Dust may have controlled ancient human civilization

Date:
September 15, 2020
Source:
Geological Society of America
Summary:
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 Sea called the Levant served as a critical
gateway between northern Africa and Eurasia. A new study shows that
the existence of that oasis depended almost entirely on something
we almost never think about: dust.



FULL STORY ==========================================================================
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 Sea called the Levant served as a critical gateway between northern Africa and Eurasia. A new study, published in Geology, shows
that the existence of that oasis depended almost entirely on something
we almost never think about: dust.


==========================================================================
Dr. Rivka Amit, at the Geological Survey of Israel, and her team
initially set out with 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 critical role
in forming thick soils in the Levant, but also that had the source of
dust not changed 200,000 years ago, early humans might have had a much
tougher time leaving Africa, and parts of the Fertile Crescent wouldn't
have been so hospitable for civilization to take root.

Thick soils tend to form in areas with wet, humid climates, and thin
soils form in arid environments with lower weathering rates. But in the Mediterranean, where much of the bedrock is dissolvable carbonate, the
opposite is true: wetter northern regions have thin, unproductive soils,
and more arid southeastern regions have thick, productive soils. Some scientists have attributed these patterns to differences in the rates of erosion, driven by human activity. But for Amit, who has been studying
the area for years, a high erosion rate alone didn't make sense. She
challenged the existing hypotheses, reasoning that another factor --
dust input -- likely plays a critical role when weathering rates are
too slow to form soils from bedrock.

To assess the influence of dust on Mediterranean soils, Amit and her team needed to trace the dust back to its original source. They collected dust samples from soils in the region, as well as nearby and far-flung dust
sources, and compared the samples' grain size distribution. The team
identified a key difference between areas with thin and thick soils:
thin soils comprised only the finest grain sizes sourced from distant
deserts like the Sahara, whereas the thicker, more productive soils had
coarser dust called loess, sourced from the nearby Negev desert and its
massive dune fields. The thick soils in the eastern Mediterranean formed 200,000 years ago when glaciers covered large swaths of land, grinding
up bedrock and creating an abundance of fine-grained sediments. "The
whole planet was a lot dustier," Amit said, which allowed extensive dune
fields like those in the Negev to build up, creating new sources of dust
and ultimately, thicker soils in places like the Levant.

Amit, then, had her answer: regions with thin soils simply hadn't received enough loess to form thick, agriculturally productive soils, whereas
the southeastern Mediterranean had. "Erosion here is less important,"
she said.

"What's important is whether you get an influx of coarse [dust] fractions.

[Without that], you get thin, unproductive soils." Amit didn't stop
there. She now knew that the thickest soils had received a large flux
of coarse dust, leading to the area's designation as the "land of milk
and honey" for its agricultural productivity. Her next question was, had
it always been like this? She was surprised at what they found. Looking
below the loess in the soil profile, they found a dearth of fine-grained sediments. "What was [deposited] before the loess were very thin soils,"
she said. "It was a big surprise... The landscape was totally different,
so I'm not sure that 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 formation of the
Negev dune field, then, 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 soils aren't accumulating any more. "The
dust source is cut off," Amit explained, since the glaciers retreated
in the Holocene, "now we're only reworking the old loess." Even if there
were a dust source, it would take tens of thousands of years to rebuild
a soil there. That leaves these mountainous soils in a fragile state,
and people living there must balance conservation and agricultural
use. Employing responsible agricultural practices in the region, as
terracing has been used for thousands of years, is critical for soil preservation if agriculture is to continue.


========================================================================== Story Source: Materials provided by Geological_Society_of_America. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Rivka Amit, Yehouda Enzel, Onn Crouvi. Quaternary influx of proximal
coarse-grained dust altered circum-Mediterranean soil
productivity and impacted early human culture. Geology, 2020;
DOI: 10.1130/G47708.1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200915152446.htm

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