In the ninth century A.D., the Maya abandoned the great city of Tikal after hundreds of years of prosperity and expansion. Researchers have long sought to explain how and why the city collapsed, but despite extensive study of the site, unanswered questions remain.
Commonly cited explanations for Tikal’s downfall center on a confluence of overpopulation, overexploitation of the surrounding landscape and a spate of withering megadroughts. Now, reports Kiona Smith for Ars Technica, a new study of the ancient city’s reservoirs outlines evidence that mercury and toxic algae may have poisoned Tikal’s drinking water at a time when it was already struggling to survive the dry season.
Located in northern Guatemala, Tikal dates back to the third century B.C. Once among the most powerful city-states in the Americas, the rainforest metropolis boasted multiple stone temples standing more than 100 feet tall and, at its zenith in the mid-eighth century, supported upward of 60,000 inhabitants, according to David Roberts of Smithsonian magazine.
Tikal’s residents built reservoirs to collect and store water after rainfall slowed to a trickle during multi-decade droughts in the ninth century. These reservoirs were essential during the dry season, as the city had no access to lakes or rivers, and the local water table, or level at which the ground reaches saturation, lies more than 600 feet underground.
Per the study, published last month in the journal Scientific Reports, the Maya sought to collect as much water as possible during the region’s rainy season, developing huge, paved plazas that were sloped to send water sluicing into the reservoirs for storage. As the researchers argue, this system inadvertently contributed to the city’s undoing.
To assess the factors at play in Tikal’s demise, the team took samples of sediments at the bottom of four of Tikal’s reservoirs. Chemical and biological analyses of layers dated to the mid-800s revealed the grim history of the lakes’ contents: As Ruth Schuster reports for Haaretz, two of the largest reservoirs were not only dangerously polluted with the heavy metal mercury, but also carried traces of enormous toxic algal blooms.
The researchers attribute the mercury pollution’s presence to the mineral cinnabar, or mercuric sulfide. Members of the Maya civilization mined this mercury-based ore and combined it with iron oxide to create a bloodred powder used as a versatile pigment and dye. The brilliant red—found coating the interiors of almost every high-status burial in Tikal—may have held special significance for the Maya. One grave unearthed by archaeologists contained roughly 20 pounds of powdered cinnabar.
Tikal residents’ widespread use of cinnabar, especially in and around the city’s temples and main palace, likely resulted in dangerous quantities of the mercury-laden powder washing into the reservoirs during heavy rainfall.
“The drinking and cooking water for the Tikal rulers and their elite entourage almost certainly came from the Palace and Temple Reservoirs,” the researchers write in the study. “As a result, the leading families of Tikal likely were fed foods laced with mercury at every meal.”
Another factor in Tikal’s decline was an explosion of toxin-producing blue-green algae. The team found traces of DNA from two such algae species in the reservoirs’ sediments.
“The bad thing about these is they’re resistant to boiling,” says lead author David Lentz, a paleobiologist at the University of Cincinnati, in a statement. “It made water in these reservoirs toxic to drink.”
During the late 800s, sediments from Tikal’s two central reservoirs were loaded with phosphate, a nutrient that blue-green algae needs to proliferate. The study’s authors write that these high levels of phosphate accrued after centuries of “smoky cooking fires and ceramic plates washed in the reservoir added organic material to the waters.”
The researchers also note that a midden, or trash heap, filled with food waste was located close enough to one of the reservoirs that “during the rainy seasons, effluent from this trash pile would have washed directly into the reservoir.”
When the city’s phosphate-filled reservoirs erupted in blooms of toxic blue-green algae, locals were probably able to tell that something major had gone wrong.
“The water would have looked nasty,” says co-author Kenneth Tankersley, an anthropologist at the University of Cincinnati, in the statement. “It would have tasted nasty. Nobody would have wanted to drink that water.”
Even without the poisoned drinking supply, losing the use of two huge water stores would have been devastating for Tikal. Prior research has identified a period of drought between 820 and 870—a timeframe that corresponds with the layers of sediment in which the blue-green algae and mercury were found.
Taken together, the dry weather and befouled water supply may have led the Maya to suspect their rulers had failed to adequately appease the gods.
“These events ... must have resulted in a demoralized populace who, in the face of dwindling water and food supplies, became more willing to abandon their homes,” the authors write.
Poisoned water wasn’t the sole cause of Tikal’s downfall, but as the researchers conclude, “The conversion of Tikal’s central reservoirs from life-sustaining to sickness-inducing places would have both practically and symbolically helped to bring about the abandonment of this magnificent city.”
According to Ars Technica, the researchers may pursue similar tests at other former Maya settlements to determine if the phenomena documented at Tikal influenced the decline of other cities across the empire.