Neolithic farmers impacted sedimentation

by Mary Caperton Morton
Monday, September 18, 2017

The Neolithic Revolution, also known as the Agricultural Revolution, started in the Middle East about 11,500 years ago when people moved away from nomadic hunting and gathering 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 Project, part of the International Continental Scientific Drilling Program, harvested a 450-meter-long sediment core. Radiocarbon and uranium-thorium dating revealed that the core spanned the last 220,000 years, making it one of the longest 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 archaeological record of human occupation in the region suggests that the Neolithic Revolution began there about 11,500 years ago, about the same time that sedimentation rates began increasing.

“In this relatively dry landscape, constant grazing pressure and intensive fodder harvesting would have continuously depleted and reduced the natural plant and tree communities, and disturbed 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, grazing 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 Mediterranean 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 earthquake 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.”