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Wednesday, July 24, 2024

Turtles Carry Signs of Humanity’s Nuclear History in Their Shells

On a spring day in 1978, a fisherman caught a tiger shark in the lagoon surrounding Enewetak Atoll, part of the Marshall Islands in the north Pacific. That shark, along with the remains of a green sea turtle it had swallowed, wound up in a natural history museum. Today, scientists are realizing that this turtle holds clues to the lagoon’s nuclear past—and could help us understand how nuclear research, energy production, and warfare will affect the environment in the future.

In 1952, the world’s first hydrogen bomb test had obliterated a neighboring island—one of 43 nuclear bombs detonated at Enewetak in the early years of the Cold War. Recently, Cyler Conrad, an archeologist at Pacific Northwest National Laboratory, began investigating whether radioactive signatures of those explosions had been archived by some particularly good environmental historians: turtles.

“Anywhere that nuclear events have occurred throughout the globe, there are turtles,” Conrad says. It’s not because turtles—including sea turtles, tortoises, and freshwater terrapins—are drawn to nuclear testing sites. They’re just everywhere. They have been mainstays of mythology and popular culture since the dawn of recorded history. “Our human story on the planet is really closely tied to turtles,” Conrad says. And, he adds, because they are famously long-lived, they are uniquely equipped to document the human story within their tough, slow-growing shells.

Collaborating with researchers at Los Alamos National Laboratory, which was once directed by J. Robert Oppenheimer, Conrad was able to use some of the world’s most advanced tools for detecting radioactive elements. Last week, his team’s study in PNAS Nexus reported that this turtle, and others that had lived near nuclear development sites, carried highly enriched uranium—a telltale sign of nuclear weapons testing—in their shells.

Turtle shells are covered by scutes, plates made of keratin, the same material in fingernails. Scutes grow in layers like tree rings, forming beautiful swirls that preserve a chemical record of the turtle’s environment in each sheet. If any animal takes in more of a chemical than it’s able to excrete, whether through eating it, breathing it in, or touching it, that chemical will linger in its body.

Once chemical contaminants—including radionuclides, the unstable radioactive alter egos of chemical elements—make their way into scute, they’re basically stuck there. While these can get smeared across layers in tree rings or soft animal tissues, they get locked into each scute layer at the time the turtle was exposed. The growth pattern on each turtle’s shell depends on its species. Box turtles, for example, grow their scute outward over time, like how humans grow fingernails. Desert tortoise scutes also grow sequentially, but new layers grow underneath older layers, overlapping to create a tree ring-like profile.

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Because they are so sensitive to environmental changes, turtles have long been considered sentinels of ecosystem health—a different kind of canary in the coal mine. “They’ll show us things that are emergent problems,” says Wallace J. Nichols, a marine biologist who was not involved in this study. But Conrad’s new findings reveal that turtles are also “showing us things that are distinct problems from the past.”

Conrad’s team at Los Alamos handpicked five turtles from museum archives, with each one representing a different nuclear event in history. One was the Enewetak Atoll green sea turtle, borrowed from the Bernice Pauahi Bishop Museum in Honolulu, Hawaii. Others included a Mojave desert tortoise collected within range of fallout from the former Nevada Test Site; a river cooter from the Savannah River Site, which manufactured fuel for nuclear weapons; and an eastern box turtle from Oak Ridge, which once produced parts for nuclear weapons. A Sonoran desert tortoise, collected far from any nuclear testing or manufacturing sites, served as a natural control.

While working at Los Alamos, Conrad met isotope geochemist and soon-to-be coauthor Jeremy Inglis, who knew how to spot even the most subtle signs of nuclear exposure in a turtle shell. They chose to look for uranium. To a geochemist, this might initially feel like an odd choice. Uranium is found everywhere in nature, and doesn’t necessarily flag anything historically significant. But with sensitive-enough gear, uranium can reveal a lot about isotope composition, or the ratio of its atoms containing different configurations of protons, electrons, and neutrons. Natural uranium, which is in most rocks, is configured very differently from the highly enriched uranium found in nuclear labs and weapons.

To find the highly enriched uranium hidden among the normal stuff in each turtle shell sample, Inglis wore a full-body protective suit in a clean room to keep his uranium from getting in the way. (“There’s enough uranium in my hair to contaminate a picogram of a sample,” he says.) Inglis describes the samples like a gin and tonic: “The tonic is the natural uranium. If you add lots of natural uranium tonic into your highly enriched uranium gin, you ruin it. If we contaminate our samples with natural uranium, the isotope ratio changes, and we can’t see the signal that we’re looking for.”

The team concluded that all four turtles that came from historic nuclear testing or manufacturing sites carried traces of highly enriched uranium. The Sonoran desert tortoise that had never been exposed to nuclear activity was the only one without it.

They collected bulk scute samples from three of their turtles, meaning that they could determine whether the turtle took in uranium at some point in its life, but not exactly when. But the researchers took things a step further with the Oak Ridge box turtle, looking at changes in uranium isotope concentrations across seven scute layers, marking the seven years of the turtle’s life between 1955 and 1962. Changes in the scutes corresponded with fluctuations in documented uranium contamination levels in the area, suggesting that the Oak Ridge turtle’s shell was time-stamped by historic nuclear events. Even the neonatal scute, a layer that grew before the turtle hatched, had signs of nuclear history passed down from its mother.

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It’s unclear what this contamination meant for the turtles’ health. All of these shells were from long-dead animals preserved in museum archives. The best time to assess the effects of radionuclides on their health would have been while they were alive, says Kristin Berry, a wildlife biologist specializing in desert tortoises at the Western Ecological Research Center, who was not involved in this study. Berry adds that further research, using controlled experiments in captivity, may help figure out exactly how these animals are taking in nuclear contaminants. Is it from their food? The soil? The air?

Because turtles are nearly omnipresent, tracing nuclear contamination in shells from animals living at various distances from sites of nuclear activity may also help us understand the long-term environmental effects of weapons testing and energy production. Conrad is currently analyzing desert tortoise samples from southwestern Utah, collected by Berry, to better relate exposure to radionuclides (like uranium) to their diets over the course of their lives. He also hopes that these findings will inspire others to study plants and animals with tissues that grow sequentially—like mollusks, which are also found in nearly all aquatic environments.

The incredible migratory patterns of sea turtles, which sometimes span the entire ocean (as anyone familiar with Finding Nemo may recall), open up additional opportunities. For example, sea turtles forage off the Japanese coast, where in 2011 the most powerful earthquake in Japan’s history caused a tsunami that led to a chain reaction of failures at the Fukushima Daiichi Nuclear Power Plant. With lifespans of up to 100 years, many of those turtles are likely still alive today, carrying traces of the disaster on their backs.

Recently, the Japanese government started slowly releasing treated radioactive water from the Fukushima Daiichi plant into the Pacific Ocean. Scientists and policymakers seem to hesitantly agree that this is the least bad option for disposing of the waste, but others are more concerned. (The Chinese government, for instance, banned aquatic imports from Japan in late August.) Through turtle shells, we may better understand how the plant’s failure, and the following cleanup efforts, affect the surrounding ocean.

The bodies of these creatures have been keeping score for millennia. “For better or for worse, they get hit by everything we do,” Nichols says. Maybe, he adds, “the lesson is: Pay more attention to turtles.”

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