New AMS dates for Machu Picchu: results and implications

Written by cambridge.org on August 10, 2021. Posted in Current News

Machu Picchu, in Cuzco, is one of the most famous of the Inca archaeological sites in South America. The precise dating of the monumental complex, however, relies largely on documentary sources.

Samples of bone and teeth from individuals buried in caves at four cemeteries around Machu Picchu form the basis for a new programme of Accelerator Mass Spectrometry radiocarbon-dating.

The results show that the site was occupied from c. AD 1420–1532, with activity beginning two decades earlier than suggested by the textual sources that associate the site with Emperor Pachacuti’s rise to power in AD 1438. The new AMS dates—the first large set published for Machu Picchu—therefore have implications for the wider understanding of Inca chronology.

This article presents the results of the AMS radiocarbon analysis of 26 human bone and tooth samples recovered from burial contexts at Machu Picchu, Peru. This suite of measurements provides the basis for a scientifically derived estimate for the foundation of Machu Picchu and the duration of its occupation. Since its ‘scientific discovery’ in 1911, this Inca country palace has become widely recognised and is now probably the best-known archaeological site in South America (Salazar & Burger 2018).

In 1983 Machu Picchu was designated as a UNESCO World Heritage Site and—before the start of the COVID-19 pandemic—it was visited by over a million travellers each year. Machu Picchu has been the focus of considerable scholarly attention, but despite this, it has never been directly dated using AMS radiocarbon analysis based on a large number of samples from secure contexts.

An earlier attempt in the 1980s using radiometric radiocarbon methods yielded problematic results (Berger et al. 1988; Tables S1–2 & Figures S1–2 in the online supplementary material (OSM)). The subsequent development and refinement of the AMS technique now makes it possible to produce high-precision measurements on human skeletal material with low organic content.

Another recent effort to establish the chronology of Machu Picchu using AMS dating was constrained by the small number of samples (n = 3) from the site itself, and due to issues regarding sample context and the organic material analysed (Ziółkowski et al. 2020; Table S3).

The scarcity of reliable radiocarbon measurements for Machu Picchu was the result of a widely held opinion among archaeologists working in the Andes that such analyses were unnecessary because the accurate dating of Inca sites such as Machu Picchu could be established on the basis of Spanish historical accounts. Until recently, archaeological work at Cuzco has produced few radiocarbon dates for Inca-period sites, and even recent in-depth studies have relied on Spanish chronicles for dating (e.g. Nair 2015).

An ‘absolute chronology’ for the Inca period based on historical records was published by John Rowe in 1945 and remains the dominant chronological framework (Rowe 1945). According to Rowe, after repelling an invasion by the Chanka ethnic group in AD 1438, the army of the Emperor Pachacuti conquered the lower Urubamba Valley, an area that includes Machu Picchu, before expanding imperial rule over much of the central Andes (Figure 1; Rowe 1945: 270).

Sixteenth-century legal documents published by Luis Glave and María Remy (1983) contain information interpreted by Rowe to indicate that Machu Picchu was built as part of a royal estate known as ‘Picchu’. Rowe argued that this royal estate belonged to the Emperor Pachacuti, and suggested that the palatial complex would have been constructed to commemorate his conquests of the lower Urubamba (Rowe 1990: 140 & 142–43).

In Rowe’s historical framework Machu Picchu was built sometime after Pachacuti’s accession to power in AD 1438 and before his son, Tupac Inca Yupanqui, took command of the imperial army in AD 1463 or ascended to the throne in AD 1471. Although Rowe acknowledged the lack of consistency regarding imperial chronology among Spanish accounts and the difficulty of disentangling them, he relied on dates reported in the 1586 chronicle by Cabello de Balboa because “they are the most plausible ones we have or, indeed, ever likely to have” (Rowe 1945: 277).

The source of Cabello’s dates has never been determined (Covey 2006: 171), but at the time of the publication of Rowe’s chronological model, no technique besides historical analysis was available to produce an absolute chronology. Even after the introduction of radiocarbon-dating to Andean archaeology, the method’s limitations and the scarcity of measurements available undermined acceptance of it as an alternative to Rowe’s historicist reading.

In recent years, the publication of a large number of radiocarbon measurements from Inca sites including in northern Chile, Argentina and Ecuador, far from the Cuzco heartland, has prompted a shift in opinion within Inca studies (D’Altroy et al. 2007: 92–93; Ogburn 2012; Marsh et al. 2017; Covey 2018). Many of these new dates appear to be inconsistent with the Rowe chronology, and these discrepancies have resulted in growing scepticism about the historically based dates proposed for Pachacuti’s establishment of the Inca Empire and its expansion.

