Four types of obsidians from the Monte Arci volcanic complex (Sardinia) were used by Neolithic men in the North Tyrrhenian area of the western Mediterranean. A map of their occurrences from primary sources (mother rocks) to distant secondary deposits in the surrounding plains is presented. Some 1457 specimens were macroscopically characterized and in addition ~15% of them fingerprinted from their elemental compositions as determined by electron microprobe, neutron activation analysis or ion beam analysis. The results show that secondary sources, up to now largely neglected in provenance studies of ‘archaeological’ obsidians will have from now to be taken into account. .
L'obsidienne, présente dans de nombreux sites néolithiques de Méditerranée occidentale, provient de gîtes situés dans les îles Lipari, Palmarola, Pantelleria ou de Sardaigne (voir par exemple
L'importance des gîtes secondaires d'obsidienne, peu considérés jusqu'alors, a été récemment soulignée
L'obsidienne du Monte Arci a été caractérisée visuellement et instrumentalement. La reconnaissance visuelle des types SA, SB1, SB2 et SC a été établie à partir de critères discutés précédemment
Un secteur d'environ 400 km2, centré sur le Monte Arci, a été prospecté par l'un d'entre nous (C.L.) et 1430 échantillons d'obsidienne de localisation précisément obtenue par repérage GPS, collectés. Alors que les sources primaires (d'altitudes comprises entre 120 et 720 m) et sub-primaires sont toutes situées à l'intérieur du massif, les sources secondaires sont essentiellement distribuées dans les plaines environnantes plio-quaternaires. Les obsidiennes y affleurent en surface de dépôts d'origine colluviale à alluviale, souvent remaniés, ou sur les berges des rios Mannu et Mogoro. Des déterminations de composition élémentaire ont été effectuées sur 217 échantillons, dont 190 positionnés par GPS. Elles ont concerné principalement les obsidiennes de type SB1, SB2 et SC, dont les gîtes sont plus étendus que ceux des obsidiennes SA. En ce qui concerne les obsidiennes qualifiées de SB par l'analyse en activation neutronique, leur subdivision en SB1 et SB2, pour celles des sources secondaires, a été rendue possible par leurs apparences visuelles très caractéristiques
Par ailleurs, chaque type de composition élémentaire est très homogène (Le Bourdonnec et al., en préparation). Si l'on considère que l'échantillonnage a concerné des obsidiennes mises au jour aussi bien récemment par l'érosion (dans les sources primaires) qu'à différents moments (depuis la formation de ces obsidiennes, il s'est passé plus de 3 Ma), cela signifie que les roches mères présentaient une composition très homogène, une information importante pour les études de provenance des obsidiennes néolithiques.
Bien que la présence d'obsidienne dans les plaines proches du Monte Arci ait été signalée depuis longtemps (voir par exemple
La confrontation entre ces données et les données archéologiques sardes atteste l'utilisation de sources secondaires du Monte Arci, pour les sites de la « zone d'approvisionnement directe » (
Il est devenu d'usage de relever les proportions relatives d'obsidiennes de type SA, SB1, SB2 et SC dans les séries néolithiques. Les résultats présentés ici montrent que l'appartenance à un type géochimique ne permet pas de préciser l'origine géographique d'une obsidienne taillée. C'est la combinaison caractérisation par la composition élémentaire/observation visuelle, avec notamment la présence ou non de cortex résiduel, qui pourra éventuellement permettre de remonter au type de source utilisée. Il est ainsi clair que les proportions relatives des types d'obsidienne dans un site ou un niveau culturel n'auront pas la même signification au Néolithique ancien et moyen, d'une part, et au Néolithique tardif, d'autre part.
