First dates from the Guaibituguçu archaeological site, Japaratinga, Alagoas, Brazil

Abstract

The archaeological site, Guaibituguçu, located in the State of Alagoas, Brazil, is an open air site that revealed artifacts indicating an indigenous presence in context with objects of European and local non-indigenous manufacture. This study seeks to define chronological parameters for the site and to correlate the pottery chronology with the stratigraphic layers identified during excavations. Four pottery fragments were dated using Thermoluminescence techniques (TL). Each fragment was subject to mechanical treatment that removed approximately 2 mm of the artifact’s surface and the samples were ground and the grains sieved to obtain a size of >150μm. Chemical treatment to screen and select quartz grains of 75-150μm employed hydrogen peroxide, chloric acid and fluosilicate acid. The samples were used to determine the accumulated dose (Da) using multiple aliquots by way of regenerative dose method (MAR). The sediment collected associated with each fragment was sealed in a plastic container for a period of four weeks to determine the level of U, Th and K using gamma spectrometry. From these values, the external dose was inferred. Additionally, the internal dose was determined using the neutron activation technique from the U, Th and K concentrations of the artifact fragment. The sum of these two values and the cosmic radiation permitted determination of the annual dose (TD), and the calculation of sample age from the Da and TD values. All of the TL measurements were conducted using the Harshaw TLD 3500 Reader, heat rate of 5 oC/s at the Departamento de Energia Nuclear (DEN) of the Universidade Federal de Pernambuco (UFPE). A Canberra Hyperpure Germanium (GeHP) detector, coupled with and Eagle 5004 multichannel analyzer in laboratory conditions with low background radiation was used to determine the concentration of the levels of U, Th and K. The individual fragments were sent to the Instituto de Pesquisas Energéticas e Nucleares (IPEN) of the Universidade de São Paulo (USP) to determine the concentrations of U, Th and K of the samples. Analysis and discussion of results took place at the Laboratório de Estudos Arqueométricos (LEARQ) of UFPE. At this time, the age of three samples has been estimated, with accumulated doses of: 0.74±0.36 Gy; 0.79±0.12 Gy; 1.03±0.13 Gy and the TD dose levels of 0.669 mGy/year; 1.108mGy/year; 1.419 mGy/year. The resultant ages are 1148, 712, 723 and 739 years AP, considering samples S-949, S-1107 and S-1109, respectively. Sample S-5747 has not been finalized, though the ages already estimated indicate a direct relation between the age and depth where the sample was obtained. Provenance of S-949 at the deepest level and the other two at the same level may indicate two occupations. Ongoing investigations involving excavations, sampling and laboratory analysis continue with the objective of refining the chronology of Guaibituguçu.