Not So Clear-Cut: Soil erosion may not have led to Mayan downfall

Hand-planted maize, beans, and squash sustained the Mayans for millennia, until their culture collapsed about 1,100 years ago. Some researchers have suggested that the Mayans’ very success in turning forests into farmland led to soil erosion that made farming increasingly difficult and eventually caused their downfall. But a new study of ancient lake sediments has revealed that most erosion happened well before the culture collapsed and likely played only a small role in disrupting the civilization.

“When you clear a forest, you open up the soil and expose it to rainfall and weathering,” says Flavio S. Anselmetti, now of the Swiss Federal Institute of Aquatic Science and Technology in Duebendorf. In the tropical lowlands, he explains, clay soils can drift substantially, but only after forest clearing.

To assess soil erosion during the Mayan era, Anselmetti’s team measured how much clay had washed into Guatemala’s Lake Salpetén. The layer of clay, 7 meters thick at some spots, contrasts sharply with nutrient-rich organic material below and above it, which was deposited before and after the Mayan period.

By analyzing the ages of different layers within the clay, the researchers determined that most of it accumulated 2,700 to 4,000 years ago, early in the Mayan era. Erosion must have begun as soon as the Mayans started clearing land for agriculture, say Anselmetti and his colleagues in the October Geology.

Vernon L. Scarborough of the University of Cincinnati says that the new results make him rethink the role erosion played in the collapse of the Mayans. He notes that the Mayans’ complicated irrigation and terracing schemes would have taken a vast amount of maintenance, and that this upkeep might have faltered with any initial crumbling of the society. The resulting erosion and damage to agriculture could have further weakened the societal framework.

“One of our thoughts was that [the Mayans] terraced and manipulated the landscape so dramatically that when they left it basically all slid away,” says Scarborough. But that would have deposited a second wave of clay in the lake basin about 1,100 years ago, and no such layer is evident.

The state of the soil could still have played a role. Erosion over thousands of years could have thinned the soil so much that growing anything at all may have been difficult during the decline of the civilization, says Anselmetti.

“This is just a puzzle piece in a multicausal cascade that made their lives miserable,” he adds.

He stresses that the ancient course of erosion might hold important modern-day lessons. Since the 1950s, Guatemala’s population has exploded, and people are once again clear-cutting land that had just barely recovered from the agriculture of the Mayans. Understanding what happened 2,000 years ago could help minimize new damage to the land, says Anselmetti.

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