Ancient Maya communities in Mexico may have faced centuries of severe mercury pollution after toxic concentrations accumulated in lakes used as sources of water and aquatic food, a new study has found.
Sediment cores collected from Lago Cobá and Cenote Xcach on the northern Yucatán Peninsula revealed mercury concentrations that exceeded modern toxicity thresholds during periods of ancient Maya occupation.
At Cenote Xcach, concentrations reached approximately 18 micrograms per gram around AD 720 — far above the modern sediment threshold at which severe effects on aquatic organisms would be expected.
Researchers also found evidence that algae, bacteria and animals within the lakes declined or disappeared during periods of extreme mercury contamination, suggesting that pollution disrupted entire aquatic food webs.
The findings add deteriorating water quality to the environmental pressures faced by Maya communities during the Classic Period, alongside better-known problems including drought, deforestation and soil erosion.
Mercury reached extreme concentrations
Researchers analysed sediment cores collected in 2023 from Lago Cobá and Cenote Xcach, two bodies of water associated with ancient Maya settlements.
Lago Cobá lies beside the ruins of the major Maya city of Cobá, which had an estimated population of between 50,000 and 90,000 during the Classic and Early Postclassic periods.
The 6.4-metre sediment core recovered from the lake preserves thousands of years of environmental history.
Mercury concentrations at Cobá began increasing above background levels around AD 40 and reached approximately 2.3 micrograms per gram between AD 250 and 300 before gradually declining.
The evidence from Cenote Xcach was even more dramatic.
Researchers identified three major episodes of elevated mercury. The most severe occurred between approximately AD 535 and 820, when concentrations averaged 8.4 micrograms per gram and reached a maximum of around 18 micrograms per gram at about AD 720.
For comparison, the modern probable-effect threshold used by the researchers is 1.06 micrograms per gram, while a concentration of 2 micrograms per gram represents a level at which severe effects on aquatic organisms are expected.
Mercury at Cenote Xcach therefore remained above the severe-effect threshold for almost three centuries.
Pollution appears linked to Maya activity
The researchers argue that the contamination was probably associated with mercury brought into the region by ancient Maya communities.
Mercury was valued in Mesoamerica both in its elemental liquid form and as cinnabar, a mercury sulphide mineral prized for its vivid red colour.
Cinnabar was used as a pigment in burials and to decorate pottery and buildings, while evidence for elemental mercury has been discovered at several Maya archaeological sites.
The material may have been transported over considerable distances from mercury-producing regions elsewhere in Mesoamerica.
To determine whether the contamination recorded in the two lakes represented a broader natural phenomenon, researchers also examined sediment cores from two other lakes in the northern Yucatán.
Neither contained similarly elevated concentrations.
This allowed the researchers to rule out a widely distributed atmospheric source and led them to propose that elemental mercury and/or cinnabar had been imported into the Cobá and Xcach watersheds before eventually being washed into the lakes.
Soil erosion may have played an important role in transporting the contaminant from surrounding landscapes into the water.
Aquatic ecosystems collapsed
The sediment cores also preserve biological evidence allowing researchers to reconstruct how the lakes responded to the contamination.
As mercury reached toxic concentrations, indicators of algae, cyanobacteria and purple sulphur bacteria declined substantially.
At Lago Cobá, researchers found an interval during the period of toxic mercury contamination in which shells belonging to ostracods and gastropods disappeared completely from the sediment record.
Cenote Xcach produced a different but similarly striking signal.
Fossilised remains of chironomids, or non-biting midges, declined as mercury concentrations increased. The composition of the surviving chironomid community also changed dramatically, with one previously dominant genus almost disappearing and another becoming predominant.
Taken together, the changes occurred across several levels of the aquatic food web.
The researchers conclude that the timing of these biological declines alongside extreme mercury concentrations provides evidence for ecosystem-wide stress associated with mercury toxicity.
Maya populations may have been exposed
The findings also raise questions about possible human exposure.
Water bodies such as Lago Cobá were important sources of water and aquatic food for nearby populations, while fish and shellfish formed part of the ancient Maya diet.
Both can accumulate mercury within their tissues.
The researchers therefore suggest that people living around Cobá and Xcach could have been exposed to mercury through drinking water and consuming aquatic foods over periods lasting decades or even centuries.
However, the study did not directly measure mercury concentrations in ancient inhabitants from these two communities.
The possibility of chronic human exposure is consequently inferred from the environmental evidence rather than demonstrated through human remains.
Mercury contamination has previously been identified elsewhere in the Maya world, including archaeological soils, buildings and reservoir sediments, while mercury has also been detected in human remains from the broader region.
An overlooked environmental pressure
Environmental stress affecting ancient Maya societies has traditionally focused heavily on changes in water availability.
Periods of drought, combined with deforestation, erosion and intensive land use, have been investigated as pressures affecting Maya communities at different times and places.
The new findings suggest that water quality could have presented another problem.
At Cenote Xcach in particular, extreme mercury concentrations persisted through several centuries of the Classic Period.
The researchers note that archaeological knowledge of the settlement surrounding Cenote Xcach is limited so “at present it is not possible to associate the pollution with a specific phase of urban development or activity”.
Nor does the study say that mercury pollution was responsible for the broader transformations Maya societies underwent.
Instead it sees toxic contamination as an additional environmental stressor that might have affected those particular communities alongside other ecological and climatic pressures.
The results suggest that the Maya use and movement of mercury-bearing materials went far beyond their ritual or decorative ends to contaminate aquatic environments on which local populations depended.
Sources : Science Advances