Records show that deaths have historically oscillated with seasonal conditions. And unpacking this seasonality for people of any age, geography, or era can document a more vivid anthropological history. Some clusters may even reveal ancient epidemics or forgotten histories of seasonal violence.
Eerkens adds that death typically comes with rituals to help people cope—the trauma of many deaths concentrated in a particular time of year can iron a crease into cultural and archaeological legacies. His method offers a new way to identify these annual waves of death.
Isotopes are distinctly heavier or lighter forms of chemical elements; scientists can use instruments called mass spectrometers to identify and quantify isotopes in a sample of hair or bone. With human remains, archaeologists see isotopes as caches of interactions between that individual and their environment.
Hair can be a particularly useful source of isotopes in archaeology. Bones and teeth also carry isotopes, but they stop growing in a person’s 20s. Hair doesn’t. Millimeter by millimeter, follicles assemble keratin proteins with environmental carbon, nitrogen, and hydrogen. “Hair is a fantastic record because it grows so fast,” Eerkens says. “We can get this sort of neat record into someone’s behavior—someone’s life.”
The catch is that hair doesn’t last as long as bone in many burial conditions. “It’s a very, very rare case that we’d ever be finding this hair,” says Siân Halcrow, a bioarchaeologist at the University of Otago in New Zealand, and an expert in infant and child remains. (Halcrow wasn’t involved with the study.)
Edith’s airtight metal casket mummified her skin and prevented her hair from decomposing over the 140-some years she spent underground. Eerkens, a veteran practitioner of isotope analysis, got permission to detach two locks of hair in 2016, when the construction crew unearthed her remains.
He and his colleagues first analyzed carbon and nitrogen isotopes in her hair, and showed, in 2017, that the unidentified child had died battling an illness that starved her over time. That evidence helped identify Edith, but Eerkens had enough strands remaining to investigate hydrogen isotopes in her hair, too. Eerkens knew that the girl had died in San Francisco on October 13, 1876. But he had a hunch that the embedded specks of hydrogen could, in a sense, prove that Edith’s hair “knew” that as well.
Eerkens’ team began by compiling data on hydrogen isotopes’ fluctuation in San Francisco’s water in a given year. Climate and altitude deal out hydrogen and its less common “heavy” isotope, hydrogen-2, differently. Their ratio in hair, therefore, represents specific geographies and annual patterns. Water from a creek in southern Florida, for example, likely flows with more heavy hydrogen isotopes than does water in central California—and isotope ratios in both places will rise and fall with the seasons.