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Climate History Links MesoAmerican Monsoon to Agricultural Emergence

Cave deposits reveal 22,000 years of North American monsoon rainfall.
Research  |  May 24, 2013  |  By Shane Bevell
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UNLV geoscientist Matthew Lachniet explores stalagmites in caves in southwestern Mexico.

The emergence of New World agriculture can be linked to monsoon strengthening that occurred 11,000 years ago, according to a new study by UNLV geoscientist Matthew Lachniet.

Findings from Lachniet and a team of researchers show that rainfall in the summer monsoon in Mesoamerica is sensitive to changing climate conditions, and may experience rapid and abrupt collapses. Thus, the strength of the summer monsoon - which supplies water for agriculture in Mexico - should be considered potentially unstable in the face of additional human-caused climate changes. The findings were recently published online this week in the journal Proceedings of the National Academy of Sciences.

Lachniet and the team determined the results by the measuring naturally occurring oxygen isotopes in southwestern Mexico cave stalagmites. Stalagmites are common cave formations that act as ancient rain gauges to record historic climate data. Stalagmites grow upward at rates of inches every few hundred years 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.

"We were surprised to see that rainfall amounts can change dramatically in just a few hundred years," said Lachniet, whose research is part of a six-year long project to investigate the North American Monsoon in Mexico. "This is a worrying sign for future water resources as most of Mexico and the southwestern United States is projected to become drier in the future."

The study area is important to scientists because it served as the epicenter for the domestication of maize (corn) by early humans at the emergence of New World agriculture.

"These results show that the monsoon can collapse in as little as a few decades, which must have required some flexibility in the pre-Columbian human response to summer rainfall changes," Lachniet said. "For example, the early farmers used lake-margin settings for their maize crops, possibly because these areas were wetter and monsoon rainfall wasn't always reliable enough for their annual plantings."

The wet conditions beginning 11,000 years ago were associated with an intensification of the summer rains, a climate system known as the Mesoamerican monsoon, which followed a millennia-long dry and cold period known as the Younger Dryas. Although humans were first present in Mexico during the Younger Dryas interval, climatic conditions were not yet favorable for agriculture.

Another finding of the study is that the Mesoamerican monsoon was active, and conditions were wet, during the last Ice Age approximately 20,000 years ago when an extensive ice sheet covered much of Canada and the United States. This result is surprising because previous studies had invoked a collapse of the Mesoamerican monsoon during full glacial periods.

However, Lachniet's team has shown that the monsoon "collapse" occurred later, around 17,000 years ago, a time interval that coincided with a massive disintegration of the northern hemisphere ice sheets. This ice sheet disintegration event produced groups of icebergs into the North Atlantic Ocean, like the one that sunk the Titanic, and caused ocean currents to shut down and the monsoon rains to stay south of Mexico.

Further, the rainfall reconstruction shows that the strength of today's monsoon is weaker than 11,000 years ago, a result that is attributed to decreasing solar radiation reaching the subtropical regions of Mexico due to the regular change in the configuration of Earth's orbit around the sun, a parameter called insolation. Typically, stronger insolation during the summer months leads to a stronger monsoon because of enhanced ocean and land surface heating.

About the Study

The study, "Orbital pacing and ocean circulation-induced collapses of the Mesoamerican monsoon over the past 22,000 years" was published in the "Early Edition" of the journal Proceedings of the National Academy of Sciences and was supported by grants from the National Science Foundation and the National Geographic Society. Analyses were completed at the Las Vegas Isotope Science Laboratory, housed in the UNLV Science and Engineering Building.

The study sites included Juxtlahuaca Cave, in the Sierra Madre del Sur of southwestern Mexico. Partnering with Lachniet were Juan Pablo Bernal of the Universidad Aut?noma de Mexico (UNAM) in Quer?taro; Lorenzo Vazquez-Selem of UNAM in Mexico City; and Yemane Asmerom and Victor Polyak from the University of New Mexico.