Redating Mexico’s Toloquilla Footprints with optically stimulated luminescence

The peopling of the Americas is one of my favorite subjects in anthropology. Lately, we’ve seen a whole slew of studies that focus on this topic but through a genetic lens, the most impactful of which indicates that people began migrating to the Americas roughly 16,000 years ago.

But there are some inconsistencies with this date, especially the 325 foot-like prints found in Valsequillo Basin’s Toloquilla rock quarry in Mexico. These footprints were within a soft, damp volcanic ash along a lakeshore shortly after a volcanic eruption. This type of volcanic tuff is known as the Xalnene. Samples of the Xalnene tuff were sent off to the Berkeley Geochronology Center for argon-argon and paleomagnetic dating. The results, published in this 2005 Nature study, “Age of Mexican ash with alleged ‘footprints’,” indicated the ash layer to be 1.3 million years old, making many wonder as to whether the indentations were even made by humans.

The team that initially found the prints have revisited the dating of the Xalnene tuff at Toloquilla. They say that the ‘dating of the ash is complicated by the fact that an eruption occurred underwater.’ The team reanalyzed the tuff by optically stimulated luminescence (OSL), a method that requires sampling deep within the tuff and in complete darkness. The samples are then irradiated with an atomic reactor and when ultraviolet light is shone onto the irradiated samples, the resulting fluorescence reveals how long it has been since the rock was last exposed to sunlight—or volcanic heat.

The results of the OSL redating were shared at last week’s meeting of the American Geophysical Union in Florida. Right below the ash, the sediments date to 70,000 and 100,000 years old. The sediments above, date to 9,000 to 40,000 years old. With this time range, it is possible to confer the OSL dates with carbon-14. The team did just that, radio-carbon dating the ages of shells in the sediments above and below the ash layer. The dates of all three layers therefore suggest the footprints were made about 40,000 years ago.

This date indicates humans were in the Americas 25,000 years before the coalescence dates from the most recent genetic studies, and 27,000 years before the Clovis culture. So it begs one to ask if the foot prints are even made by humans? The team scanned the foot prints with a laser scanner and constructed images to compare to footprints made by volunteers trotting on a beach in the United Kingdom.

Ultimately, this study challenges argon-argon and paleomagnetic dating. Paul Renne, of the University of California, Berkeley’s Geochronology Center and Rafael Suárez of the Museo Nacional de Historia Natural y Antropología in Montevideo, Uruguay both are critical… they wonder why there isn’t an archaeological record for 27,000 years.

    Renne, P.R., Feinberg, J.M., Waters, M.R., Arroyo-Cabrales, J., Ochoa-Castillo, P., Perez-Campa, M., Knight, K.B. (2005). Geochronology: Age of Mexican ash with alleged ‘footprints’. Nature, 438(7068), E7-E8. DOI: 10.1038/nature04425

5 thoughts on “Redating Mexico’s Toloquilla Footprints with optically stimulated luminescence

  1. There are reasons to be very critical. You don’t normally date human presence on controversial footprints and all the rest is against. I have read on other isolated odd datations being discarded almost rutinarily by prehistorians: 12,000 BP Acheulean in NW Spain, a Bell Beaker from Soria apparently from almost 1,000 years before Bell Beaker existed… Things like that happen, you need something more solid to build a theory.

    Btw, I have not even clear they are human-made. Your pic shows an apparent trail but they looked bear like to me. The image at National Geographic is not much better but guess I can discard the bear. What about Bigfoot? ;)

    It could be anything: specially if the eruption happened underwater. Why would any human (or any other animal) step on fresh (I mean: hot) volcanic ash anyhow.

  2. A few years back I observed moose footprints in a snowbank that appeared to be huge human footprints. The moose put each foot into the snow leaving a large entry hole. It then moved forward leaving a “waist,” or narrowing where its lower leg had moved forward through the snow. As the foot was removed from the snow for the next step, a larger hole was left at the other end of the “waist.” After fine snow almost filled the deep tracks, the resulting “footprints” appeared to have been left by a bipedal hominine. Many supposed “footprints” may, indeed, be left by other animals. This can only be determined in the same way that I determined a moose rather than a hominine made the footprints in the snow, by digging deep to find the real prints well below the surface!

  3. Assuming they are indeed hominid footprints, is it possible humans were in the Americas much longer ago than previously thought? Perhaps in much smaller numbers and populations than the generally accepted populations of 15K years ago or so. Maybe for some reason these hominids in the new world became extinct only to be replaced by the newer cultures of 15K years ago.

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