Down in the dark of an ancient sinkhole in South Dakota, more than 60 Columbian mammoths met their end — mostly young males who slipped down slick walls into warm, steamy springs they could never climb back out of. For decades, scientists thought these ice-age giants had died about 26,000 years ago. But new dating, just published in the journal Quaternary Research, tells a far bigger story: these mammoths lived more than 160,000 years ago, pushing the site's age back by more than 100,000 years.
This matters because the Mammoth Site, discovered in 1974 when workers building a housing project stumbled on a pile of giant tusks, holds the largest concentration of mammoth remains in North America. It is now one of only a few places dated so deep into the Northern Great Plains' distant past — a hint that these animals lived in a colder chapter of the Pleistocene, a long stretch of time known as the ice age.
For years, Jim Mead, an emeritus researcher at the Mammoth Site and the University of Arizona, had doubted the old dates. As a graduate student working in a radiocarbon lab, he found the numbers odd. "I have always wanted to get a different chronology method to date the site," he said, "and a decade later here we are."
The trouble was the bones themselves. They had no collagen, the protein normally used for radiocarbon dating, so researchers had to rely on bone apatite — a method known for being unreliable in watery places. Six dates came back between 23,000 and 42,000 years. Two other methods suggested something much older, around 101,000 to 150,000 years.
So the team turned to the sand. They used a technique called luminescence dating, which figures out when mineral grains were last hit by sunlight. While buried, grains soak up radiation like a ticking clock; sunlight resets it. The usual grains, quartz, were useless — they had already soaked up so much radiation they could only say "at least 75,000 years." So researchers switched to feldspar grains.
The sampling happened right inside a museum built over the excavation, so visitors could wander past and chat with the scientists. "It was the purest form of science education," said Shannon Mahan, a research geologist with the U.S. Geological Survey, "because it did not matter what that person's religion was, what their political affiliation was, or even their educational level."
Two separate labs analyzed the samples and returned matching results: the site is roughly 160,000 to 232,000 years old. Why were the old radiocarbon numbers so wrong? Emma Krolczyk, a U.S. Geological Survey geologist, explained that old carbon in bone apatite can dissolve and reform with fresh carbon from groundwater. "This essentially resets the radiocarbon signal," she said. The mammoths' true date had been hiding in the sand all along — they just needed the right grains to tell it.