. “Erosion here is less important,” she said. “What is important is whether you get an influx of rough [dust] fractions. [Without that], you get thin, unproductive soils. “
Amit did not stop there. She now knew that the thickest soils had received a large stream of coarse dust, leading to the area being designated as “the land of milk and honey” for its agricultural productivity. Her next question was, had it always been that way?
She was surprised at what they found. When looking under the landslide, they found a lack of fine-grained sediments. “What was [deposited] before loess was very thin soil, “she said. It was a big surprise … The landscape was completely different, so I’m not sure people would [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, the fertile area that served as a passage for early humans may have been too difficult to pass through and survive.
In the modern Mediterranean, the earth no longer accumulates. “The source of dust is cut off,” Amit explained as the glaciers retreated into the Holocene, “now we are only working on the old louse.” Even if there was a source of dust, it would take tens of thousands of years to rebuild a soil there. This leaves these mountainous soils in a fragile state, and people living there have to weigh conservation and farming. The employment of responsible farming methods in the region, as terraces have been used for thousands of years, is crucial for soil conservation if agriculture is to continue.
Char application restores soil carbon and productivity
Rivka Amit et al., Quaternary influx of proximal coarse-grained dust altered the Earth’s productivity around the Mediterranean and affected early human culture, Geology (2020). DOI: 10.1130 / G47708.1
Provided by the Geological Society of America
Citation: Dust may have controlled ancient human civilization (2020, September 15) retrieved September 15, 2020 from https://phys.org/news/2020-09-ancient-human-civilization.html
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