Earth_Magazine_October_2017

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T

he Neolithic Revolution,

also known as the Agricul-

tural Revolution, started

in the Middle East about

11,500 years ago when people moved

away from nomadic hunting and gath-

ering toward more settled agricultural

communities where they raised livestock

and cultivated crops. In a new study of

the Dead Sea Basin, researchers found

that this turning point may also mark the

first time that humans made a measurable

impact on sedimentation rates.

The Dead Sea is an endorheic basin —

a closed drainage basin with no outflow

— covering parts of Jordan, Israel and

Palestine that was created by tectonic

forces along the Dead Sea Transform

Fault over the last 20 million years. In

2010, the Dead Sea Deep Drilling Proj-

ect, part of the International Continental

Scientific Drilling Program, harvested a

450-meter-long sediment core. Radio-

carbon and uranium-thorium dating

revealed that the core spanned the last

220,000 years, making it one of the lon-

gest time spans covered by a continental

core drilled to date.

In the new study, published in Global

and Planetary Change, researchers led by

Yin Lu of Tel Aviv University in Israel

focused on the upper 110 meters of the

core, a portion deposited over the last

22,000 years, spanning the period since

the last glacial maximum. “[The Dead Sea

Basin] serves as a natural laboratory for

understanding how sedimentation rates in

a deep basin are related to climate change,

tectonics and anthropogenic impacts on

the landscape,” Lu’s team wrote.

To study how the Neolithic Revolution

may have affected basinwide erosion rates,

the team charted the thickness of layers of

seasonal sediments preserved in the core.

The researchers found that over the last

11,500 years, sedimentation rates were on

average 4.5 times higher than they were

between 21,700 and 11,600 years ago. The

team found no direct signs of humans at

any point within the core sediments, but

correlation with the extensive archaeo-

logical record of human occupation in the

region suggests that the Neolithic Revolu-

tion began there about 11,500 years ago,

about the same time that sedimentation

rates began increasing.

“In this relatively dry landscape, con-

stant grazing pressure and intensive

fodder harvesting would have continu-

ously depleted and reduced the natural

plant and tree communities, and dis-

turbed the soil, making it more erodible,”

Lu says. “We suggest that human impact

on the landscape was the primary driver

causing the intensified erosion, and that

the Dead Sea sedimentary record serves

as a reliable record of this impact since

the Neolithic Revolution.”

However, not everyone sees such a

clear correlation between humans, graz-

ing animals and sediments. “The evidence

here for an early anthropogenic signal

is suggestive, but not definitive,” says

William Ruddiman, a paleoclimatologist

at the University of Virginia who was not

involved in the new study. “The peak of

the last interglacial [around 130,000 years

ago] has sedimentation rates just as high,

even though humans could not have been

a factor back then,” he says. Other studies

have shown that the Eastern Mediterra-

nean climate has been prone to very wet

winters throughout the Holocene, which

may have also led to higher erosion rates,

he says.

Next, the team plans to study the earth-

quake record preserved by the sediment

core. Previously drilled Dead Sea sediment

cores have revealed as many as 36 major

earthquakes in this region over the past

50,000 years. The new core may reveal

an even longer history of the tectonically

active region. “The Dead Sea core will

provide us with a 220,000-year record,”

Lu says, “which will be the most extensive

earthquake record in the world.”

Mary Caperton Morton

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