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Ancient lake holds secrets to Mayan civilization’s collapse

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Laguna Chichankanab. Photo: Wanderlum

Sediment under Lake Chichankanab backs a long-held theory about what caused the demise of Mayan civilization.

Scientists have several theories about why the collapse happened, including deforestation, overpopulation and extreme drought. New research, published Thursday in Science magazine, focuses on a drought 1,000 years ago.

Ancient Mayans, primarily concentrated in what is now the Yucatán Peninsula, were among the most advanced civilizations of their time. They were some of the first to build cities, they used astronomy to advance agricultural production, and they created calendars and used advanced mathematics. But despite all of their progress, the Mayan empire, built over thousands of years, may have crumbled in just a few hundred.

While analyzing sediment under Lake Chichankanab (also spelled Chichancanab), just over the state line in José María Morelos, Quintana Roo, scientists found a 50 percent decrease in annual precipitation over more than 100 years, from 800 to 1,000 A.D. At times the decrease was as much as 70 percent.

The study, the first to quantify the rainfall, relative humidity and evaporation at that time, analyzed sediment cores, similar to what researchers have done on the Gulf of Mexico to learn about the dinosaur extinction. Each layer of sediment buried deep underground contains evidence of rainfall, temperature and even air pollution which can be analyzed layer by layer to form a timeline.

During dry periods, the lake volume would have shrunk, said Nick Evans, a graduate student at Cambridge University and first author of the study. As the water evaporated, lighter particles would have evaporated first, leaving behind heavier elements.

If the drought was intense and long-lasting, gypsum crystals formed and incorporated existing lake water directly into their structure. The “fossil water” inside the crystals allowed Evans and his co-authors to analyze the properties of the lake water during each period.

“It’s as close as you’ll ever get to sampling water in the past,” Evans said.

The drought coincides with the beginning of the Medieval Warm Period, thought to have been caused by a decrease in volcanic ash in the atmosphere and an increase in solar activity. The Mayans’ deforestation may have also contributed, as well as changes to the atmospheric circulation and decline in tropical cyclone frequency, Evans said.

Source: Washington Post

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