Periodic Reporting for period 1 - DIACHRON (Assessing the effect of diagenesis on dateable materials: implications for the chronology of the Levantine Middle Palaeolithic)
Okres sprawozdawczy: 2022-09-01 do 2024-08-31
The Levant is a key area to study past human dynamics: due to its strategic position, connecting Africa to Europe and Asia, it has been the main route of the most ancient dispersals of modern humans out of Africa and a privileged environment for Neanderthal occupation. It is known that several human species occupied this area between around 200,000 and 40,000 years, including our own species, Homo sapiens, during the so-called cultural period of the Middle Palaeolithic. For sites older than 50,000 years, beyond the range of radiocarbon, trapped-charge dating methods such as combined electron spin resonance/uranium-series (ESR/U-series) dating of fossil tooth enamel and luminescence dating of sediment are commonly applied to date prehistoric occupations. They are based on the property of the mineral (carbonate hydroxyapatite for ESR, quartz and feldspars for luminescence) to record a dose of ionising rays produced from radioactive disintegrations in the sample and the sediment and from cosmic rays. The dose absorbed is a function of the burial time and radioactivity: minerals can be dated by dividing the total dose absorbed by the sample since burial (the “equivalent dose”) by the annual dose (or “dose rate”) measured in the field and in the laboratory. However, little is known about how much diagenesis, which are the physico-chemical processes that cause alteration in a material after burial, may affect the age results. The DIACHRON project is based on an innovative approach combining material characterisation and dating. It aims at investigating the degree of preservation of the sample using characterisation tools such as scanning electron microscope (SEM), Fourier transform infrared (FTIR) and Raman spectroscopy, to evaluate the effect of diagenesis on age determinations. DIACHRON meant to provide a high-resolution chronological framework for Palaeolithic occupation in the Levant, to shed more light into questions related to migration routes, adaptation to climate change, social interactions, technological and subsistence behaviour of past human populations.
The DIACHRON project focused on the thorough chronological study of Besor 27 (B27), a Middle Palaeolithic open-air site located in the Besor basin, Southern Israel. Lithic artefacts typical of the Middle Palaeolithic were excavated, together with poorly preserved bones and teeth (see figure 1), due to unfavourable environmental conditions. Teeth samples were selected for a detailed characterisation study combining the use of scanning electron microscopy, Raman and FTIR spectroscopy, followed by a complete dating study including electron spin resonance spectrometry and U-series measurements. Modern samples of similar species (caprine, cattle, and equid) were also studied in order to build a reference analogue (see figure 7-9) and highlight diagenetic changes in fossil samples, as well as fossil samples from other archaeological contexts. The characterisation study showed that diagenetic processes, such as dissolution and recrystallisation, were present in the dated samples and that the precipitation of secondary mineral phase (such as calcite) may also have happened during burial in some of the samples. For instance, none of the samples from B27 preserved the dentine, the internal tissue whose composition is similar to bone mineral (ca. 70% mineralised phase). The enamel, which is mainly composed of hydroxyapatite, the mineral component of bones and teeth, was still preserved but alterations were visible to the naked eye, and at the microscopic level (see figure 4-6). It is known that teeth are prone to diagenetic alteration as stated above, but also the uptake of trace elements in dental tissues (e.g. enamel, dentine, and cementum). In particular, the uptake of uranium (U), which is radioactive, can affect significantly the age determination. Indeed, U follows different uptake modes that can be modelled using U-series analyses (230Th/U), but the reason for such diversity is not well understood. The U-series analyses conducted on the dated samples showed that U was heterogeneously distributed in the enamel. In particular, the external and internal sides (both in contact with the sediment because no dentine was preserved), contain a greater quantity of U, confirming that diagenesis may occur at different levels within a sample and that the dose rate is affected by such diagenetic processes (see figure 3). All these observations support the necessity of using characterisation tools prior to dating, in order to take into account such complex processes for the age calculation. The ESR analyses were performed on powder aliquots, from which an average dose of ionising rays received during burial was calculated (see figure 2). The equivalent doses are included within the same range, suggesting that those samples belong to a similar radioactive environment. Finally, the age calculation was obtained combining ESR, U-series and the radioactive composition of the sediment (determined by mass spectrometry), framing the human occupation at the site to ca. 90,000-70,000 years, in agreement with luminescence dating results obtained on the sediment.
The results contributed to both methodological and archaeological issues. First, they highlight the difficulty of dating teeth samples that are heavily altered, and that the use of characterisation techniques is essential to have all the necessary information to analyse samples and interpret dating results, and to adjust the strategy for age computation and modelling. These results also suggest that not only dating analyses may be impacted by such diagenetic processes, but that other geochemical signals, which are generally preserved in teeth samples and used for other purposes such as environmental reconstructions, may be also altered (i.e. stable isotopes geochemistry). Secondly, they frame human activity at the site during the second half of marine isotopic stage (MIS) 5, which is an interglacial period ranging from ca. 130,000 to 70,000 years. This gives new insights into the occupation of the lower Besor Basin, which was known to be occupied at later stages and during cooler phases (e.g. ca. 49,000 years at the nearby site Far’ah II). DIACHRON shows that humans occupied this semi-arid environment earlier than previously thought, in agreement with luminescence ages on sediment. The age results allows contextualising the lithic technology found at the site, and contribute to our understanding of the diachronic variation of human behaviour in the southern Levant. Indeed, this area was the main corridor connecting Africa to Eurasia, where different human groups may have met and interacted, boosting exchange and cultural complexity during the late Middle Palaeolithic.