Enfin, si les caractéristiques optiques suffisent, en pratique, à déterminer le type d'une obsidienne prise dans un gîte naturel associé au Monte Arci, il n'en est plus de même pour les obsidiennes taillées, parce qu'il peut s'agir, soit de petits objets à l'allure pétrographique apparente peu caractéristique, soit d'objets plus volumineux, mais opaques, et qu'il est peu judicieux, voire inadmissible, de fragmenter afin d'obtenir de meilleures conditions d'observation. Ainsi, des examens menés à la fois visuellement et instrumentalement ont montré que la part de succès dans l'attribution d'un type à une obsidienne sarde pouvait n'atteindre que 65–69%
Obsidian artefacts are often present in Neolithic sites of the Tyrrhenian Islands (western Mediterranean) and of surrounding continental regions, from eastern Algeria to Italy and southern France. Provenance studies based on fission track dating and/or elemental analyses have shown that the raw materials of this industry were exclusively coming from the four Italian islands of Lipari, Palmarola, Pantelleria and Sardinia (see, e.g.,
Sardinian obsidians were formed some 3.2 Ma ago in the Plio-Pleistocene Monte Arci volcanic complex dominating the coastal plain to the East of the Oristano Gulf
The importance of distant secondary sources in Neolithic man raw material procurement strategies in Sardinia was recently pointed out
All the obsidians sampled were first visually characterized. In most cases a type (SA, SB1, SB2 or SC) attribution could be obtained directly from visual examination with the naked eye and/or with the help of a stereomicroscope (100 ×). Our visual criteria for obsidian-type identification are discussed elsewhere
A majority of samples were analysed by instrumental neutron activation analysis (INAA); the elemental compositions were performed using the standard techniques adopted in the Radiochemistry Laboratory of the Department of General Chemistry of the University of Pavia
Although the Ba content is in itself a good discriminator of the SA, SB and SC obsidian types of Hallam et al.
A second group of samples were analysed from polished sections with the SX50 (CAMECA) electron microprobe (EMP) of IFREMER (Brest, France). Analyses were performed on polished sections with a 20-nA and 15-keV defocused (5-μm diameter) electron beam. To insure internal consistency between analytical sessions, the Monte Arci SA type sample SA-66 was repeatedly measured. The element Na, Mg, Al, Si, P, K, C, Ti, Mn and Fe contents were computed with the PAP software
Finally, several obsidians were treated also in the form of polished sections by ion-beam analysis. Analyses by particle induced X-ray emission (PIXE) were carried out with the AGLAE facility (external beam) of the ‘Centre de recherche et de restauration des musées de France’ (C2RMF, Paris, France). As two Si(Li) detectors were available, light elements Na, Al, Si, K, Ca, Ti, Mn and Fe and heavy elements Zn, Ga, Rb, Sr, Y, Zr and Nb were determined simultaneously with a 3-MeV proton beam
An area of some 400 km2 centred on the Monte Arci was systematically prospected and sampled by one of us (C.L.). The localisations of the 1430 obsidians collected during this survey were systematically GPS-recorded. Whereas primary sources, at altitudes between 120 and 720 m a.s.l. and sub-primary sources are all located in the Monte Arci massif s.s., the secondary sources are almost entirely situated in the Plio-Quaternary sediments of the surrounding plains. Obsidian pebbles often outcrop on the surface of these more or less reworked colluvial to alluvial deposits. The secondary source samples collected were taken either in such terrains or along river (Rio Mogoro, Rio Mannu) banks. Internal reference values for the SA, SB1, SB2 and SC types of elemental compositions were obtained from samples taken in primary to sub-primary sources. In all, 217 samples were analysed, of which 190 with GPS-recorded localisations. As can be seen in
As discussed elsewhere, each one of the instrumentally determined type of elemental composition is very homogenous (Le Bourdonnec, et al. in preparation). Considering that obsidians from primary sources may have been denuded by erosion at different times since 3.2 Ma, this shows that the corresponding mother rock compositions are also quite homogeneous, an important point for further regional Neolithic obsidian provenance studies based on elemental composition.
The presence of scattered obsidians in the Campidano Plain, south of Monte Arci, is known since long (see, e.g.,
Another question is that of the suitability to Neolithic man of these secondary obsidian resources. Few data are actually available for a detailed reconstruction of the landscape and vegetation of the Monte Arci area during these times: the most general and commonly accepted picture is that of a land entirely covered by forests and vegetation, which might have hidden the obsidian secondary sources in the plains, especially to the West and the South of the massif. Some pedostratigraphic data referring to the 6th millennium BC demonstrated the presence of a Mediterranean climate with biostatic conditions developed on sandsoils belonging to stage 2. It could thus be argued that, at least along riverbeds of the Rio Mannu–Rio Mogoro system, some superficial erosion might have brought to light obsidian-bearing alluvial deposits, offering to Early Neolithic man their first contacts with this raw material. Further agricultural advances might have enhanced the erosion and visibility of the ground, giving to Neolithic communities a more detailed outline and awareness of obsidian secondary deposits and of related outcrops as well.
