New Research Questions in Dating Ice Age Footprints in North America

New Research Questions in Dating Ice Age Footprints in North America

According to a study published in September 2021 in the journal Nature, preserved footprints found in the Lake Otero Basin in New Mexico were dated to the last ice age between 23 and 21 thousand years ago.

The footprints were found in an ancient lake bed alongside evidence of sloths and giant mammoths, which the study said was the “first unequivocal evidence of human occupation anywhere in the Americas” at most. peak of the last glacial cycle, known as the last glacial maximum.

New research from scientists at DRI, Kansas State University, University of Nevada, Reno and Oregon State University now disputes the original study, suggesting that the footprints could have been left thousands of years later than originally thought.

Charles Oviatt, professor emeritus of geology at Kansas State University and one of the authors of the new study, said: “I read the original Science article on human footprints at White Sands and I first was struck, not only by the scale of the footprints. , but how important precise dating would be. I saw potential problems with the scientific testing of the dates reported in the scientific paper.

Archaeologists and historians use a number of methods to determine the timing of historical events. Based on these methods, scientists tend to agree that the earliest known dates of human settlement of North America are between 14 and 16 thousand years ago. If the initial assertions are correct, current chronological models in fields as varied as paleogenetics and regional geochronology should be reassessed.

By studying ancient DNA from human fossils and using rates of genetic change (a kind of molecular clock using DNA), paleogeneticists speculate that the American Southwest was first occupied 20 years ago. 000 years. If the fingerprints are older, this calls into question the use and integrity of these genetic models. It is possible that ages from a single-site study in a New Mexico lake basin are valid and that age estimates from various other areas are invalid, the authors write, but stronger evidence is needed to confirm the assertions.

At the center of the debate are tiny seeds used to date footprints using radiocarbon dating methods, in which researchers examine carbon-14 that comes from the atmosphere and is taken up by plants through photosynthesis.

These carbon isotopes decay at a constant rate over time, and by comparing the amount of carbon-14 in the atmosphere to the amount present in fossilized plant matter, scientists can determine their approximate age.

However, the seeds of the plant come from Ruppia cirrhosa, an aquatic plant that grows underwater and therefore obtains much of its carbon for photosynthesis not directly from the atmosphere as land plants do, but from carbon atoms dissolved in water.

“Although researchers recognize the problem, they underestimate the basic biology of the plant,” says Rhode. “For the most part, it uses the carbon it finds in the waters of the lake. And in most cases, that means it’s taking in carbon from sources other than the contemporary atmosphere — sources that are usually quite old.

This method is likely to skew radiocarbon-based age estimates, as the results will be much older than the plants themselves. Ancient carbon enters groundwater in the Lake Otero basin from eroded bedrock in the Tularosa Valley and surrounding mountains and occurs in extensive calcium carbonate deposits throughout the basin.

The authors demonstrated this effect by examining known-age Ruppia plant material from the same region. Botanists collected live Ruppia plants from a nearby spring-fed pond in 1947 and archived them at the University of New Mexico Herbarium. Using the same radiocarbon dating method, plants that were alive in 1947 returned a radiocarbon date suggesting they were around 7400 years old, a mismatch resulting from the plant’s use of ancient groundwater.

The study suggests that if Ruppia seeds dated from human footprints were also off by about 7,400 years, their true age would be between 15 and 13 thousand years old – a date that matches the age of several other archaeological sites north- known Americans.

Dating of footprints can be solved by other methods, including radiocarbon dating of land plants (which use atmospheric carbon, not groundwater carbon) and optically stimulated luminescence dating of quartz found in sediments, write the authors.

“These leads are really a great resource for understanding the past, there’s no doubt about that,” says Rhode. “I would love to see them myself. I’m just cautious about the age that researchers give them.


Desert Research Institute

https://doi.org/10.1017/qua.2022.38

Header image credit: Jeff Pigati and Kathleen Springer, USGS

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