Some ancient Mayan cities have dangerously high levels of mercury pollution.
A review in Frontiers in Environmental Science has shown that this mercury pollution comes from the ancient Mayans, who appeared to use a lot of the compound at certain points in their long history.
“Mercury pollution in the environment is usually found in contemporary urban areas and industrial landscapes,” says lead author Dr Duncan Cook, an associate professor of geography at the Australian Catholic University.
“Discovering mercury buried deep in soils and sediments in ancient Maya cities is difficult to explain, until we begin to consider the archaeology of the region which tells us that the Maya were using mercury for centuries.”
The international team of researchers reviewed all the available data on mercury contamination at ten different archaeological Mayan sites.
Seven of the ten sites had mercury contamination in at least one location. These sites were mostly from the Late Classic period, during the latter half of the first millennium CE. All of the sites had been abandoned by the 10th Century.
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Mercury levels range from 0.016 parts per million (ppm) at Actuncan, to 17.16 ppm at Tikal.
“The levels of total mercury found in some ancient contexts at Maya sites today are equal to or greater than modern guidelines we have in place for safe exposure limits for mercury in soils, such as the WHO’s recommended safe limit for mercury in soils for agriculture, which is 0.05 ppm,” says Cook.
“Our review shows that numerous Maya sites have total mercury levels that, if found in a playground or a building site, would be cause for concern.”
He adds that it’s “very difficult to say” how dangerous any given Maya site is, however.
“We simply do not know enough yet about the mercury that is being detected at ancient Maya sites, how it got there, and what forms it takes today in the environment, 1000 or more years after the Maya.”
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The researchers suggest several reasons for the mercury contamination.
Vessels of pure liquid mercury have been found at several Mayan sites. At other sites, the bright red mineral cinnabar, which is made from mercury and sulphur, is a favoured pigment.
“The brilliant red pigment of cinnabar was an invaluable and sacred substance, but unbeknownst to them it was also deadly and its legacy persists in soils and sediments around ancient Maya sites,” says co-author Dr Nicholas Dunning, a professor at the University of Cincinnati, US.
It’s not obvious what effect this level of mercury had on the Mayan civilisation, or the health and behaviour of the residents of these cities.
“What we need now is new multidisciplinary research on the Maya mercury to try and get closer to answering this question,” says Cook.
“There already exists several excellent studies showing mercury present in ancient human remains from the Maya world, but we need to connect the sites with high mercury levels in buried soils and sediments to the Maya who lived there.
“This means mercury analysis on human remains from the very same sites where geoscientists have detected instances of elevated mercury still present today.”
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Naturally occurring mercury is rare in the limestone around the Mayan region, suggesting it was imported.
“We conclude that even the ancient Maya, who barely used metals, caused mercury concentrations to be greatly elevated in their environment,” says Co-author Dr Tim Beach, a professor at the University of Texas at Austin, US.
Cook tells Cosmos he was one of the first researchers to identify mercury at a Maya site, back in the early 2000s – a surprising result which has “haunted him ever since”.
“My colleagues and I remain fascinated by the story of how a pre-industrial society, 1000-2000 years ago, in the tropical forests of Central America, were using mercury in such a way that its chemical signature has persisted in the environment until today,” he says.