Mercury from Maya cities left Coba and Xcach lakes toxic for centuries

Mercury from cinnabar pigment reached lake sediments near two ancient Maya cities at levels that a new study says far exceed modern toxicity limits. The research, published in the journal Science Advances, examined the lakes of Coba and Xcach on the Yucatán Peninsula in Mexico, and it points to contamination that lasted for centuries.

The team, led by Matt Waters, professor of environmental science at Auburn University in Alabama, collected sediment cores in May 2023. Cores are long columns of lake mud that record the deposits of past centuries, and the researchers used them to trace how the metal changed over time. Waters and his colleagues tested the layers and found sharp peaks that matched the Classic Period of Maya civilization.

At Coba, one of the largest Maya settlements in the northern lowlands, mercury levels rose from around 250 CE and stayed at toxic levels for about 150 years. The lake sits near the Nohoch Mul pyramid, one of the best-known monuments of the site. At Xcach, a smaller site with several pyramids and structures on private land, the record shows three separate spikes: one from 125 CE to 200 CE, a second around the time of the Coba peak, and a third from 535 CE to 820 CE.

The source of the metal was cinnabar, a naturally occurring mercury sulfide mineral with a vivid red color. The Maya prized it for paint on temple walls, palace murals, funerary objects, ceremonial floors and pottery, and they linked it to blood, vitality and the sacred. Rain washed the pigment from building surfaces, and the runoff carried particles into the rivers and lakes that supplied water and fish to nearby families.

Lake cores show algae, shellfish and midges vanishing

The metal did more than sit in the mud. The sediment layers from the periods of highest contamination show sudden drops in aquatic life, and some groups disappeared altogether from the most polluted layers. Shelled bottom-dwelling organisms and mosquito larvae vanished from the most contaminated strata of the lakes.

The team identified several groups whose populations fell in the sediment record:

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• Bacteria
• Algae
• Cyanobacteria
• Shelled organisms
• Insect midges

Waters said the team used the markers to test whether the metal moved up the food chain. “Modern toxicology and pollution regulations were far exceeded at the most toxic levels for centuries,” he said, referring to the levels around Coba and Xcach. The study also found that mercury levels in some of the sediment layers rose several times above current sediment quality guidelines.

The researchers concluded that mercury moved into fish and other animals that people ate. Fish and crustaceans from the lakes were a primary source of protein for farmers and elites, the study indicates. Repeated consumption of fish and water from the contaminated basins would have kept people exposed to the metal for long periods.

Mercury’s health risks and the limits of the new evidence

Mercury is a potent neurotoxin, and exposure through breathing or eating can damage the central nervous system, the brain, the kidneys, the lungs and the cardiovascular system. Tremors, confusion, organ failure and birth defects are known effects of long-term exposure to compounds derived from cinnabar. In pregnant women, the metal can cross the placenta and cause lasting developmental problems in newborns.

Savanna Wooten, a postdoctoral researcher at the Jones Center at Ichauway in Georgia and a co-author of the study, said the lake records do not prove that the Maya were poisoned. “While we do not show this in our sediment records,” she said, the pattern suggests that people “might have been exposed to mercury via consumption of water or fish.”

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The study also has clear limits. The researchers did not measure methylmercury, the form most harmful to living things, which forms in oxygen-poor mud at the bottom of lakes. Wooten said the work does little to link mercury directly to the decline of the Classic-era cities, although it adds one more pressure to a list of environmental and climate stresses that occurred at the same time.

Precautions for visitors and archaeologists at Maya sites

Stone structures painted with red pigments can keep mineral residues for centuries after abandonment, so the guidance for visitors and field teams focuses on contact and dust. Tourists, field researchers and heritage conservation teams are advised to follow a few basic steps at active digs and closed monuments in Mesoamerica.

• Avoid touching ancient murals decorated with red paint, and do not touch the mouth or eyes after walking through ruins.
• Use masks with particle filters in confined excavations, underground burial chambers and newly opened tombs.
• Do not drink untreated water from natural springs or lakes near preserved archaeological sites.
• Wash boots, trail shoes and expedition equipment with running water before returning to inhabited areas.

The National Institute of Anthropology and History of Mexico, known as INAH, is continuing geochemical mapping of lake basins nearby. The work aims to establish how far the metal anomalies reach into other Maya provinces.