January 25, 2012 - Deep in the caves of Southern Mexico, UNLV geoscientist Matthew Lachniet hopes to reveal why some of North America's most prosperous early civilizations died out.
The evidence he's looking for isn't hidden among ancient cave paintings, and it's not the type of thing you'd see treasured in an Indiana Jones movie. Nope, he's looking for a stalagmite, a pile of mineral deposits rising from a cave floor.
These common cave formations act as ancient rain gauges to recording historic climate change. Lachniet and an international team of researchers used them to establish 2,400 years of new climate history from southwestern Mexico, which, coupled with archaeological evidence, links the rise and fall of ancient Mesoamerican civilizations to changing rainfall. The findings were published online January in the journal Geology.
The Science of Stalagmites
Stalagmites grow as mineral-rich waters seep through the ground above and drop from the tips of stalactites on cave ceilings. Calcite minerals from tiny drops of water accumulate over thousands of years and, much like tree rings, accurately record the rainfall history of an area. Three natural forms of oxygen are found in water, and the quantity of one form decreases as rainfall increases. This information is locked into the stalagmites over time.
"Only recently have scientists started to unlock the scientific secrets of stalagmites for climate change," said Lachniet. "Stalagmites from tropical regions preserve thousands of years of rainfall history, providing a much longer record than tree rings. They also grow rapidly which allows us to pinpoint climate variations on near-annual timescales."
Civilizations Run on Water
Little is known about what contributed to the growth and downfall 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.
To find the answers, Lachniet and his team collected and analyzed a stalagmite from Juxtlahuaca Cave in the Mexican state of Guerrero. The cave is located in the core region affected by the North American Monsoon, a climate phenomenon primarily responsible for rain in most of Mexico and parts of Arizona, New Mexico and Southern Nevada.
Researchers first verified the rainfall record of the stalagmite by comparing deposits from the tip of the stalagmite with known rainfall amounts from the more recent past. Water samples were also collected deep within the caves to calibrate chemical variations in the stalagmites and unravel the climate history hidden within.
"Mexico may seem far removed from Southern Nevada, but the two regions are in fact linked by climate processes in the Pacific Ocean. Our new record shows that dry conditions, likely linked to El Ni?o processes, recurred frequently over time," Lachniet said. "The point to be made is that civilization runs on water. Take away a water supply and the civilization may fail."
History's Mysteries Revealed
Next, Lachniet and his colleagues correlated the region's cultural milestones with measured rainfall amounts. Above average rainfall between the first and third centuries, for example, coincided with the rise of the largest early Mesoamerican city of Teotihuacan. At its peak, more than 125,000 people lived in massive buildings 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 dry-land agriculture practices in the semi-arid Mexican Highlands.
Researchers argue that another drought, this one from 690-860 CE, made it difficult for the Basin area to sustain large urban areas. Archaeological evidence from this dry period also includes smashed "Storm God" artifacts, which may have signified abandonment of the civilization's rain god.
"We can't say with certainty that other social factors weren't drivers of the cultural change, but we now have well-dated and robust climate information to compare," Lachniet said. "Climate change was the norm for Mexico and the region dating back thousands of years, and similar variations are to be expected for the future."
The team also found that wet conditions were timed with the rise of the Toltec people between 900 and 1150 CE, and to the later rise of the Aztec city of Tenochtitlan during one of its wettest periods of the past millennium.
"Nature doesn't always reveal secrets easily, so we are very fortunate to have found the right place to work," said Lachniet.
Ê×Ò³| Â鶹´«Ã½Ó³» the Study
The study, "A 2400-year Mesoamerican Rainfall Reconstruction Links Climate and Cultural Change," was published in as a "pre-issue publication" of the journal Geology and was supported by grants from the National Science Foundation and the National Geographic Society. Partnering with Lachniet were Juan Pablo Bernal of Mexico's Centro de Geociencias in Juriquilla; Yemane Asmerom and Victor Polyak of the University of New Mexico; and Dolores Piperno of the Smithsonian National Museum of Natural History.
