Over 2 centuries, the Maya abandoned city after city in their empire. By about A.D. 950, their civilization had more or less disappeared. Archaeologists have suspected that drought was one of the factors, and in the 14 March issue of Science, a team of geoscientists gives them the most detailed evidence yet to back up that idea.
Water was crucial for the Maya. Even though the lowlands of Central America have abundant rain in the summer, winter can be bone dry. To be able to live away from lakes or rivers, the Maya constructed reservoirs and irritation systems for their cities.
The pronounced seasonality in rainfall also left a more subtle geological record, far away. That's because of an atmospheric pattern called the Intertropical Convergence Zone. In the summer, the rivers of Central America dump sediment into the Caribbean that washes to the Cariaco Basin off northern Venezuela. When winter comes, the rain clouds head south, and light-colored algae drift into the basin. This regular, alternating influx has created one of the best climate records for the tropics, says Gerald Haug of the Geoforschungszentrum Potsdam in Germany. By examining cores from the Ocean Drilling Program, Haug and his colleagues could figure out how much sediment had entered the basin and get a fix on the relative amounts of rainfall in Central America. They'd already deciphered a pattern of drought, with a 4-year resolution, but archaeologists wanted more details.
Now they have them. Haug's team was able to measure the amount of titanium--a reliable indicator of how much sediment was entering, because it doesn't get altered by geochemical changes--every 50 micrometers, equivalent to a 2-month period. This strengthens the evidence for a century-long decline in rainfall, which would have parched winter fields, Haug says. Worse yet were four severe droughts that each lasted several years. This could have led to extreme conditions, such as drinking-water shortages, that could have helped spell the end for crowded cities, Haug speculates.
Interestingly, these droughts match the times proposed for phases of city abandonment, based on the dates of final carvings. Although it's difficult to definitively date abandonments by carvings, cautions archaeologist Lisa Lucero of New Mexico State University, the new findings will help solve the mystery. "Now that we have these refined dates, we can start to get specific histories," she says. "It's really exciting." Lucero is planning to use the new climate record to test her idea that cities more dependent on reservoirs were the first to go under.