Strofilas (Andros Island, Greece): new evidence for the cycladic final neolithic period through novel dating methods using luminescence and obsidian hydration

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Strofilas (Andros Island, Greece): new evidence for the cycladic final neolithicperiod through novel dating methods using luminescence and obsidian hydration

Ioannis LiritzisLaboratory of Archaeometry, Department of Mediterranean Studies, University of the Aegean, Rhodes 85100, Greece

a r t i c l e i n f o

Article history:Received 26 July 2008Received in revised form26 December 2009Accepted 31 December 2009

Keywords:AegeanFinal NeolithicFortificationObsidian hydrationTL datingRock art

a b s t r a c t

The recently excavated coastal prehistoric settlement of Strofilas on Andros Island (Cyclades, Greece) inthe Aegean sheds new light on the transitional phase from the Final Neolithic to Early Cycladic periodregarding masonry, fortification, and richly engraved rock art. The fortification possesses early evidenceof preserved defensive architecture, as evidenced from the plethora of scattered finds from within andaround the settlement. Important features are carvings on rock walls which mainly depict ships, animals,and fish. Initial archaeometric dating via the application of luminescence dating of two samples from thefortified wall bearing engraved ships, and by obsidian hydration of two blades employing the newSIMS-SS method (secondary ion mass spectrometry via surface saturation), has been undertaken todetermine the site’s chronology. The former yields an average date of 3520 (�540) BC and the latter anaverage date of 3400 (�200) years BC, both of which, within overlapping errors, suggest the mainsettlement occurred during the Final Neolithic.

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1. Introduction

Archaeologists are concerned with investigating and imple-menting a wide variety of established and newly developedarchaeometric techniques for determining the age and duration ofhuman site occupation. The dating of prehistoric settlements in theAegean has been achieved mainly through ceramic and otherartifact typologies, but also through radiocarbon dating. However,several ceramic sherd types are non-diagnostic while others do notconform to established typologies. Luminescence dating has alsobeen used, including thermoluminescence (TL) of Minoan kilns andNeolithic settlements in Thessaly and Macedonia (Liritzis, 1979,1984; Liritzis and Galloway, 1982; Liritzis and Thomas, 1980; Liritziset al., 2002) and on metallurgical slags from Siphnos and Thasos(Elitzsch et al., 1983). Obsidian hydration dating (OHD) in theAegean has not been extensively used for dating due to variousdrawbacks of the method until recently with modern advances(Stevenson et al., 2002; Liritzis, 2006; Liritzis et al., 2004), and theonly characterization work that has been done extensively is onobsidian tools to help determine their provenance (Dixon andRenfrew, 1968; Kilikoglou et al., 1996, 1997; Renfrew et al., 1965;Williams-Thorpe et al., 1984; Liritzis, 2008). Part of the issue lies inthe large uncertainties with OHD regarding the empirical equationand estimated diffusion coefficient (Anovitz et al., 1999).

Here, both luminescence and obsidian hydration are employed,to verifying previously uncertain archaeological chronologies offinds from the site of Strofilas on Andros Island (Cyclades, Greece)in the Aegean. The problem of dating has become a pressing issue;previous attempts identified a chronology for most artifacts andstructures at the site with the exception of the fortified wall andobsidians. However other archaeologists have expressed doubtsregarding these earlier dating estimates. Failing to reacha consensus, I was asked to provide additional dates as part ofa pilot study which is presented here; additional work is expectedto provide further chronological data.

The use of TL and optically stimulated luminescence (OSL) todate megalithic stone buildings and masonries made from largercarved stones has been previously reported (see Greilich et al.,2005; Liritzis et al., 1997; Liritzis and Vafiadou, 2005; Theocariset al., 1996; Theocaris et al., 1997). Regarding OHD, the advent ofhydration dating rests on a different approach which is based onthe hydrogen profile of diffused water in the hydrated rim. Thisrelatively recent technique is known as the SIMS-SS (secondary ionmass spectrometry) which uses surface saturation (see Brodkeyand Liritzis, 2004; Liritzis, 2006; Liritzis et al., 2004; Liritzis andDiakostamatiou, 2002; Liritzis and Ganetsos, 2006). This techniquehas been successfully applied on obsidian from a number of sitesworldwide, including Japan, Mexico, Easter Island, and Hungary(Liritzis et al., 2004). Both luminescence and a novel alternativeOHD method were used for first time on recently (2001) discoveredfinds at Strofilas (Fig. 1).E-mail address: [email protected]

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To facilitate the unaware readers on the operation principles ofdating methods used, their relatively recent development, and thesignificance of the site, I provide a brief outline of the archaeologyof the site followed by a description of these new methods, theirsampling and preparation, and the results obtained.

