Valsequillo Footprints Back In The News

The Valsequillo footprints are back in the news, again. National Geographic has a story on new dates on the footprints:

To attempt to date once again the indentations, Gonzalez’s team used a new method called optically stimulated luminescence.
This involved collecting samples in the dark, then irradiating them with an atomic reactor.
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.

Using this technique, the footprints were dated to 38 KYA (plus or minus 8 KY). Additional radiocarbon dates are also reported:

Just to be sure, her team also used carbon-14 dating to determine the ages of shells in the sediments above and below the ash layer.
“You can’t date just one layer,” she said. “You have to makes sure the whole stratigraphy makes sense.”
The sediments immediately below the ash were between 70,000 and 100,000 years old, Gonzalez’s team found.
The sediments immediately above the ash ranged from 9,000 to 40,000 years old. The dates of all three layers therefore suggest the footprints were made about 40,000 years ago, she said.

One thing from the story that caught my attention was the 3D mapping of the footprints. According to the Mexican Footprints Website the 3D mapping was followed by…you guessed it rapid prototyping.

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2 Responses

  1. This confuses me:

    [C]arbon-14 dating [was] to determine the ages of shells in the sediments above and below the ash layer.

    The sediments immediately below the ash were between 70,000 and 100,000 years old…

    I’m not an expert, but my understanding is 14C dating isn’t usable (or is too unreliable?) for datings of more than c.50KYA. So isn’t a dating of 70-100KY(A?) very dubious if based (solely?) on 14C dating?

  2. Yes, and the Nat Geo article mentions that:

    James Dunbar, an archaeologist at Florida’s Bureau of Archaeological Research in Tallahassee cautions that carbon-14 dating in the 40,000-year age range is often suspect, because the samples might include “dead carbon,” or carbon not originally from living organisms.

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