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.