UNLV geoscientist Matthew Lachniet and an international team of researchers used stalagmites to unveil a 2,400-year climate history from southwestern Mexico. This discovery, coupled with archaeological evidence, links the rise and fall of ancient Mesoamerican civilizations to changing rainfall. The findings were reported Jan. 23 as a “Pre-Issue Publication” of the journal Geology.

Little is known about what contributed to the fate of the prosperous ancient Mesoamerican city of Teotihuacan, though historic evidence suggests periods of above average rainfall followed by extreme drought might have played a role.

“Mexico and the American southwest are linked by climate processes in the Pacific Ocean. Our new record shows that dry conditions, likely linked to El Nino processes, recurred frequently over time,” said Lachniet. “The point to be made is that civilization runs on water. Take away a water supply and the civilization may fail.”

Lachniet and his team collected and analyzed a stalagmite from Juxtlahuaca Cave in Guerrero, Mexico. The cave is in the core region of the North American Monsoon, a climate phenomenon primarily responsible for rain in most of Mexico and parts of the American Southwest. The cave was also a great source for stalagmites – common cave formations formed over thousands of years as minerals from water droplets crystallize and accumulate. Much like tree rings, they can accurately record the rainfall history of an area.

“Only recently have scientists started to unlock the secrets of stalagmites as ancient rain gauges,” said Lachniet. “Stalagmites from tropical regions provide a much longer record than tree rings. They also grow rapidly which allows us to pinpoint climate variations on near-annual timescales.”

The team correlated the region’s cultural milestones with measured rainfall. Above average rainfall between the first and third centuries coincided with the rise of the early Mesoamerican city of Teotihuacan. At its peak, more than 125,000 people lived around the massive pyramids in the highly developed city.

Conversely, a 500-year drying trend, including a drought of more than 150 years, coincided with rapid population decline in Teotihuacan around 550 CE. The drought likely impacted spring-fed agriculture practices in the semi-arid Mexican Highlands.

“We can’t say with certainty that other social factors were drivers of the cultural change, but we now have well-dated and robust climate information to compare,” said Lachniet. “Climate change was the norm for the region dating back thousands of years, and similar variations are to be expected for the future.”

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