Anthracological study of a Chalcolithic funerary deposit from Perdigões (Alentejo, Portugal): A new analytical methodology to establish the wood burning temperature

Abstract

Anthracological analyses of charcoal samples retrieved from Pit 16 of Perdigões (Reguengos de Monsaraz, Portugal), a secondary deposition of cremated human remains dated back to the middle of the 3rd millennium BC, enabled the identification of 7 different taxa: Olea europaea, Quercus spp. (evergreen), Pinus pinaster, Fraxinus cf. angustifolia, Arbutus unedo, Cistus sp. and Fabaceae. All taxa are characteristic of both deciduous and evergreen Mediterranean vegetation, and this data might indicate that the gathering of woods employed for the human cremation/s occurred either on site, or in its vicinity. However, considering both the large distribution of the identified taxa and data about human mobility, it is not possible to conclusively determine the origin of the wood used in the cremation(s). Chemometric analysis were carried out to estimate the absolute burning temperature of woods employed for the human cremation/s. An in-lab charcoal reference collection was created by burning sound wood samples of the three main taxa identified from Pit 16, Olea europaea var. sylvestris, Quercus suber (evergreen type) and Pinus pinaster, at temperatures between 350 and 600 °C. The archaeological charcoal samples and the charcoal reference collection were chemically characterized by using mid-infrared (MIR) spectroscopy in the 1800-400 cm-1 range, and Partial Least Squares (PLS) regression method was used to build calibration models to predict the absolute combustion temperature of the archaeological woods. Results showed successful PLS forecasting of burn temperature for each taxon (significant (P <0.05) cross validation coefficients). The anthracological and chemometric analysis evidenced differences between the taxa coming from the two stratigraphic units within the Pit, SUs 72 and 74, suggesting that they may come from two different pyres or two different depositional moments.

Fig 1

A. Location map of Perdigões archaeological site in the Iberian Peninsula, B) Aerial photo of Perdigões ditch enclosures.

Fig 2. Section of Pit 16.

In dark grey: the two stratigraphic units (SUs 72 and 74) containing human remains associated with charcoals and the grave goods.

Fig 3. Sound woods transversal anatomical section of Olea europaea var. sylvestris (A), Quercus suber (B), and Pinus pinaster (C) heated at 350°C.

Samples were photographed with SEM.

Fig 4. O. europaea, Quercus spp. (evergreen) and P. pinaster archaeological charcoal samples observed and photographed with SEM.

(A, B) Transversal and radial sections of an O. europaea sample, (C, D) Transversal and tangential sections of a Quercus spp. (evergreen) sample, (E, F) Transversal and radial sections of a P. pinaster sample.

Fig 5. Cistus sp., A. unedo, F. cf. angustifolia and Fabaceae archaeological charcoal samples observed and photographed with SEM.

(A, B) Transversal and tangential sections of a Cistus sp. sample, (C, D) Transversal and tangential sections of an A. unedo sample, (E) Transversal section of a F. cf. angustifolia sample, (F) Transversal section of a Fabaceae sample.

Fig 6. A) Relative frequency of taxa found in Pit 16, B) Relative frequency of taxa found in the SU72 and in the SU74.

Fig 7. Charcoal size classes identified in Pit 16.

Fig 8. This figure represents the overlay of the infrared spectra to show possible differences among replicates.

Medium infrared spectrum (MIR) of archaeological charcoal of O. europaea (A), Quercus spp. (evergreen) (B) and P. pinaster (C) retrieved from a) SU74 and b) SU72.

Fig 9. Cross-validation plots for crematory temperature of modern charred woods taxa: A) O. europaea var. sylvestris, B) Q. suber (evergreen type) and C) P. pinaster, generated by partial least squares (PLS) regression using infrared spectral as predictors.

Fig 10. Superposition of variable importance for projection (VIP) of spectral points and average infrared spectra of modern char particles of O. europaea var. sylvestris (A), Q. suber (B) and P. pinaster (C).