The BBC reports on a BBC program covering Silvia Gonzalez and the Cerro Toluquilla "footprints". (I have a roundup of stories from the initial announcement. )
The new BBC article gives a good account of how they were found and the context they are in. The big problem is the dating:
More controversial still are the dates. Colleagues of Silvia Gonzalez at Oxford University used a technique called optically stimulated luminescence (OSL) that records the last time rocks were exposed to sunlight or heat.
That gave a variety of dates from the overlying sediments, but when applied to small fragments of what looked like brick or burnt clay within the volcanic ash, it produced a date of about 40,000 years.
That initially shocked Dr Gonzalez as it implied by far the earliest evidence of humans in the Americas. But it fitted in with dates of up to 38,000 years based on carbon 14 in shells in the sediments above.
The dispute right now boils down to a "last gasp" by Gonzalez. Paul Renne's group found that the ash layer dates to 1.3 million years by Ar-Ar. But there is always a chance that the layer was actually redeposited by water from an earlier ashfall. At 1.3 million years old, the ash layer ought to have reversed polarity, since the last period of normal polarity began 780,000 years ago. When Renne's team checked the polarity of the ash, they found it does have reversed polarity. So the evidence is very strong that the ash layer is 1.3 million years old.
Here's Gonzalez' argument:
Silvia Gonzalez' view? "We know that there are short-term 'excursions' of the magnetic field, and one of those happened 40,000 years ago, very interestingly."
Why is this special pleading? Because the ash is still 1.3 million years old! So for the "footprints" to be 40,000 years old, the ash must have been redeposited from some earlier source, and been plastic at just the time of a paleomagnetic excursion.
I love Renne's reaction:
Professor Renne: "How did I know they were going to say that? There is a finite possibility that that is correct, but the probability is extremely low."
That's why I try not to disagree with geochronologists!
It would be one thing if there were clearly consistent footprint trails. But there aren't. Since it's the floor of a quarry, I expect eventually to hear someone come forward who knows how the marks were made.