There have been repeated calls for a re-evaluation of the Rowe chronology using radiocarbon analyses, but samples from short-lived materials with well-established provenance that can be tied to Inca imperial history have been difficult to find (Burger 2007: 427–29). Thus, the measurements reported here for samples from Pachacuti’s royal estate at Machu Picchu not only shed light on the chronology of Peru’s most famous archaeological site, but also have implications for the ongoing debate over the document-based chronology of Inca imperial history.

Contextualising the bone and tooth samples from Machu Picchu

Machu Picchu is located on the eastern face of the Andes at 2450m asl on a narrow granitic ridge between two mountains (Figure 2). Steep slopes leading down to the Urubamba River 450m below make the site nearly inaccessible on three sides and with the construction of a dry moat along the southern limit of the palatial complex, the builders rendered the site almost impregnable. Despite the term ‘lost city’ used by Bingham (1948), Machu Picchu was a country palace, one of many, in the Urubamba drainage basin (Niles 2015).

Before the palace could be constructed, the irregular ridge on which it is built had to be completely reworked into a series of terraces using massive stone fills sustained by masonry retention walls (Wright et al. 2000). The resulting flat open areas on the summit of the ridge accommodated residences, shrines, open plazas and other elements typical of high-status Inca imperial architecture. An enclosure wall, with a single entrance, adjacent to the dry moat protected the vulnerable southern side of the complex (Bingham 1930).

Figure 2. Map showing the location of burial caves at Machu Picchu and highlighting the caves sampled in this study (adapted from Miller 2003: fig. 1.1).

The archaeological remains recovered at Machu Picchu are consistent with the documentary evidence, which portrays this architectural complex as part of a royal estate belonging to the Emperor Pachacuti and his descendants; this interpretation is widely accepted by most modern scholars (e.g. Valencia 2004: 82; D’Altroy 2014; Niles 2015; Quave 2018).

The architectural core of the royal estate corresponds to a palace where the royal family and their guests could escape the pressures of the capital in Cuzco and engage in recreational activities such as banqueting, hunting and gambling, as well as religious activities associated with the royal family (Salazar & Burger 2004). The hunting of animals such as deer and agouti was probably practised in the surrounding forests (Miller 2003).

Machu Picchu’s warmer temperatures and lush vegetation would have made it an attractive seasonal destination for the Cuzco elite, particularly in the winter months of May to August when the overnight temperature falls below freezing in the capital. Located only 75km north of Cuzco, the royal estate could be reached along a stone-paved Inca road in four to six days. Judging from the number of house structures at the site, it is unlikely that the number of inhabitants exceeded 500, most of whom would have been retainers and craftsmen (Salazar 2004: 30).

During the austral summer, when rains were heavier and access more difficult, Machu Picchu would have been occupied by only a few hundred inhabitants dedicated to maintaining facilities and religious activities (Rowe 1990: 145; Salazar 2007). Most residents were probably yanacona—specialised labourers who were removed from their home societies and committed to life-long service to the state—or, in this case, Pachacuti’s panaca (royal dynastic descent group) and camayoq, specialised artisans taken from their homelands and attached to the royal panaca (Rowe 1982).

Some of the camayoq at Machu Picchu may have been metal workers brought from the north coast of Peru (Salazar 2007).

The radiocarbon samples discussed here come from individuals interred in burial caves at Machu Picchu. The 1912 excavations located 104 such burials (Eaton 1916), most with few grave goods. It has been hypothesised that these interments are the remains of retainers rather than the Cuzco elite (Salazar & Burger 2004; Salazar 2007). Such retainers (camayoc or yanacona) would have been assigned to the royal estate from throughout the empire and would have reflected the history of conquests (Niles 2004, 2015; Salazar 2007).

Since this hypothesis was proposed (Burger & Salazar 1993), the suggested multi-ethnic character of the buried individuals at Machu Picchu has been confirmed by the results of research on cranium deformation (Verano 2003: 88–90), cranial morphology (Verano 2003: 90–97), analyses of strontium ⁸⁷Sr/⁸⁶Sr (Turner et al. 2009) and stable carbon (δ¹³C) isotopes (Burger et al. 2003; Table S4). Similarly, ongoing palaeogenomic analysis has produced preliminary results that confirm and expand the evidence for ethnic diversity at the site (Forst et al. 2019).

