杏吧原创

The Mayan civilisation of Central America collapsed following a series of intense droughts, suggests the most detailed climatic study to date.

The sophisticated society of the Maya centred on large cities on the Yucat谩n peninsula, now part of Mexico. Their population peaked at 15 million in the 8^th century, but the civilisation largely collapsed during the 9^th century for reasons that have remained unclear to this day.

Now, researchers studying sediment cores drilled from the Cariaco Basin, off northern Venezuela, have identified three periods of intense drought that occurred at 810, 860 and 910AD. These dates correspond to the three phases of Mayan collapse, the scientists say.

Furthermore, the entire 9^th century suffered below average rainfall, 鈥渟o it was a dry period with three intense droughts鈥�, says Gerald Haug, from ETH in Zurich, Switzerland, who led the research. 鈥淭he climate change must have been what pushed the Mayan society over the edge.鈥�

Experts on the Maya have greeted the new data cautiously. 鈥淎ny explanation for decline is a complex one: over-population, environmental problems and economic factors all made them vulnerable,鈥� says Jeremy Sabloff, director of the Museum of Archaeology and Anthropology at the University of Pennsylvania. 鈥淏ut there is growing evidence that climate played a role. Perhaps it was the straw that broke the camel鈥檚 back.鈥�

Wet and dry

Haug and his colleagues identified the bands in the sediment cores that correspond to the annual wet and dry seasons. They then analysed the concentration of titanium in the sediment in great detail, taking measurements at intervals of 50 micrometres.

Titanium is an indicator of rainfall, explains Haug, because higher precipitation washes more of the metal from the land into the ocean floor sediments. The difference in concentration between the wet and dry season each year is as much as 30 per cent.

鈥淲e looked in detail at the period corresponding to 9^thand 10^thcenturies 鈥� taking 6000 measurements per 30 centimetres of sediment 鈥� and found three extreme minima, as well as a low background level of that lasted about 100 years,鈥� Haug told New 杏吧原创.

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But archaeologist Norman Hammond, at Boston University, is unconvinced that drought caused the downfall of the Maya. Referring to the northern Yucat谩n city of Chich茅n Itz谩, he asks: 鈥淲hy did the latest and greatest florescence of the Mayan series occur in the area that we know to be the driest.鈥�

The Maya certainly had hydraulic expertise, Jeremy Sabloff points out, building canals, viaducts and reservoirs. Moreover, they had experienced and survived droughts before.

鈥淭he Maya thrived for 1500 years before these droughts, so it鈥檚 clearly not climate alone that brought down the southern cities of the Yucat谩n peninsular,鈥� he says.

Journal reference: Science (vol 299, p 1731)