News ID: 270889
Published: 0841 GMT July 01, 2020

Mercury and algal blooms poisoned Maya reservoirs at Tikal

Mercury and algal blooms poisoned Maya reservoirs at Tikal
The ancient Maya city of Tikal in northern Guatemala (GERD EICHMANN/CC BY-SA 4.0.)

For centuries, Tikal was a bustling Maya city in what is now northern Guatemala. But by the late 800s CE, its plazas and temples stood silent, surrounded by mostly abandoned farms.

A recent study suggests a possible explanation for its decline: Mercury and toxic algal blooms poisoned the water sources that should have carried the city through dry seasons, reported.

Tikal’s Maya rulers built the city’s reservoirs to store water from rain and runoff during the winter months. The pavement of the large plazas in the heart of the city tilted slightly, helping funnel rainwater into the reservoirs. Over the centuries, dust and litter settled into the bottom of the reservoirs, too, providing a record of what the environment around Tikal was like — and what was washing into the city’s water supply.

University of Cincinnati biologist David Lentz and his colleagues sampled layers of sediment dating back to the mid-800s, and they found that two of Tikal’s central reservoirs would have been too polluted to drink from.

An X-ray fluorescence spectrometer (which identifies the chemicals in a sample based on how they react to being zapped with an X-ray light) revealed that the sediment on the bottom of the reservoirs was laced with dangerous amounts of mercury. Lentz and his colleagues also found ancient DNA from blue-green algae, or cyanobacteria, which can produce deadly toxins.

The loss of these reservoirs came at the worst possible time for Tikal. In the mid- and late 800s, most of the great cities of the Maya world were already faltering under the weight of growing populations, degrading farmland, and decades of drought. Tikal simply couldn’t survive the collapse of even part of its infrastructure.

But how did the reservoirs get so polluted in the first place?

 “Color was important in the ancient Maya world,” said University of Cincinnati anthropologist Kenneth Tankersley, a coauthor of the study. The Maya were especially fond of paint made from a blood-red mineral pigment called cinnabar. Painters used it in colorful murals, builders painted the plaster walls of palaces and temples with it, mourners decorated ceremonial burials with it, and potters even used it to decorate ceramics.

Unfortunately for the Maya, cinnabar is poison. The pretty red mineral, which produces such lovely shades when mixed with iron oxide, is actually mercury sulfide. And Lentz and his colleagues say that over the centuries, mercury leached out of the vibrantly painted walls of Tikal’s most magnificent buildings and flowed straight into its reservoirs.

It had been building up in two of Tikal’s reservoirs for centuries before the city’s final decline. Lentz and his colleagues found toxic levels of mercury in sediment layers dating from 600 CE to 900 CE, based on radiocarbon dating of bits of organic matter mixed into the sediment.

“We were able to find a mineral fingerprint that showed beyond a reasonable doubt that the mercury in the water originated from cinnabar,” said Tankersley.

Most of the time, polluted urban water supplies are a problem for the poor — think of the London cholera outbreak that kickstarted modern epidemiology, or modern lead pollution in the water supply of Flint, Michigan. But the reservoirs in Tikal watered the political and ceremonial heart of the city, as they resided next door to a palace complex and major temples.

“The drinking and cooking water for the Tikal rulers and their elite entourage almost certainly came from the Palace and Temple Reservoirs,” wrote Lentz and his colleagues. “As a result, the leading families of Tikal likely were fed foods laced with mercury at every meal.”

And ironically, the wealth and power that surrounded the reservoirs poisoned their waters but left the rest of the city’s water supply untouched. The plazas that drained into the Palace and Temple Reservoirs were surrounded by palaces, temples, ballcourts, and cemeteries, all decorated with murals and cinnabar-painted plaster. Two other large reservoirs in less prestigious areas of the city were mercury-free, according to Lentz and his colleagues.

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