@article{Renne2005,
	title = {Age of the Xalnene Ash, Central Mexico and Archeological Implications},
	author = {P. Renne and J. Feinberg and M. Waters and J. Cabrales and P. Castillo and M. Campa and K. Knight},
	day = {xx},
	journal = {AGU Fall Meeting Abstracts},
	month = {Dec},
	note = {Renne, P. R.; Feinberg, J. M.; Waters, M. R.; Cabrales, J. A.; Castillo, P. O.; Campa, M. P.; Knight, K. B. (Thulefoth 2008-05-12 05:25:10)},
	pages = {4},
	url = {http://adsabs.harvard.edu/abs/2005AGUFM.U42A..04R},
	year = {2005},
	biburl = {http://www.bibsonomy.org/bibtex/2267b12cc13d07f3a244630d11195c493/thulefoth},
	abstract = {Human footprints {\textasciitilde}40 ka old have been reported from the Toloquilla quarry near Valsequillo Reservoir, ca. 15 km south of the city of Puebla in central Mexico (http://www.mexicanfootprints.co.uk/default.htm). If correct, this would be important evidence for early peopling of the Americas. The indentations interpreted as footprints and other ichnofossils occur on the surface of an indurated basaltic lapilli tuff within a several meter thick sequence of thinly bedded (1-10 cm) tuffs of similar character, lacking paleosols, erosional features or interlayered sediments, informally known as the Xalnene ash. A sample was collected at 18{\textdegree}55.402` N latitude and 098{\textdegree}09.375` W longitude from the surface on which the purported footprints occur. Lapilli were separated and analyzed by incremental heating 40Ar/39Ar methods, yielding 9 indistinguishable plateau ages averaging 1.30 {\textpm}0.03 Ma (2$\sigma$) for single lapilli (N=6) and multiple lapilli (N=3) subsamples. Though some minor discordance (presumably due to 39Ar recoil) is manifest in 5 of the age spectra, all plateaux comprise >60\% of the 39Ar released and 4 or more consecutive steps. Paleomagnetic data from azimuthally unoriented bulk samples of 11.25 cm3 reveal a reverse polarity (I = -32.1{\textdegree}) thermoremanent component carried by titanomagnetite and a normal polarity component carried by goethite. Measurements on individual matrix-free lapilli lack the goethite component, which is presumed to be associated with the clay-rich cement. Consistency of the reverse component implies deposition of the lapilli at supra-Curie temperatures, with no postdepositional reworking. Reverse polarity is consistent with deposition during chron C1r.2r (1.77 to 1.07 Ma) as indicated by the 40Ar/39Ar data. If the features observed on the tuff are indeed footprints, their 1.3 Ma antiquity would be truly remarkable, predating by far any other evidence for human presence in the Americas and in fact predating the evolutionary emergence of Homo sapiens (in Africa) by more than 1 Ma. We conclude that the identification of these features as syn-depositional human footprints is likely erroneous.},
	keywords = {paleomagnetic }
}