The Boston Globe, March 14, 2003
WASHINGTON (AP) -- A study of southern Caribbean sediments suggests that a century-long dry trend may have been the killing blow in the demise of the Mayan civilization that once built pyramids and elaborate cities in Mexico.
Konrad A. Hughen, a geochemist at the Woods Hole Oceanographic Institution, said sediments from the Cariaco Basin in northern Venezuela clearly record a long dry siege that struck the entire Caribbean starting in about the seventh century and lasting more than 100 years.
Within this dry period, said Hughen, there were years of almost no rainfall. It was in those periods of extra dryness, he said, that the Mayan civilization went through a series of collapses before its final demise. Hughen is co-author of a study appearing in the latest issue of the journal Science.
The Cariaco Basin is on the southern Caribbean; the Maya lived for about a thousand years on the Yucatan, now part of Mexico, on the northwestern edge of the Caribbean. Hughen said both areas share the same climate, with a wet season and a dry season, so the dry trend detected in the Cariaco Basin sediments is thought to reflect the same climate experienced on the Yucatan.
Hughen said the Maya flourished in what is known as the preclassic period before 700 A.D., building cities and elaborate irrigation systems to support a population that soared above 1 million. The civilization collapsed and many of the sites were abandoned early in the 800s. They were reoccupied only to collapse again, with some cities deserted in 860 and others in 910.
''Those abandonments occur synchronously with the timing of the droughts in our record [from the sediments], suggesting the droughts were causing those events,'' Hughen said.
The sediment records show that the gradual drying started about 1,200 years ago, but there was still enough rain for the Maya to flourish.
''They were still getting rain, but clearly it was less than their grandparents did,'' Hughen said. ''Then, all of a sudden, there were periods of nine, three, and six years when there were very dry conditions.''
He said the populations were already stressed by a trend of sparse rainfall and the ''exceptionally severe'' periods were enough to cause the collapses.
''A severe event didn't have to be long'' to force the Maya to abandon some sites, Hughen said. ''Each one of those dry events resulted in the collapse of a certain portion of the Mayan population.''
A severe dry spell in 910, he said, ''was the last straw.''
Mayan communities in the southern and central lowlands collapsed first, while those in the northern highlands lasted for another century before the final collapse.
''The northern areas had access to more groundwater resources,'' Hughen said. ''They were able to weather the first and second dry periods, but not the third.''
This story ran on page A12 of the Boston Globe on 3/14/2003.
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.