Medieval bear’s teeth shed light on historic heavy metal pollution

Abnormally high levels of lead in the teeth of a brown bear, which died 1000 years ago down a cave shaft in the Southern Carpathian mountains, represent the earliest known evidence of human-induced heavy metal pollution in a wild animal. The finding pushes back the date of such pollution affecting terrestrial wildlife by several centuries before the Industrial Revolution began around 1760.

Heavy metal pollution is usually associated with modern, large-scale industrialisation. However, humans have processed metals for millennia and growing evidence shows that early mining and smelting practices caused localised lead and other heavy metal pollution, which likely affected ecosystems by releasing particles into the atmosphere, contaminating water and accumulating in the food chain.

A report published last month provided the earliest known evidence of environmental lead pollution. Analyses of sediment cores revealed such pollution had already begun at low levels at the onset of the Bronze Age 5200 years ago in northern Greece. Then, 2150 years ago the pollution intensified, aligning with when the Roman Empire began to occupy Ancient Greece.

However, although historic heavy metal pollution likely affected ecosystems, little is known about its impacts due to a lack of direct evidence. Now, palaeontologist Marius Robu at the Emil Racoviţă Institute of Speleology in Bucharest, Romania, and his colleagues have found the earliest evidence yet of lead pollution in a wild animal after analysing the teeth of a bear that lived around 1000 years ago.

Robu and his colleagues discovered the bear’s bones along with those of two other bears in 2011 inside a cave that can only be accessed down a vertical shaft. If the 20-metre fall didn’t kill the bears, then starvation did because there was no way out. Fast forward to 2020 and Robu teamed up with Jeremy Martin at the University of Lyon, France, and other researchers to analyse the teeth of one of the bears in order to understand its diet as part of a project investigating the paleodiet of extinct cave bears, a sister-species.

However, after scanning the bear’s teeth for various trace elements that could shed light on the bear’s diet, the team observed unusually high levels of lead in the dentine layers of teeth in the lower jaw. ‘This discovery came as an unexpected result,’ says Robu. ‘We realised that we needed to research the historical context of the bear’s life, its exact age and to compare our data with what was previously published to try to explain this unnatural lead occurrence.’

Further analyses of the teeth revealed that the bear was between five and six years old and that it died in early autumn shortly before its denning period. What’s more, the lead concentrations in the dentine followed a seasonal pattern across the years, being high during the bear’s active months and low when the bear hibernated. This confirmed that the lead must have originated from food, water and air intake.

The researchers found that dentine lead levels peaked during the bear’s final summer season at over 15 parts per million, three times higher than the threshold for lead neurotoxicity in humans. However, the neurotoxic threshold for modern bears is only known through blood samples, which aren’t comparable to dentine content, making it difficult to know if this bear’s high lead intake affected its general health and behaviour.

What the researchers did establish, however, is that such high levels of lead in the environment align with previous findings that have shown increased fire activity and metal production during the same time period and geographical area encompassing parts of Romania, Serbia and Bulgaria. In particular, the region of Banat between 30 and 70 kilometres from the cave site was known for its medieval tradition in extracting metals, smelting and alloying. The team speculate that westerly winds common to the region could have transported and deposited the resulting pollution into the bear’s habitat.

’Although difficult to extrapolate from one individual to an entire brown bear population, we do not exclude that the anthropogenic lead pollution affected wildlife at a larger scale,’ says Robu. ‘It’s highly improbable that only a mammal individual was experiencing lead pollution whereas the other member of the species or different species did not.’

‘This study is a significant contribution to our knowledge of historical environmental pollution and its effects on wildlife, pushing back the earliest evidence of heavy metal contamination in terrestrial wildlife by several centuries.’ comments Florent Rivals at the Catalan Institute of Human Paleoecology and Social Evolution (IPHES) in Tarragona, Spain. ‘It suggests that even before large-scale industrialisation, localised pollution was already affecting ecosystems and highlights the importance of considering long-term anthropogenic impacts when assessing modern conservation and pollution challenges.’