This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Deep within an ancient sinkhole lie the remains of dozens of mammoths, prehistoric wanderers who waded into the warm springs to their deaths. These deaths had long been accepted as having occurred roughly 26,000 years ago, but new dates published in the journal Quaternary Research suggest these ice-age giants were more than 160,000 years old.
More than 60 mostly young male Columbian mammoths have been found at Mammoth Site in South Dakota since its discovery in 1974, the largest concentration in North America. The new luminescence dates suggest these mammoths lived in a far deeper, likely colder chapter of the Pleistocene, making the site one of a rare few dated so deep into the Northern Great Plains' past. Jim Mead, an emeritus researcher at Mammoth Site and the University of Arizona, had doubted those dates for years.
Having worked as a graduate student in the University of Arizona's radiocarbon lab, Mead found the dates researchers got from the mineral part of bone, called apatite, to be unusual. "I have always wanted to get a different chronology method to date the site…" he said, "and a decade later here we are." The Mammoth Site was discovered by accident when work on a housing project revealed a collection of giant tusks. Soon after, Mead and the late Larry Agenbroad, a former professor at Northern Arizona University and site director, got to work digging up one bone after another, the study authors wrote.
Their investigations revealed an ancient sinkhole likely formed when the ground above collapsed as underlying groundwater slowly dissolved the supporting rock, creating a 4- to 5-meter-deep (13- to 16-foot-deep), 90- to 95-degree-Fahrenheit (32- to 35-degree-Celsius) death trap. According to the study, the slick, steep sides meant unwary mammoths had no chance of escape once inside, sharing their fate with at least one short-faced bear, North American camel, wolf and coyote. Dating the remains was a dilemma even then.
The bones had no collagen, a protein typically used for radiocarbon dating, so researchers had to use bone apatite, which is notorious for yielding dubious ages in similar water environments. Six dates were taken, yielding ages between 23,000 and 42,000 years, the study reported. Two other dating methods challenged these dates, suggesting ages of 129,000, 150,000 and 101,000 years.
As the bones provided irreconcilable results, the researchers turned to the surrounding sand. They used luminescence, which dates when mineral grains in the sediment were last exposed to sunlight. This works by measuring how much radiation they absorb while buried, but exposure to light resets that clock.
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