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| Titel |
40Ar/39Ar dating of the Valsequillo volcanic deposits, Central Mexico: Resolution of an ongoing archaeological controversy and implications for the first human colonization of the 'New World' |
| VerfasserIn |
Darren Mark |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250034559
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| Zusammenfassung |
| It is currently accepted that the Clovis culture was the first to migrate into the New World at
13.1 ka [1]. However, archeological evidence in the form of stone tools, linguistics,
craniometrics and genetics suggest that the first Americans were ethnically diverse and a few
sites dated to 15-16 ka BP challenge the ‘Clovis First’ model. Perhaps the biggest challenge
to the ‘Clovis First’ model was the reported presence of human footprints within a basaltic
ash (Xalnene Ash) dated to 38.04 ± 8.57 ka using optically stimulated luminescence (OSL)
[2]. However, Renne et al. [3] challenged the validity of the footprints by dating lapilli from
the Xalnene ash using 40Ar/39Ar and reported an age of 1.30 ± 0.03 Ma (2Ï). They also
reported a reversed palaeomagnetic polarity for the ash, consistent with deposition during
chron C1r.2r. Such antiquity casts considerable doubt on the interpretation of the
impressions as human footprints. Gonzalez et al. [4] questioned the validity of the
40Ar/39Ar age and highlighted the heterogeneous nature of the lapilli as a potential
problem for 40Ar/39Ar geochronology. The lapilli contain abundant phenocrysts and
xenocrysts. Olivine phenocrysts can be contaminated with excess Ar (40ArE) [5] and
hence the dating of 40ArE-bearing lapilli and xenocrystic material may potentially
produce anomalously old 40Ar/39Ar ages. Gonzalez et al. [4] also dismissed the
significance of the reversed palaeomagnetic polarity as the proposed age of the ash (38.04
± 8.57 ka) overlapped with the Laschamp Geomagnetic Excursion at 40.4 ± 1.1
ka.
Subsequently there has been support for both sides of the debate. The OSL age presented was
questioned [6] and reconfirmed by [7]. The OU 40Ar/39Ar laboratory showed the presence of
40ArE in the samples although they were unable to date the ash [2]. Palaeomagnetic
data has both supported emplacement of the Xalnene Ash during the LGE [8,9]
and at 1.3 Ma [10]. The age of the ‘alleged’ footprint-bearing Xalnene ash and
hence the timing of the first human colonization of the Americas remain highly
controversial.
New 40Ar/39Ar ages for basaltic lava flows and silicic tuffs throughout the Valsequillo
stratigraphic sequence resolve the ongoing archaeological controversy. The data have direct
implications for the first human colonization of the ‘New World’.
[1] Waters, M.R. & Stafford, T.W., Jr. (2007) Science, 315, 1122-1126.
[2] Gonzalez, S. et al. (2006a) Quaternary Science Reviews, 25, 201-222.
[3] Renne, P.R. et al. (2005) Nature, 438, E7-E8.
[4] Gonzalez, S. et al. (2006b) World Archaeology, 38, 611-627.
[5] McDougall, I. et al. (1969) GCA, 33, (12), 1485-1520.
[6] Duller, G.A.T. (2006) Quaternary Science Reviews, 25, 3074-3076.
[7] Schwenninger, J.L. et al. (2006) Quaternary Science Reviews, 24, 3077-3080.
[8] Goguitchaichvili, A. et al. (2007) Geofisica Internacional, 46, 85-87.
[9] Goguitchaichvili, A. et al (2009) Earth Planets Space, 61, 205-211.
[10] Feinberg, J.M. (2009) Geology, 37, (3), 267-270. |
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