News and Views Dating of the Valsequillo volcanic deposits: Resolution of an ongoing archaeological controversy in Central Mexico Darren F. Mark a, * , Silvia Gonzalez b , David Huddart b , Harald Böhnel c a NERC Argon Isotope Facility, Scottish Universities Environmental Research Centre, Rankine Avenue, East Kilbride G75 0QF, UK b School of Natural Sciences and Psychology, Liverpool John Moores University, Byrom Street. Liverpool, L3 3AF, UK c UNAM, Centro de Geociencias, Campus Juriquilla, Queretaro 76230, Mexico article info Article history: Received 19 September 2009 Accepted 2 February 2010 Introduction The timing and origin of the earliest human colonization of the Americas has been the subject of great debate over the last 100 years and is still a matter of heated discussion today (Renne et al., 2005; Gonzalez et al., 2006a). It is widely accepted that the Clovis culture was the first to migrate into the New World at 13.1 ka (Waters and Stafford, 2007). However, archaeological evidence, in the form of stone tools, linguistics, craniometrics and genetics all suggest that the first Americans were ethnically diverse, and a few sites dated to 15e16 ka challenge the ‘Clovis First’ model (Goebel et al., 2008). Perhaps the most spectacular challenge to the ‘Clovis First’ model was the reported presence of human footprints within a basaltic ash (Xalnene Ash, Valsequillo Basin, Central Mexico), dated to 38.04 Æ 8.57 ka using optically stimulated luminescence (OSL; Gonzalez et al., 2006a). However, Renne et al. (2005) chal- lenged the validity of the footprints by dating lapilli from the Xal- nene Ash using 40 Ar/ 39 Ar and reported an age of 1.30 Æ 0.03 Ma (2s). Renne et al. (2005) also reported a reversed palaeomagnetic polarity for the ash, consistent with deposition during chron C1r.2r (Cande and Kent, 1995). Such antiquity casts considerable doubt on the interpretation of the impressions as human footprints. Gonzalez et al. (2006b) questioned the 40 Ar/ 39 Ar age and high- lighted the heterogeneous nature of the lapilli as a potential problem for 40 Ar/ 39 Ar geochronology. The lapilli contain phenocrysts and include xenocrysts. Olivine phenocrysts can be contaminated with excess Ar ( 40 Ar E ; McDougall et al., 1969) and hence the dating of 40 Ar E -bearing lapilli and xenocrystic material may potentially produce anomalously old 40 Ar/ 39 Ar ages (although it should be noted that 40 Ar E was not evident in the data of Renne et al., 2005). Gonzalez et al. (2006b) also dismissed the significance of the reversed palae- omagnetic polarity, as the proposed age of the ash (38.04 Æ 8.57 ka) overlapped with the Laschamp Geomagnetic Excursion (LGE) at 40.4 Æ 1.1 ka (Guillo et al., 2004). Subsequently, there has been support for both sides of the debate. The OSL age presented by Gonzalez et al. (2006a) was questioned (Duller, 2006) and reconfirmed (Schwenninger et al., 2006), and interpretations of palaeomagnetic data support both deposition of the Xalnene Ash during the LGE (Gogichaishvili et al., 2007, 2009) and at 1.3 Ma (Feinberg et al., 2009). In attempt to resolve the controversy surrounding the age of the Xalnene Ash, we report new 40 Ar/ 39 Ar data for the volcanic rocks of the Valsequillo Basin (Fig. 1A). We sampled basaltic lava flows and alkaline tuffs from the Valsequillo stratigraphic sequence to produce a chronology for the basin fill. Samples The volcanic deposits from the Valsequillo (most sampled for this study) comprise the following major units, which are arranged stratigraphically from oldest to youngest (Fig. 1B): (1) Caulapan Tuff: rhyolitic pyroclastic deposit with pumice (sample V20, C1 and C2), (2) Ixcalo lava: basaltic andesitic, subaerial lava (not dated), (3) Guadalupe lava: basaltic subaerial lava (sample V9), (4) Flank Tol- uquilla lava (sample V7) basalt andesitic, subaerial lava, (5) Xalnene Ash: alleged footprint-bearing olivine basalt ash produced during an explosive hydromagmatic eruption (sample TL4). The eruption started subaqueously but the summit Toluquilla lava was emplaced subaerially (samples V2, CT1 and CT2), and (6) Valsequillo Gravels: include several volcanic ashes (e.g., the Hueyatlaco Ash). Gonzalez et al. (2006a) provide a detailed description of the basin stratigraphy. Results To reduce the possibility of analyzing 40 Ar E -bearing material, basaltic ash and lavas were carefully prepared to remove pheno- crysts and xenocrysts (details in supplementary material [file #1]). * Corresponding author. E-mail address: d.mark@suerc.gla.ac.uk (D.F. Mark). Contents lists available at ScienceDirect Journal of Human Evolution journal homepage: www.elsevier.com/locate/jhevol 0047-2484/$ e see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jhevol.2010.02.009 Journal of Human Evolution 58 (2010) 441e445