Turtle Shells Keep a Record of Humans’ Nuclear History

Scientists can measure uranium isotopes in tortoise and turtle shells to understand the environmental impact of past nuclear events, a new study reports

A desert tortoise
Shells of turtles and tortoises can accumulate uranium isotopes. Scott Smith via Getty Images

Nuclear explosions can have far-reaching consequences on the environment and people surrounding a detonation site. After a nuclear weapon test, for example, particles and gases from an explosion called nuclear fallout contaminate the environment and leave lingering radionuclides—atoms with unstable nuclei.

These radioactive particles produced by human activity remain in the environment for a long time, spread across the globe and accumulate in plants and animals. In the United States, an estimated 30 million to 80 million cubic meters of soil and roughly 1.8 billion to 4.7 billion cubic meters of water are contaminated with radionuclides from past nuclear activity.

Scientists have been looking for a reliable way to track these particles in the environment. In the past, measuring radionuclides in animals has been challenging, writes Jake Buehler for New Scientist. But now, researchers have discovered a new avenue for understanding the consequences of nuclear activity: turtle shells.

In a study published last week in PNAS Nexus, a team of researchers describes how turtle and tortoise shells retain evidence of previous nuclear explosions—much like how tree rings hold information on the past environment.

a pointed turtle scute with red markings showing the layers
Sequentially grown layers of a turtle's scute. Conrad et al., PNAS Nexus 2023 under CC BY 4.0

As they grow, the horn-like, bony plates atop the reptiles’ shells—also known as scutes—form discrete layers that store environmental information chronologically, acting as a “reservoir” of data, per the paper.

The scientists examined scutes from four turtle shells collected in hotspots that may have accumulated uranium from nuclear fallout or waste: a green sea turtle collected in 1978 from the Republic of the Marshall Islands; a Mojave desert tortoise from southwestern Utah collected in 1959; a river cooter from 1985 collected in South Carolina; and an eastern box turtle collected in 1962 from the Oak Ridge Reservation in Tennessee. As a control, they also examined a shell from a Sonoran desert tortoise collected in 1999 from an area of Arizona not associated with nuclear testing. 

While some level of uranium is naturally found in the environment, the scientists discovered levels of the element that matched the history of nearby nuclear events in all specimens except for the control, the Sonoran desert tortoise.

Notably, the green sea turtle shell was collected at Enewetak Atoll, which, together with the neighboring Bikini Atoll, was the site of 67 nuclear weapons tests between 1946 and 1958. The specimen was not collected until 20 years after nuclear testing ended, and researchers concluded the turtle was unlikely to have been alive during the testing. Still, its shell had evidence of uranium from the tests. Scientists write that it likely foraged there after the tests occurred. 

a person squats next to a turtle on a beach and holds a device over its shell in a black and white archival image
A person uses a Geiger counter to examine a green sea turtle for potential radioactivity in the Republic of the Marshall Islands in 1957. National Archives

The researchers suggest turtle shells could be an even better indicator of radionuclides in the past environment than tree rings are: Sometimes, the elements in a tree’s wood can diffuse between rings, making them an unreliable record, as lead author Cyler Conrad, an Earth scientist at the Pacific Northwest National Laboratory, tells New Scientist

The study also shows that the United States’ use of nuclear weapons has had profound impacts not just in Japan where it detonated bombs, but in American ecosystems, particularly on or near Indigenous lands where testing occurred, Laura Martin, an environmental historian at Williams College who was not involved with the new research, tells Meghan Bartels of Scientific American

“This paper points us to how nuclear colonialism is not just a human history,” she tells the publication. “But it has and continues to impact the whole biosphere.”

The researchers conclude that other organisms with sequentially grown tissue may be useful for tracking nuclear activity—corals, plant tissues, cactus spines, mollusks and bird feathers might also hold a record of the past.

“Combining analysis of a variety of organisms from nuclear sites in conjunction with localized sediments, plants and water will also help inform the exact pathway of contamination,” they write in the study.

Turtle shells might also help scientists track radioactive contamination in more modern contexts, the authors write—for example, near the site where Japan is releasing treated wastewater from the Fukushima Daiichi nuclear power plant into the Pacific Ocean, a step toward cleaning up the plant that began last week.

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