2. The archaeology of Strofilas

The recent excavation of Strofilas is one of the most excitingdevelopments in the archaeology of the prehistoric Cyclades.Excavated Cycladic communities of pre-Bronze date remain rela-tively rare, and this one is large, fortified and located on Andros,a mojor island stepping stone from/to Atica, yet one about whichfrustratingly little is known archaeologically. Other major sitesdated to the Late/Final Neolithic period are scarce (Ftelia atMykonos, Saliagos at Antiparos, Za cave in Naxos, Koukounaries atParos, Kefala at Kea) (see relevant chapters in Sampson, 2006).

According to excavator Televantou (2001, 2005, 2006, 2008), thesettlement provides finds that date back to the Late Neolithic (i.e.,the Final Late period, ca. 4500 to ca. 3200–3000 BC) to the latterpart of the Early Bronze Age, on the west coast of the island. It isa large settlement dated to this period yet found in the Aegean,stretching roughly 2.5–3.0 ha in the center of the valley-plateau ofStrofilas (37� 460 5000 of latitude, 24� 510 2100 of longitude, 143� 3 mabove sea level) (see Table 1 for chronological issues dealing withthe dating of Strofilas). (Fig. 2)

2.1. Architecture and fortification

Most excitingly of all, the site includes along with domesticarchitecture and fortifications, an unparalleled wealth of rock art

pecked onto both geological rock surfaces and the face of thefortification wall. This fortified wall is the earliest known to exist inthe Aegean Sea with clear evidence of the fortification defenseconcept, it is also unique for the Final Neolithic period. It shows thatthe known Protocycladic (II and III) fortifications of the 3rdmillennium BC in Markiani (Amorgos), Kastri (Syros) and Panormos(Naxos) were preceded by those in the Neolithic period (Aslanis,1998; Darcque and Treuil, 1990; Dickinson, 1994; Doumas, 1977;Maragou et al., 2006; Sampson, 2006).

The settlement is situated on the western coast of the island atthe top of a small peninsula adjacent to a large valley – a natural fort,with two safe ports that ensure the protection of ships and maritimeactivities. This peninsular valley is vulnerable to attack only frominland with easy access that endangers the settlement (Figs.1 and 2).

The town’s spatial arrangement is exceptionally dense, withquadrangular and apsidal buildings. They have thick walls rangingbetween 60 and 80 cm wide, suggesting there may have beena second floor. This may be indicative of the connections thatAndros had in trade routes and the movement of ideas, technology,and various products between the Aegean and continental Greece.

Fig. 1. Position of the settlement in the Aegean Sea. Strofilas settlement is encircled.

Table 1Aegean Neolithic and Bronze Age.

Phase Date, B.C.

Aceramic Neolithic ca. 7000/6800–6500Early Neolithic ca. 6500–5800Middle Neolithic ca. 5800–5300Late Neolithic ca. 5300–4500Final Neolithica ca. 4500–ca. 3200Early Bronze/Early Minoan/Early Cycladicb ca. 3100/3200–ca. 2000Middle Bronze/Middle Minoan/Middle Cycladic ca. 2000–ca. 1650Late Bronze/Late Minoan/Late Cycladic ca. 1650–ca. 1070

(Sources: Barber, 1987; Demoule and Perlis, 1993:366, their Fig. 2; Dickinson, 1994;Manning, 1995, 1994; Treuil, 1983).

a The term ‘‘Final Neolithic’’ means the very last, transitional phase of theNeolithic, in which stone tools were in use along with elements of the succeedingmetal age. The terms ‘‘Chalcolithic’’, ‘‘Copper Age’’ and ‘‘Sub-Neolithic’’ clearly fallinto this category. The lower end is set somewhere between 3000 and 3200 B.C.

b In general absolute dates for the Aegean Neolithic and Bronze Ages are not yetvery reliable and many different sets of dates are often in use for one and the samephase or period. The Bronze Age Cultures within the central and western Aegeanislands are termed ‘‘Cycladic’’. At any rate, here ‘Cycladic culture’ or ‘the Cycladicperiod’ stands for the EBA in the Aegean, and ‘Protocycladic’ for the earlier part ofEBA.

Fig. 2. View of Strofilas settlement plateau. Access from the sea is hindered by thesteep cliffs, on the left by the two natural parallel cliffs and to the right of the picture bythe fortified wall.

