Footprints in ash layers, from from web site of Sarah Metcalfe, University of Nottingham
The debate began in July 2005 when Dr. Silvia Gonzalez and her colleagues at Liverpool John Moores University reported that they had found 40,000-year-old human and animal footprints preserved in volcanic ash. The ash had fallen along a shoreline of a shallow lake in the Valsequillo Basin, about 80 miles southeast of Mexico City. If the dates are correct, it would make them the earliest evidence for people in the Americas. Gonzalez’s team based their analysis on accelerator mass spectrometry dating, electron spin resonance, Argon-Argon dating, and optically simulated luminescence dating. Knowing that the dates would be controversial, Gonzalez said at the time “It’s going to be an archaeological bomb and we’re up for a fight.”
By the end of 2005, a team at Berkeley Geochronology Center at the University of California Berkeley had responded. They reported that the ash dated to 1.3 million years old. Either the tracks belonged to our ancestor Homo erectus or, more likely, the impressions were not footprints but marks that resulted from quarrying. The California team also relied on Argon-Argon dating, as well as a paleomagnetic data. Their most recent and detailed analysis of the quarry site ash appeared in the March 2009 issue of Geology.
Numerous small quarries dot the Valsequillo Basin. The ashy layers split into thin sheets that can be used in roads and walkways. The track-rich quarry was abandoned relatively recently, according to Joshua Feinberg, formerly a graduate student at UC-Berkeley and now a professor at the University of Minnesota, but not before workers had removed several feet of lake sediments to get to the underlying hard beds of ash. As happened with the quarries examined by Gerta Keller for evidence that gases released by voluminous basalt flows led to the extinction of the dinosaurs, the Mexican quarry workers made the story possible by exposing the ash.