The individuals recovered from the burial caves show little evidence of involvement in heavy physical labour such as agriculture or construction, and do not exhibit pathologies resulting from violence (Verano 2003: 97–104). Thus, the population of Machu Picchu was dominated by retainers and would have been ethnically diverse and cosmopolitan, and fundamentally different from the agricultural villages in the Cuzco heartland (Salazar 2007: 174; Andrushko & Torres 2011).

The burials at Machu Picchu were placed beneath boulders, under cliff overhangs and in shallow caves, and were sealed for protection with coarse, clay-plastered masonry walls (Figure 2; Eaton 1916). The grave goods include ceramics in the imperial style of Cuzco and the provinces, as well as bronze and silver shawl pins and tweezers, and other typical Inca artefacts.

Significantly, many of the non-local vessels had been broken and repaired in antiquity, and their inclusion in the graves suggests that they were valued heirlooms, perhaps from an ethnic homeland (Salazar & Burger 2004). Among the assemblages of grave goods, as in those from the architectural core of the site, there is a conspicuous lack of pre-Inca and post-Spanish conquest artefacts (Bingham 1930: 115–116; Salazar & Burger 2004), a pattern suggesting that the occupation of Machu Picchu was limited to the reign of Pachacuti and his immediate successors.

The ceramic styles preceding the classic Cuzco Inca style, such as Killke and Lucre (Bauer 1999), are absent from the large ceramic assemblage recovered in the 1912 excavations (Salazar & Burger 2004).

Most of the known burials are clustered in areas on the margins of the country palace (Figure 2); one cluster (cemetery one) is situated among the boulders to the north-east of the architectural core, another (cemetery two) on the slopes to the east and a third (cemetery three) along the northern slopes of Machu Picchu mountain to the south (Bingham 1930: 102–105). Additional burials have been recovered on the western terraces and are referred to here as cemetery four (Salazar 2001, 2007: 171).

In some of the burial caves there are multiple interments, in some only a single burial. The social relationship between the cemeteries is unknown, as is the rationale for the presence of multiple or single individuals in the burial caves.

The present study of AMS radiocarbon-dating of bone and tooth samples forms part of a wider, ongoing ancient DNA project on osteological samples that were formerly curated at the Yale Peabody Museum of Natural History. All human remains and other archaeological materials from Machu Picchu have subsequently been returned to Cuzco where they are now conserved at the Museo Machu Picchu, which is administered by the Universidad Nacional San Antonio Abad Cuzco (UNSAAC) (Salazar & Burger 2016).

The osteological samples for this study were chosen to represent burial contexts from all known cemeteries and outliers, to represent burials with multiple individuals in order to clarify the extent of the period in which caves were used for burial, and to consider the chronological dimension of the diversity observed in the grave goods, cranial modifications and other indicators suggesting the diverse geographic origins of the buried individuals (Verano 2003; Salazar 2004, 2007).

A summary of the age and sex determinations for the individuals in the sample, along with relevant data on cranial deformation, ⁸⁷Sr/⁸⁶Sr measurements and grave goods is provided in Table S4.

An analysis of the Yale Peruvian Scientific Expedition Collections by Verano (2003) concluded that a minimum of 174 individuals were represented in the cave burials. Investigations have been carried out over the last half century by Peruvian government archaeologists and investigators from UNSAAC, but only about two dozen additional burials have been documented.

Assuming that Verano’s estimate for the number of buried individuals recovered in 1912 is correct, a reasonable estimate of the total buried population at Machu Pichu can be made by adding those individuals recovered by archaeologists after 1912 to this number. The 26 samples analysed here may represent over ten per cent of the individuals buried at Machu Picchu.

The samples come from cemetery one (n = nine), cemetery two (n = six), cemetery three (n = seven), and cemetery four (n = three); the remaining sample is an outlier from near the banks of the Urubamba (Figure 2). Interments with multiple individuals are represented by samples from burial caves 4 and 84.

As it is believed that the bone samples relate to site retainers, these interments should span the history of Machu Picchu from its early use as a palace until its abandonment. They are unlikely to reflect the period during the construction of Machu Picchu. As noted, the skeletons recovered show little evidence of heavy labour and it is likely that the labour for estate-building was provided by a rural non-retainer population that would not have left its dead at the site.

The objects buried with the dead at Machu Picchu are consistent with this assumption. Finally, the initial construction date estimated for Machu Picchu should be considered as a terminus ante quem relative to the death of the retainers who served there.

This is taken from a long article. Read the rest here: cambridge.org

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