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The houses seem to belong to more than one phase. Current small-scale excavations (2009) on the base of fortified wall has reachedbedrock verifying the use of mortar between the slabs.

2.2. Engravings

The earliest monument engraving representation in the Aegeanassumed to be of religiously significant includes spirals, eight-shaped human foot-like and ring-shaped figures, similar to knownNeolithic pendants/figurines over the floor’s entire surface. Thefour stone pendants of Strofilas, apart from the incised ones foundin different locations around the settlement (pictured togetherwith a spiral made of punctured holes) are the only ones known toexist in the Cyclades. These ring-shaped figurines are similar to theenigmatic Protocycladic frying-pan shaped utensils which,amongst other purposes, are attributed to some sort of ritual andare considered as a symbol in the ‘universal’ code of communica-tion for that period. Similar figurines are found in the Peloponesse(Alepotripa cave), Neolithic Dimini (Thessaly), and elsewhere, up asfar as the Balkans and Varna in the Black Sea (see Sampson, 2006).On the other hand, there is a similar correspondence of figures withpercussion on stones from Korfi t’ Aroniou (Naxos) dated to theEarly Cycladic II period (Doumas, 1965). Many cases of Naxian rockart dispersed in the fields of the island show that the art is not easilydated, but that this artistic practice continued in the Cycladesduring the Early Bronze Age.

2.3. Rock art of ships

The art depicts much of the same imagery that is known fromthe following, better-known Early Bronze Age (3rd millennium BC)Cyclades, though with some clearly slightly earlier elements.Included are images of large and small canoes that have thepotential to be the earliest depictions of seacraft (as opposed toriverine vessels) anywhere in the Mediterranean – itself a heartharea of global seafaring from a far earlier date, as is indicated bymovements of materials and the colonisation of insular areas. Untilnow, the earliest Mediterranean depictions have come from otherCycladic islands, Malta, and Egypt, dating to the 3rd millennium BC,with a few possible late 4th millennium BC images from the Levant,too, but no assurance that these last depict seacraft. (Broodbank,1989; Zois, 1973; Doumas, 1964; Woolner, 1957). Most ships seemto be directed towards the gate and perhaps functioned as markersindicating the entrance to the settlement. But the 37 unique shipimages of various sizes and shapes so far discovered on the bedrockand the fortified stones in Strofilas are of particular importance, notonly for artistic and symbolic reasons, but because they providenew significant insights on the sophisticated degree of ship-building during the Late Neolithic period – as the present resultsconfirm – by which time it is accepted that large sea-going canoesand extended dugouts are plausible (even if not previously attesteddirectly by imagery), perhaps linked to the proliferation of metaltools (see, for example, Broodbank, 1989, 2000: 99). On thecontrary, the ships take on variable shapes, size, and type, capableof holding only a small crew or one much larger in size – many ofthese ships have a cabin and a helm (Televantou, 2005:214;Fig. 291; Televantou, 2006: Fig. 178). This might have been expec-ted, taking into consideration the abundance of trees on islands,such as, Andros which must have played a key role in the art ofship-building. In the central bastion there appears to be a convoy offour such ships, probably reflecting a standard practice for thatperiod in terms of collective marine activities such as fishing andsea trade.

However, these finds need to be anchored chronologically withclear associations with other archaeological materials. The themes

of these petroglyphs are related to marine figures but also tocontinental images (see also, Moutsopoulos, 1969). This highlightsthe two most significant branches of activity of the Strofilas resi-dents: farming (particularly livestock farming as well as thehunting of wild animals), but also navigation, fishing, and seafaring.

2.4. Ceramic typology

Ceramics were typically found under the floors in abundance,while those found above the floors of the final residence weresparse, giving the impression that residents took all utensils withthem during a peaceful abandonment of the settlement. The clayfabric is monochrome, melanochrome, usually polished, and eitherincised with white pigment or pattern-burnished. Typologicallythey compare well with those from Kefala (Kea), Attica, AlepotripaCave in Mani, Aria in Argolid, Emporio Chios, and cave Za in Naxos(Coleman, 1977, 1985; Sotirakopoulou, 1998; Zachos, 1999). Fromthis comparison Christina Televantou the excavator suggests that itbelongs to the so called ‘‘Attica-Kefala culture’’, which spread fromAttica to the central and northeastern Aegean w4500–3300 BC(Televantou, 2006). Nevertheless, these clay vases are not easilyattributable to this phase as they include basic characteristics ofProtocycladic analogues. Also, several ceramic sherds clearly belongto an early stage of EBA: the ‘‘Grotta Pylos’’ phase (Broodbank, 2000;Sotirakopoulou, 1998). A large number of other stone artifactsinclude figurines, obsidian blades and flints, grinders, axes, andmetal finds such as copper objects. It is possible that the inhabitantsmoved to a more southerly location at cape Plaka, where a largetown has been discovered from the Early to Middle Bronze Age,similar to those of Philakopi on Melos and Aghia Irini on Kea(Televantou, excavation work in progress, pers. comm., 2007).

