Turtles and tortoises can store a decades-long record of past exposure to radioactive contamination on their backs.
When exposed to fallout from nuclear weapons testing or accidental waste releases, the reptiles accumulate radioactive uranium isotopes in their shell scales. The finding could be useful for long-term monitoring of radionuclides – radioactive variations of elements – in nature.
Radionuclides from nuclear activities have spread widely and stick around in ecosystems for a long time. In the US alone, up to 80 million cubic metres of soil and 4.7 billion cubic metres of water are estimated to be contaminated by past nuclear activities.
Testing for radionuclides’ accumulation in organisms has its challenges, says Cyler Conrad at the Pacific Northwest National Laboratory in Washington state. Tree rings, for instance, are created sequentially and can carry radionuclides. But the elements can diffuse in the wood between rings, he says, and thus make an unreliable chronological record.
Conrad and his colleagues wondered if the tough scales growing on turtle and tortoise shells – called scutes – might be a more promising option. These also grow in layers, but once the nail-like scale material is deposited and separate from other bodily tissues, it is effectively time-stamped.
The researchers sampled scutes from four museum specimen turtles, each from a different species in a different location historically exposed to nuclear materials. The shelled subjects included a green sea turtle ( Chelonia mydas) from the Marshall Islands and a Mojave desert tortoise (Gopherus agassizii) from Nevada. Both sites experienced nuclear weapons testing in the mid-20th century.
The other two turtles were from fuel processing sites that have contaminated surrounding areas via nuclear waste. The researchers also looked at a desert tortoise from an area not associated with nuclear activity.
Chemical analyses of tiny scute fragments showed that the four turtles from historic nuclear sites had small but elevated levels of uranium radionuclides in their shells. An eastern box turtle (Terrapene carolina carolina) that lived near the Oak Ridge National Laboratory in Tennessee recorded a uranium signature in its scute growth rings between 1955 and 1962 that coincided with the timing of airborne releases of waste at the site.
The team thinks these shambling uranium chronicles could be used to reconstruct histories of the nuclear contamination of ecosystems.
Germán Orizaola at the University of Oviedo in Spain says the study could open a field of radioecology research using zoological collections in museums, where researchers “assess the concentration of radionuclides in feathers, bones and other tissues in specimens collected before and after nuclear tests and accidents”.
Considering that the researchers only needed a very small amount of shell tissue for their analyses, Clare Bradshaw at Stockholm University in Sweden wonders if the technique could be used relatively non-invasively on living turtles and tortoises too.
Conrad also identified this as a potential application of the findings. “Are [the turtles] still accumulating [radionuclides]? Can they be measured and studied to understand what’s going on in our modern environment?”