Single-grain OSL dating at La Grotte des Contrebandiers

Jacobs Z., Meyer M. C., Roberts R. G., Aldeias V., Dibble H.,El Hajraoui M. A. (2011). Single-grain OSL dating at La Grotte des Contrebandiers (‘Smugglers’ Cave’), Morocco: improved age constraints for the Middle Paleolithic levels. Journal of Archaeological Science, 38 (12): 3631-3643.

Optically stimulated luminescence (OSL) measurements of individual quartz grains are reported for Middle Paleolithic (MP) or Middle Stone Age (MSA) deposits in La Grotte des Contrebandiers, Morocco. Single-grain measurements enable rejection of grains that may lead to under- or over-estimation of age due to malign luminescence properties or remobilisation of grains after burial. We identified the former using a range of experimental procedures and objective rejection criteria. Three post-depositional factors influenced the distribution pattern of equivalent dose (D_e) values for the remaining single grains: the disintegration of roof spall liberating unbleached, older grains into otherwise well-bleached sediments; the intrusion of a small number of younger grains from the overlying Upper Paleolithic deposits; and small-scale differences in the beta dose received by individual grains. Roof spall contamination was a significant problem, so we characterised the OSL signals and obtained D_e values for the calcareous sandstone in which the cave is formed, which allowed bedrock-derived grains in the MP/MSA deposits to be identified and rejected prior to age determination. We collected 31 samples from the MP/MSA deposits, and were able to calculate ages for 30 of them. These ages suggest that MP/MSA occupation of Contrebandiers was restricted to the interval between about 120 and 90 ka ago (Marine Isotope Stages 5d to 5b). Weighted mean OSL ages were obtained for samples associated with archaeologically sterile deposits at the base of the excavations (126 ± 9 ka), followed by two phases of the Moroccan Mousterian (122 ± 5 and 115 ± 3 ka), interspersed with archaeologically sterile deposits (112 ± 4 ka). The base and top of the Aterian were dated to 107 ± 4 and 96 ± 4 ka, respectively. Based on the grand weighted mean ages of 116 ± 3 and 103 ± 3 ka for the Moroccan Mousterian and Aterian at Contrebandiers, we suggest that these two industries may be separated by an occupation hiatus of up to 13 ± 3 ka at this site. Our single-grain OSL chronology confirms previous age estimates obtained for Contrebandiers and other sites in the vicinity, but are generally more precise and stratigraphically more coherent, due to the inherent advantages of measuring individual quartz grains and rejecting contaminant and other unsuitable grains before age determination.

Datações de radiocarbono e dados climáticos para a Neolitização no Próximo-Oriente

Mais um novo estudo de um tema tão badalado da arqueologia internacional: a Neolitização no Próximo-Oriente.

Desta vez vários autores propõem-se a rever todos os dados climáticos e as datações de 14C disponíveis para a área do Levante. Para além de um evidente determinismo ambiental com o qual pretendem explicar as grandes mudanças ocorridas na região, os autores concluem que é urgente obter novas datações para alguns sítios-chave pois as que estão hoje disponíveis não podem ser consideradas válidas. Aliás, a visão que constroem com o presente artigo só foi possível excluindo inúmeras datações que consideram terem sido obtidas de forma errada e que somente provocam barulho em toda a discussão deste tema - datas descontextualizadas, com intervalos de tempo demasiado grandes ou com materiais não sujeitos aos devidos procedimentos laboratoriais. Eis os critérios utilizados na filtragem, ou melhor, na exclusão de datações:

"1. Dates where there was not secure association in the literature to the event in question,

2. Dates where we could not ascertain what material had been directly dated, this is very important given the well known problems with radiocarbon dates on mixed material.

3. Dates where we could not ascertain a likely reliable pre-treatment strategy, such as older dates on bone, where successful collagen extraction and removal of contaminants was unlikely, again this is a significant problem and the dates of poorly treated bone have been shown to be incorrect on the order of several thousand years (Jacobi and Higham, 2008).

4. Dates that were undertaken before the late 1980s where large laboratory measurement uncertainties meant that the ages were likely to cause more confusion than add value (note most of these dates were already rejected by other specific quality assurance criteria)."

Este problema deve ser entendido como crucial na interpretação de um evento tão importante como a Neolitização. Este problema é uma realidade também em Portugal, por exemplo, na datação do fenómeno Campaniforme na Estremadura portuguesa onde em alguns casos são utilizadas datas obtidas sobre carvõe de proveniência duvidosa no interior dos povoados (e.g. Penha Verde e Verdelha do Ruivos).

Fica aqui o resumo do artigo que motivou esta mensagem:

" This paper re-examines the chronology and environmental context for the transition to agriculture in the Southern Levant, seen as the likely starting point for the adoption of agriculture in Europe and the Near East. The role in this process of abrupt late Quaternary climate change has been discussed widely, but limitations on the archaeological and palaeoenvironmental chronologies have led to varying interpretations. Here we attempt to clarify the situation by first testing the available radiocarbon database for the archaeological transitions from the Natufian through to the PPNA. We apply internationally accepted radiocarbon quality assurance procedures and find that a significant number of the published dates fall bellow acceptable standards. The cleaning process significantly clarifies and constrains the reported time ranges for the Natufian, Late Natufian and PPNA. We then apply the new IntCal09 calibration curve and Bayesian calibration methods, using the archaeological phasing to constrain the data and calculate the most likely timing of the transitions between each phase. We then compare the onset and duration of archaeological phases to data representing the key Northern Hemisphere climatic transitions, using the new GICC05 Greenland Ice core timescale and the timing of transitions between wet and dry phases in the southern Levant from published high precision isotopic analyses of Speleothem data. The results of this exercise present the currently best available chronology for these events and suggest that during the second part of the Lateglacial interstadial, drying of the southern Levant may have triggered the transition to the Late Natufian, when hunter-gatherer communities resorted to a more mobile lifestyle. The Late Natufian culture appears to have disappeared from the southern Levant during the Younger Dryas, as drying intensified. There is then a gap in well dated evidence for human occupation until a reappearance of humans at the onset of the Pre-Pottery Neolithic A (PPNA) period at the beginning of the Holocene. Thus the onset of the Holocene can be hypothesised to be the driver behind the onset of the Neolithic in this region."

Referência:

Blockley SPE, Pinhasi R A revised chronology for the adoption of agriculture in the Southern Levant and the role of Lateglacial climatic change. Quaternary Science Reviews In Press, Corrected Proof:

Datação por carbono não destrutiva

"Non-destructive carbon dating"

é a designação de uma nova forma de datação por carbono 14 (C-14) desenvolvida por uma equipa de investigação dirigida por Marvin Rowe. Esta técnica já foi testada em mais de 20 substâncias orgânicas tendo oferecido resultados congruentes com os da forma convencional da datação por C-14. A técnica continua em processo de aperfeiçoamento e poderá a vir usada sem que implique destruição das matérias analisadas.


Aqui fica o link para a notícia:
http://www.eurekalert.org/pub_releases/2010-03/acs-nmc031210.php