Ancient Roman Bowl Shows Modern Pesticide Damage

Typically, it’s the past that affects the present (and future), not vice versa. But in a scientific twist, UK researchers have found that modern chemicals are harming yet-undiscovered ancient artifacts.

In a new study, researchers from the University of Oxford link the deterioration of a copper bowl dating from the Roman period to two modern pesticides known to accumulate in soil and water sources.

In 2016, a metal detectorist discovered a copper bowl at a farm near Wingham in Kent, UK. After the discovery, the bowl was left protected in situ so it could be excavated a month later by archeologists. According to the archeologists, the bowl was buried at a depth of 40 cm in clay soil. Features revealed during the excavation indicated that the bowl was deliberately placed in a ditch within a settlement site previously unknown to archaeology—about mid-way between the Roman settlements of Richborough and Canterbury.

The archaeology team from the Canterbury Archaeological Trust and Dover Archaeological Group dated the Roman bowl to the Late Iron Age—between 43 and 410 AD. But, during conservation, they noted unusual brown and green corrosion on both the interior and exterior surface of the bowl.

The bowl was then given to Luciana da Costa Carvalho and colleagues at the University of Oxford for analysis of the corrosion. They used X-ray powder diffraction for the identification of mineral phases, FTIR for the identification of the chemical fingerprint, and gas chromatography with quadrupole time-of-flight mass spectrometry for recovery and identification of organic molecules.

According to the study results published in Scientific Reports, the researchers found elements indicative of changes over time in the soil caused by human activities. For example, in the green-colored corrosion, the authors detected chlorobenzenes. Chlorobenzenes are synthetic compounds that are relatively resistant to chemical degradation. Hexachlorobenzene (HCB), the most unreactive and well-studied chlorobenzene, was introduced as an agricultural pesticide in 1945 with emissions peaking in mid-1960s. Given its toxicity to humans, HCB is considered a persistent pollutant and is now banned globally under the Stockholm Convention. However, the organochloride is known to accumulate in the environment and living organisms.

In the brown-colored corrosion present on the bowl, the researchers detected diethyltoluamide (DEET), a compound that is still used in insect repellents. Currently, DEET is considered an emerging pollutant in England’s groundwater systems, with the highest abundance found in natural settings.

Overall, the study concludes that soil polluted with chlorobenzenes may pose a continuing threat to the preservation of archaeological material still in the ground.

“Our study provides the first evidence that chlorobenzenes are associated with accelerated corrosion mechanisms linked to archaeological material and demonstrates that they are a threat to the preservation of archaeological metals in the ground,” write the authors.

da Costa Carvalho and colleagues say their findings present an opportunity to systematically evaluate corrosion products from freshly excavated objects to provide a clearer picture of and further monitor the impact of soil pollutants. The researchers propose undertaking this evaluation using samples found and submitted by metal detectorists who “are often marginalized by heritage professionals.”

“[This offers them] an opportunity to contribute to cultural and environmental heritage management policies,” say the study authors. “Publishing material to raise awareness about the effect of pollutants on copper alloy objects among this community and mechanism for reporting it would be the first step to facilitate engagement.”