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 virtually 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 Friday in the journal Science.
The Cariaco Basin is on the southern Caribbean; the Mayan 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 pre-classic period before 700 A.D., building cities and elaborate irrigation systems to support a population that soared above a million. The civilization collapsed and many of the sites were abandoned early in the 800s. They were later 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," said Hughen.
The sediment records show that the gradual drying started about 1,200 years ago, but there was still enough rain for the Mayans to flourish.
"They were still getting rain, but clearly it was less than their grandparents did," said Hughen. "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 Mayans to abandon some sites, said Hughen. "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 ground water resources," said Hughen. "They were able to weather the first and second dry periods, but not the third."
T. Patrick Culbert, a professor emeritus at the University of Arizona and a noted authority on the Mayan culture, said the climate study offers a plausible explanation of what happened to the Mayans.
"They were so vulnerable that anything could have knocked them over," said Culbert. "If there were these severe droughts, it would have been a disaster for them."
By Paul Recer