Several lines of evidence, notably the eventual presence of cortex remnants on artefacts, show that Early to Middle Neolithic man of the so-called ‘supply zone’, up to 30 km far from Monte Arci, used these secondary sources. The shape and size of their cobbles were in effect convenient to produce a toolkit exhibiting a strongly microlithic character. This small-size industry is generally and almost exclusively obtained from flakes detached with direct percussion reduction techniques and counts for the largest part of the archaeological assemblages. At the end of Middle Neolithic and during the Late Neolithic (late 5th–4th millennium BC), one observes both a shift in the obsidian reduction strategies, which became more oriented towards blade production and a high increase in the production rate. The secondary deposits seem then to be only scarcely exploited, for local and expedient production essentially, whilst large obsidian workshops appear directly close to the lavas outcrops, chiefly of the SA and SC types. This new acquiring strategies and scales of production are reflected in the coeval increase and more diversified circulation of SC obsidian from these workshops to the whole northward Tyrrhenian area.
In the western Mediterranean region, it has become usual, since the late 1970s, to note the percent relative abundances of SA, SB (or SB1, SB2) and SC Monte Arci obsidians in provenance studies. The belonging of a group of archaeological obsidians to a given geochemical type was in practice equated to their common provenance of the same ‘source’. The results presented here show that the relative abundances of SA, SB1, SB2 and SC obsidians in a Neolithic series cannot be interpreted in the same way in the Early/Middle Neolithic and in the Late Neolithic. It also shows the need to closely associate technological and ‘sourcing’ studies on the same archaeological pieces.
Finally, we would point out the usefulness of instrumental type determinations of archaeological artefacts. We observed here a one-to-one agreement between visual and instrumental determination of obsidian types SA, SB1, SB2 (or SB1+SB2 for NAA) and SC. This was possible because the only geological samples collected were those of a size large enough to be potentially used for the production of at least a microlithic industry. Moreover, these samples could be broken, to best reveal to the naked eye its mineral inclusions, fresh fractures brightness, transparency, etc. Neolithic artefacts may be too small or too thick to allow the observer a good determination. Combined visual and instrumental determinations have shown that, depending on the series, from about 15%
The authors wish to thank the AGLAE (‘Centre de recherche et de restauration des musées de France’, Paris) team for their help with the PIXE facilities. One of us (F.-X. L.B.) was partly supported during field work by a CNRS ‘PCR’ funding. Gérard Poupeau and François-Xavier Le Bourdonnec thank the generous invitations of the ‘Regione autonoma della Sardegna’, the ‘Amministrazione provinciale di Oristano’ and the ‘Amministrazione comunale di Pau’ (Italy). The PIXE analyses were partly funded by the French GDR ‘CHIMART’ of CNRS and by the European Community Eu-ARTECH program.
Schematic map of western Mediterranean showing the obsidian source-islands.
Fig. 1. Carte schématique de la Méditerranée occidentale montrant les îles-sources d'obsidienne.
Geochemical map showing the extension of SA, SB1, SB2 and SC obsidian sources. All primary and sub-primary sources are localised inside the Monte Arci massif. Between parentheses, the number of samples characterized.
Fig. 2. Carte géochimique montrant l'extension des sources d'obsidiennes de types SA, SB1, SB2 et SC. Les sources primaires et sub-primaires des obsidiennes sont toutes situées à l'intérieur du massif du Monte Arci. Entre parenthèses, le nombre d'échantillons caractérisés.
Samples characterized by instrumental methods
Tableau 1
Échantillons caractérisés par méthodes instrumentales
SB1 and SB2 types sorted visually from the type determined by NAA as SB (see text).
Types SB1 et SB2 différenciés visuellement à partir du type déterminé par NAA comme SB (voir texte).
Samples without GPS positioning, not reported in the map of Fig. 2.
Échantillons non référencés par GPS, non reportés sur la carte de la Fig. 2.