Tickling out human-induced changes from climate records has often been a tortuous game for scientists. Now researchers working in ancient Mayan stomping grounds are warning that in some cases climate records may be an illusion, recording instead the human touch.

Lake Salpetén in Petén, Guatemala, is a small lake, only 30 meters deep, that has often been used as a major water source. Those living in the lowland depended on the lake and its bigger neighbor, Lake Petén Itzá, with 150-meter depths, for their families and farms. The lakes lay in the middle of the Mayan heartland, says Michael Rosenmeier of the University of Florida.

[At right: Remnants of a pier along the shores of Lake Salpetén. Michael Rosenmeier]

Although enormous populations were subsisting around both lakes, the smaller Lake Salpetén was more sensitive to changes occurring along its banks. At a meeting of the American Geophysical Union on Dec. 15, Rosenmeier and colleagues showed how human deforestation within the watershed has confounded climate reconstructions.

“What you see in these records is not necessarily wet or dry in the sense of a climatic wet or dry, drought vs. increased rainfall,” Rosenmeier says. The researchers compared the oxygen isotope record from sediment cores in the lake with pollen records and saw a clear link between inferred “wet” periods and deforestation. “As you remove that forest did not impact the watershed.  cover you have a lot more surface runoff directly into the lake,” he says.

The records reach back 400 years prior to Mayan occupation, when a lush, green canopy of Moraceae trees sopped up rainfall from the soil like a sponge. In response, the lake recorded conditions that were drought-like in comparison to its later deforested days.

Much of the controversy surrounding the Mayan culture is what caused their demise. At the same AGU meeting Richardson Gill, an independent anthropologist in San Antonio, Texas, blamed the downfall on drought. Other studies have looked at isotopic records from lakes in northern Yucatán, where occupation did not affect the watershed.

Although Rosenmeier agrees that drought may have been responsible for the demise of the Mayas, he cautions that environmental degradation was also a factor. At the time of the Mayan collapse in the ninth century, the record from Lake Salpetén again showed the forest recovering and causing drought-like conditions.

As more isotopic studies begin to emerge from the tropics it will be critical to keep in mind how human activities influence the climate record, Rosenmeier says. In the case of Lake Salpetén, “it’s a red flag.”

Christina Reed

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