The initial peopling of the Americas is a contested and evolving topic 1 , with the exact timing of the first arrivals still unknown. Historically, Mexico's understudied and controversial archaeological record has remained on the periphery of First Americans research 2 . However, in recent years, investigations have shown reliable evidence of a late-Pleistocene and early-Holocene human presence in the northwest region 3,4 , the Chiapas Highlands 5 , Central Mexico 6 , and the Caribbean coast [7][8][9] . Evidence of human presence at Chiquihuite Cave extends this antiquity and attests to the cultural variability of older-than-Clovis sites [10][11][12][13][14][15][16][17] and the earliest humans on the continent. Site settings and excavation context.

The complex geographical scenario of Mexico allowed the cultural diversification and development of multiple cultures such as Tolteca, Teotihuacan, Mexica, and Maya, among others. Despite this rich cultural heritage, radiometric dating of Mexican cultural samples with radiocarbon (14C) began only in the 1980s and with accelerator mass spectrometry (AMS) in 2013. Analysis of 14C with AMS is the most widely used technique to date archaeological objects and cultural heritage. Since 2013, the Accelerator Mass Spectrometry Laboratory (LEMA) facility of the Institute of Physics at UNAM (IF-UNAM) has supported archaeological research in Mexico, but also investigation in other areas such as geology, physics, chemistry, and environmental sciences through the analysis of 14C, 10Be, 26Al, 129I, and Pu. The absolute dating with 14C continues to be the core of LEMA’s work, where different geographical scenarios of the country and climatic conditions present very diverse analytical challenges. This work presents a basic description of the AMS system of the LEMA laboratory and describes some applications that are currently being developed.

In 2003, flooding occurred in the Ciudadela (Citadel) of Teotihuacan and saltpeter began to damage the Temple of the Feathered Serpent. Work done to solve this problem led to one of the most important archaeological discoveries made in this site in recent years: an intact tunnel sealed for more than a thousand years. The project created to study the tunnel was named Tlalocan or Path to the Underworld. More than 60,000 objects have been recovered after years of exploration and removing huge amounts of soil and stones. This paper presents the first results of accelerator mass spectrometry (AMS) radiocarbon dating performed on some of those materials recovered from the tunnel. With these findings, in combination with the archaeological data, based on stratigraphy and ceramic typology, a chronology of several events is proposed concerning the construction phases and ceremonial use, as well as partial and definitive closures of the tunnel. Every closure was accompanied by a deliberate and structured deposition of offerings and ritual refuse along the tunnel. The range of ages that covers the Bayesian calibration of samples collected along the tunnel is around 115 yr, from AD 125 to 240. Material collected at the surface of the chamber located at the end of the tunnel and under the pyramid gave ages in the interval between AD 400 and 534. All samples analyzed fall within the interval of time that covers the period of occupation of Teotihuacan.

The Nevado de Toluca is a stratovolcano located in the southwest of the Toluca Valley in central Mexico. At a height of around 4200 m there are two crater lakes: El Sol and La Luna. Since Precolumbian times, people in the surrounding valleys carried out rituals and deposited offerings into the lakes. After the Spanish conquest, these rituals were kept alive clandestinely. Currently, reminiscent of Mesoamerican rituals subsist. Due to the long duration of the ritual at the Nevado de Toluca, it is important to date the materials recovered in the underwater and terrestrial archaeological explorations. This article proposes a chronology of Prehispanic ritual activities performed in the Nevado de Toluca based on the characterization and radiocarbon (14C) dating performed to materials from the volcano’s lakes.

The Mayan Codex of Mexico (MCM), the only Mayan codex found in the 20th century, was unveiled in 1971 during the Ancient Maya Calligraphy exhibition at Club Grolier. The codex comprises 10 pages of bark paper in accordion format, coated with a layer of plaster on both sides. It illustrates the synodic cycles of Venus, with its four phases. Since its discovery, the MCM has been subject to controversy and discussions about its authenticity. In 2016, a group of specialists led by Baltazar Brito chief of the National Library of Anthropology and History, carried out an exhaustive study of the codex with the purpose of determining its temporality and authenticity. In this work, the pre-Columbian authenticity of the codex is verified by the radiocarbon (14C) technique using AMS. Two cleaning procedures were contrasted: the standard acid-base-acid (ABA) protocol and a second one with Soxhlet plus ABA. Results obtained when samples were prepared following ABA protocol only, placed the age of the bark paper between 991 and 1147 cal AD. The second cleaning method with Soxhlet plus ABA, resulted in younger ages, between 1159 and 1261 cal AD. However, we consider that when Sohxlet is used as part of the cleaning protocol, organic contaminants are reduced to a minimum, and 14C dates are more reliable. These results indicate that the vegetal support of the MCM belongs to Postclassical Mayan period and place it as the oldest known manuscript of America found to date.