9 June 2008

Geoarchaeology: Footprints in ash and the first people in North America

Posted by Jessica Ball

National Geographic News has published an article about the results of new dating methods used on footprints in volcanic ash in Mexico.

The quarry where the footprints were discovered in 2003 is located in Mexico’s Valsequillo Basin (and near the Cerro Toluquilla volcano). The footprints were originally dated in 2005, when an international team of geoarchaeologists concluded that the footprints had been created more than 40,000 years ago.

(Image from the NatGeo website, credited to Silvia Gonzalez, Liverpool John Moores University, United Kingdom.)

There seems to be some confusion between various articles about the ash itself – in another 2005 article, the ash is described as being 1.3 M.Y.O. – far too old for anyone to have created the footprints in it while it was still fresh. The new NatGeo article raises this question, but says that “dating the ash is complicated by the fact that an eruption occurred underwater, said Silvia Gonzalez of Liverpool John Moores University, in the United Kingdom.” What’s strange is that in the Bristol article, the explanation is this:

“The footprints were preserved as trace fossils in volcanic ash along what was once the shoreline of an ancient volcanic lake. Climate variations and the eruption of the Cerro Toluquilla volcano caused lake levels to rise and fall, exposing the Xalnene volcanic ash layer. Early Americans walked across this new shoreline, leaving behind footprints that soon became covered in more ash and lake sediments. The trails became submerged when the water levels rose again, preserving the footprints.”

Both quotes were made by Dr. Gonzales, which is somewhat confusing. I don’t quite understand why the discrepancy exists, but I suspect that at least one of the articles has fallen victim to some bad editing.

Anyway, the short of all this editing mess is that the 1.3 Ma age is a combination of the age of the ash-producing magma and bedrock pulverized by the eruption. The new study used “optically stimulated luminescence” to determine when the hardened ash was last exposed to sunlight (or volcanic heat), and came up with an age of 40,000 years. Sediment below the ash layer was dated to 70-100,000 years, and above it to 9-40,000 years. This means that the footprints should be at least 40,000 years old, and possibly more, since the ash layer could have sat around for a good long time before being buried by the younger sediments.

I find all this pretty exciting because it has to do with an ongoing (but hopefully dying) debate in the archaeological community about when the first humans arrived in North America. It was a favorite topic of the professor who taught my Intro to Archaeology class, and it’s been going on for a good long time now – since the late 1960s, and there are still people arguing about it, although I think the Clovis holdouts pretty much just clinging to dogma by now. “Clovis First” holds that the Clovis people, a Paleoindian culture that first appears in the archaeological record about 13,000 years ago, were the first migrants to the North American continent. The first Clovis points (spear and arrowheads), which have a distinctive “flute” down the center of the point, were discovered in New Mexico in the late 1930s, and have since been found all over North America (and even in parts of Central and northern South America).

For a long time, it was accepted that the Clovis people were the first to show up. Then, in the 1970s, James Adavasio discovered material at the Meadowcroft Rockshelter near Avella, Pennsylvania, that was radiocarbon dated to 16-19 Ka. In 2004, a site in South Carolina called Topper produced charcoal fragments that were radiocarbon dated to 50 Ka, and were interlayered with what appeared to be worked stone tools. There were all sorts of disputes about the accuracy of the carbon dating – some people claimed that the site was contaminated with older material, or that the dating was done incorrectly, or that the charcoal had resulted from natural causes (wildfires or lightning strikes), or that the stone wasn’t worked but had been chipped by natural forces.

Other sites (like Monte Verde in Chile, dated to 14.5 Ka) have been discovered that pretty much negate the Clovis First theory, as they change the timing of migrations to North America over the Bering land bridge. (The Clovis people couldn’t have been the first over the bridge if the Monte Verde culture, which is more than a thousand years older, made it all the way to South America before the Clovis people did. The Monte Verde research also sparked the theory that the migrations were mainly along coastal waterways, and not limited to land, as had been previously thought.)

The whole thing is a great example of how theories evolve – and it’s neat that I can finally use something I picked up in my anthropology minor! (I also think it’s pretty cool that the evidence for really old cultures in the Americas is preserved in volcanic ash, but that’s just me.)