WHAT KILLED THE MAYANS?

How the Maya met their end.

Up to around AD 750, the Mayan civilisation was thriving. Dozens of monuments were being built every year in what is now Mexico and Central America. By AD 900, however, such building ceased altogether and some cities in the southern lowlands were abandoned. What happened?

One idea is that a prolonged drought was to blame, and now we have the best evidence yet that this was the case. Analysis of “fossil water” shows that there was half as much rain as usual between AD 800 and 1000, and that at times during this period there was 70 per cent less rainfall.

During prolonged dry periods, gypsum may precipitate out of lake waters and be deposited in sediments. The presence of such deposits in Lake Chichancanab in Mexico provided the first evidence of prolonged droughts around the time of the Mayan decline. But their severity was unknown.

To find out how significant they were, geochemist Nicholas Evans at the University of Cambridge and his colleagues extracted ancient water trapped in the gypsum and analysed its isotopic ratios. Water molecules that contain, say, an oxygen-17 atom instead of the more common oxygen-16 are heavier and less like to evaporate. Heavier water molecules therefore accumulate in the lake during times of low rainfall and high evaporation.

Although isotopic analysis has been around for decades, Evans’s team has developed a method that allows simultaneous measurement of the levels of all the different isotopes of oxygen and hydrogen. This reveals a wealth of information: rainfall, water temperature and even humidity levels.

“We get the full climate picture,” says Evans, who is using the technique to study past climate in many other places and says it could even reveal what Mars used to be like.

While this work cannot prove cause and effect, there is growing evidence from around the world that many periods of upheaval and war coincided with climate change. The drought could have had a domino effect, says Evans, with food shortages leading to unrest, warfare and political disintegration, and the eventual downfall of the Maya’s ruling elite.

“The changes [in rainfall] were considerable, but not overly dramatic,” says Eelco Rohling of the Australian National University, who has also studied how rainfall changed at this time. “In other words, they are a good illustration that no dramatic changes in climate are needed to cause enormous problems. This truly is the lesson humanity should learn for our future.

Written by Michael Le Page in "New Scientist",UK, volume 239, no. 3190, August,11-17, 2018. Digitized, adapted and illustrated to be posted by Leopoldo Costa.