Strontium is key in solving archaeological puzzles because it is one of the most geographically traceable elements: It originates in bedrock, and each type of regional rock contains a unique ratio of strontium isotopes. That isotopic ratio acts as the rock's signature—it gets passed on to the soil above and ultimately into the plants that are grown there. When people eat those plants, their bodies incorporate that same ratio into their bones and teeth. While strontium in bones can fluctuate throughout people's lives, its levels in molars are fixed in childhood, so our teeth can serve as a sort of treasure map, leading to where our food was grown when we were young.
For most Americans, that doesn't mean much: We grew up eating apples from New York, lettuce from California, and corn from the Midwest. But in the regions of Mexico responsible for the highest numbers of migrants—Guanajuato, Oaxaca, Sonora, Chiapas, and Michoacán—most people still raise their own chickens and shop at the regional produce market. Which means their teeth might be able to tell us where they're from.
With two hands, Juarez positions the tooth below a long, thin metal drill bit and rotates it to the front. Applying slight pressure to a foot pedal, she starts the drill, and it burrows in, stirring miniscule bits of dentin into the air like leaves in a mini-whirlwind. The bits float slowly down onto a thin white tissue below, which Juarez folds, and tips into a tiny transparent vial.
Juarez is creating a map of isotopic signatures throughout Mexico. To do this, she needs to gather tooth samples from all over the country, which has proven more difficult than she initially thought.
A Tucson funeral-home worker prepares an unidentified body for burial.
At first, Juarez tried getting teeth from dental clinics in Mexico. But only wealthy Mexicans get dental care. And wealthy Mexicans eat like Americans; their teeth would simply say: supermarket. She had better luck once she began visiting US dental clinics that cater to immigrants. These patients, she figured, had crossed the border at some point and survived. Perhaps the teeth of the living could bring dignity to the dead.
The tooth Juarez just finished drilling arrived in a tiny plastic bag from a dental clinic in nearby Half Moon Bay, where immigrant patients having teeth pulled can contribute their teeth to Juarez's project. A label on the bag reads:
Hometown: Jalisco, MX
Did you move as a child?
Sex: _F _xM
Once the tooth's enamel has gone through all the steps of analysis—from an acid cleaning process to drying, crushing, "digesting," freeze-drying, and then hurtling at high speeds through a mass spectrometer—Juarez will plot its isotopic signature on a map of Mexico. Once the map is complete, Juarez hopes that labs will be able to compare isotopic signatures in unidentified border crossers' teeth to those on the map to find out where the crosser grew up. Imagine those new points, then, as an overlay: the topography of poverty, the landscape of leaving home.
Juarez's father was an immigrant. He entered the country when he was 15, along with his grandmother, sister, and brother. Juarez has always been interested in border politics, but in college she realized she wanted to be a scientist. The challenge was to do work that felt relevant to the Latino community.
"I believe in doing science for the people—breaking out of academia and doing work that's useful," she says. "And even if you don't think undocumented people should be here, you can agree that dying in a strange country, and losing touch with your family, that's a tragedy."
Juarez and I relocate to the Earth and Marine Sciences building, home of the Neptune, a hunk of stainless steel and white painted metal that looks like a high-tech copy machine turned inside out. Juarez will insert the processed tooth samples into the Neptune, and it'll spit out the isotopic ratios.
Over the roar of the ventilation, I ask Juarez what her colleagues working in border forensics think of her work. They were wary at first, she explains. In 2004, Juarez attended a forensic symposium in Dallas, which focused on migrant death. There she met some of the primary people working on the issue of border death, including Parks and his colleague, Bruce Anderson, a forensic anthropologist.
"They weren't exactly like, 'Let me take you under my wing,'" she says. "But you know, they've been burned."
Juarez is referring to an incident from several years ago: The office of the Pima County ME was threatened with a lawsuit by the son of a deceased woman whose body was buried without the brain, which was still soaking in formaldehyde. Parks has learned to be particularly protective of all the human materials under his care.
Chelsey Juarez in the lab where she's been tracing the origins of migrants' teeth since 2005.
When I asked Parks about his initial reluctance to work with Juarez, he replied that his loyalty is to the families of the deceased.
"If a technique is helpful, then we'll do all we can to help," he said. "But I can't just be giving body parts to people for research. We have to remember that even if these [bodies] are unidentified, they're still people—they still have families." Of late, however, the Pima County crew has become more receptive to Juarez's work; they have also worked with a molecular anthropologist at Baylor University on a DNA database that tries to match unidentified remains with family members searching for loved ones.
The spectrometer is churning out paper now, each white sheet printed with line after line of numbers extending to the umpteenth decimal point. All of these isotopic ratios will go on her map of Mexico. For it to be truly complete, she'll need many more teeth than she's analyzed so far—especially from regions where her data is still thin.
"I will probably be working on this for the rest of my life," she says. "It's a lifelong project, a life's work." She pauses, considers what she's just said, and adds, "Sheesh."
THE OFFICE of the Pima County ME has a reputation for taking great pains to identify bodies—and in 70 percent of cases, it succeeds. But sometimes pursuing long-shot leads takes years and still it comes up short. Then the dilemma is transferred to Raymond Rodriguez, who handles "indigent burials" for the county, which means he buries prostitutes, bums, and anyone else whose family doesn't know they are dead, or who left no one willing or able to pay for their burial. In their home countries, few border crossers would fall into this category—most have migrated precisely to support the many people they've left behind.
Rodriguez has a thick head of gray hair and dresses like a high school principal: crisp collared plaid shirt, pleated navy-blue pants. I've asked him to accompany me to Evergreen Cemetery, a small out-of-the-way portion of which is reserved for the burial of indigents. We meet in downtown Tucson and drive north until the mattress superstores and taco joints of the exurbs begin to melt back into desert.
To get to the potter's field, we pass through the private burial areas within the cemetery, where scrubby green trees cast shade over sturdy engraved headstones. Those areas are lush compared to the indigent section, a wide-open stretch of bare soil, naked to the midday sun. Here, the headstones lie flat—some are plastic, some stone—a few of them crowded with tacky silk flowers and portraits of the Virgin Mary.
Body bags, many holding unidentified bodies found in the desert, sit in a cooler at the Pima County ME office.
Rodriguez clasps his hands at the small of his back, a few strides behind me, as we begin to pace the rows. He explains that the funeral home they contract with has to use concrete liners for the graves, because the desert soil shifts with the winter's flash floods. Right then I notice a sinkhole where a headstone should be—the grave marker has sunk nearly a foot below ground.
"Rainy season in Arizona is pretty much mid-June through mid-September, and it really sinks the ground," he says. "But this is tax dollars, and if you don't have enough tax dollars you can't keep up with the private cemeteries."
There aren't many Juan Doe graves here more recent than 2004—that's the year Arizona amended a law to allow Pima County to begin cremating anonymous remains.
Rodriguez is proud to show me a better-groomed part of the indigent section—the columbarium, where cremains are stored. Rodriguez's boss pushed hard for the funds to build it, and the dust of construction has barely settled. Simple stone plaques mark the openings where urns will be placed—about a dozen per slot, Rodriguez says.
As we gaze at the rows of plaques, I ask Rodriguez whether a sense of tragedy ever overwhelms him.