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How tooth isotopes help bring home unidentified soldiers

U researchers are developing methods to connect chemical signatures in teeth with soldiers’ hometowns.

One of the keys to bringing home unidentified military remains, including POW/MIAs and the more than 81,500 soldiers unaccounted for in conflicts dating back to World War II, is using science to determine where home might be. University of Utah scientists are engaged in an effort, in support of the Defense POW/MIA Accounting Agency, to develop methods that can trace the geographic origin of remains, particularly teeth.

Why teeth? Because everyone’s body, including their teeth, contains a record of where they’ve lived and traveled in the form of various stable isotopes of common elements. Through the Forensic Identification of our Nation’s Deceased with Element Mapping, or FIND-EM, project, Gabriel Bowen, professor of geology and geophysics, and colleagues are building a nationwide isotope map to help narrow down the geographic origin of unidentified remains.

“This is a small piece of the puzzle, but something that our research team is proud to contribute,” Bowen says, “If we can help investigators bring closure to even a few more families the effort will be worthwhile.”

Learn more about the FIND-EM project here.

Your isotopic travelogue

University of Utah scientists are world-renowned experts in using isotope data for a wide range of forensic applications. Recently, they’ve used isotopes to trace the origin of illegal ivory and learn about the socioeconomic status and even diet of barbershops’ clientele based on hair clippings.

Why are isotopes such a valuable source of information? For a given element—oxygen, for example—all atoms have the same number of protons. But some atoms have different numbers of neutrons. Those atoms, with very slight differences in weight, are all isotopes of the same element. Isotopes that are unstable and decay, releasing energy in the process, are radioactive isotopes. But isotopes that are stable are called, naturally, stable isotopes.

Oxygen has three stable isotopes: oxygen-16, oxygen-17 and oxygen-18. Around 99.7% of all oxygen atoms are oxygen-16. But around 0.2% are oxygen-18, and 0.04% are oxygen-17. Those trace amounts are what makes isotope science interesting.

Let’s take as an example a cloud that moves from the Pacific Ocean eastward across the United States. When the water vapor in that cloud first evaporates, the evaporation process favors lighter isotopes, changing the ratios among the three isotopes of oxygen. As the cloud moves eastward, first dropping rain on the Pacific coast and then moving inland, the precipitation process favors heavier isotopes, meaning that the isotope ratios in the rainwater keep changing as the cloud keeps moving.

Between this and other processes that act on the water as it moves through the water cycle, water in different parts of the country have different isotope signatures. So the water you drink determines the isotope ratios that will then end up in your tissues and bones.

Why the focus on teeth?

“First and foremost, teeth are often one of the best-preserved parts of the human body after death, which is critical if the goal is to identify remains that may have been buried for nearly a century,” Bowen says. And unlike hair or blood or bone, which continually replaces itself, teeth form at a certain time in a child’s life and lock in the isotopes taken from the locality where that child lives.

“That means we can go to a certain tooth in the dentition, for example a second molar, and know that the chemical signature reflects an individual’s diet and location during a certain part of childhood, in this case between about 3 and 7 years of age,” Bowen says. “Because the teeth form in a sequence, it’s even possible to piece together information from different teeth to understand relocation during childhood.”

The challenge that remains, Bowen says, is that although we know that water isotopes vary across the United States, there’s not yet enough data to build a map that can connect a tooth’s isotope signature to a location.

Bringing unidentified remains home

The potential forensic applications of such a method abound. Researchers at the Defense POW/MIA Accounting Agency, tasked with “providing the fullest possible accounting for our missing personnel to their families and the nation,” are also exploring the potential of isotope science. Bowen and his team, now numbering six researchers, staff and technicians, are working in support of that effort by building a scientific basis for connecting an isotopic signature in molars, specifically the third molars or wisdom teeth, with an individual’s location history.

“Wisdom teeth reflect the teen and pre-teen years,” Bowen says. “This is the period of life right before most service members would have joined the Armed Forces and is most immediately useful to investigators who may have limited information on the childhood history of missing service members but do know where they enlisted or enrolled for service.”

How does this help bring unidentified remains home? Suppose that a World War II-era grave site in Europe is known to contain the remains of several unidentified American soldiers. And suppose that the circumstances of the soldiers’ deaths are known to the degree that they can be traced to a single platoon. If we know the geographic origin of the as-yet-unaccounted-for soldiers in the platoon, then finding a molar that can be traced back to, say, the Pacific Northwest greatly narrows the search for other lines of evidence that can provide a positive identification.

Building an isotopic map

For several years, Bowen and his colleagues have been working on the first stage of building an isotopic map by sampling and analyzing the isotopes in drinking water and groundwater from around the country. For one of those sampling trips, Bowen flew a Cessna aircraft solo around the United States for 10 days, collecting 82 samples. The resulting isotopic map of groundwater was published in January 2022.

Now the focus of the project has turned to collect isotopic values from donated teeth. The team is working with dentists and oral surgeons, as well as individuals who’ve recently undergone a wisdom tooth extraction, to build a database of a few thousand samples out of the millions of people who undergo wisdom tooth extraction each year.

“Once we are able to compare the USA tooth database with those maps, we will learn how strong the relationship between tooth and drinking water chemistry is,” Bowen says, “and also learn about other factors, like diet or certain health conditions, that affect the water/tooth relationships and need to be considered when using tooth data for human identification.”

Potential donors complete a short survey. If the team determines that the teeth would be useful for the project, the donor receives a prepaid mailer to send the teeth to the lab.

Analyzing teeth in the lab may seem like an unconventional way to honor US military service members. But it’s in support of an effort that touches one of the military’s fundamental values: to leave no one behind.

“It is both a measure of respect for our service members and a source of strength within our Armed Forces,” Bowen says. “This commitment doesn’t expire when a conflict ends.”

Learn more about the FIND-EM project and the process for donating teeth here.