Since these finds were first announced, my overwhelmingquestion has been ‘what precise date are the images?’. Preliminaryinformation from other material culture suggests a broadly FinalNeolithic date (¼Late Neolithic II or Chalcolithic in other schemes).But this is not very helpful because (i) this overall periods runs from4500 to 3200/3000 Cal BC, albeit with subdivisions that cansometimes be discerned (ii) there seems to be some indication fromthe finds of a late phase of activity within the Early Bronze Age.Although many scholars appear to have assumed that the imagesmust go with the main phase, and some infer early dates withinthat, neither point is in fact proven. For example, if Early Bronze Agematerial is indeed present on the site it would be theoreticallyperfectly possible for the ship images to go with this phase, andtherefore to date to the 3rd millennium BC – in which case theywould be interesting but unexceptional. Therefore, good dating ofthe site, and particularly the images, is of critical importance. Giventhe largely millennial-level uncertainties (i.e. late 5th, or 4th, or3rd?) even dates using techniques with a wide error margin (as inTL and OHD) could be extremely useful.

3. Luminescence dating of the fortified wall: the rationale

The luminescence technique employed in this study has beenrepeatedly tested and published elsewhere (Liritzis et al., 1997,2008a; Liritzis and Vafiadou, 2005; Vafiadou et al., 2007), whilesurface dating in general has been successfully applied (Haber-mann et al., 2000; Huntley and Richards, 1997; Morgenstein et al.,2003; Greilich et al., 2005; see also relevant presentations in theLuminescence in Archaeology International Symposia, conferencein Delphi, LAIS2009). In masonry it involves dating the inter-blocksurfaces of building stones which relies on the optically sensitiveelectron traps responsible for TL light in the surface layer of thecarved rock, having been bleached by sunlight, prior to the blocksbeing incorporated into the structure. The surface’s exposure time

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to sunlight depends on the time taken by the stone masons to puta given block in the appropriate position overlaid by another. Fromthe moment that any surface is no longer exposed to sunlight andput in firm contact with mortar, the optically sensitive electrontraps are filled by electrons produced by the ionization caused fromnuclear radiation of natural uranium, thorium, potassium,rubidium and cosmic radiation. These isotopes are present in therock slabs and the soil surrounding the sampling point. Thus, theage can be given by equation (1):

Age ¼ Equivalent DoseðEDÞ=Annual Dose� RateðDRÞ (1)

The ED (in Grays, Gy) is measured by thermoluminescence (TL)or optical stimulated luminescence (OSL) following standardprocedures of additive doses or for the regeneration of a single dosefrom multiple aliquots (Aitken, 1985; Greilich et al., 2005; Haber-mann et al., 2000; Hong et al., 2000; Liritzis, 1994, 2000; Liritziset al., 2002; Vafiadou, 2006). However, here TL was used instead ofOSL because calcite cannot yet be analyzed using OSL. The AnnualDose Rate (in mGrays per year) denotes the radiation dose accu-mulated in a year. It is comprised of the three-radiation dosecomponents (alpha, beta and gamma radiation) derived from thenatural radioisotopes of uranium (U-238), thorium (Th-232),potassium (K-40), and rubidium (Rb-87), of the sample itself andthe surrounding environment, and includes cosmic-rays (Aitken,1985; Liritzis, 1994; Liritzis et al., 1997).

3.1. The measurements and sampling: total dose and dose rate

Sample (STR1) of calcite (with traces of muscovite) was takenfrom the fortified wall from a block beside one bearing an engravedship on its surface, using a chisel and a hammer. It was chosen, with

the aid of the excavator, in an appropriate set of large overlaid stonesfrom the lowest levels of the wall (Fig. 4). Nearby to this Strofilas 2sample was detached. Radioactivity data for the soil mortar were:K¼ 1.65� 0.03%, U¼ 1.63� 0.21, Th¼ 4.84� 0.76 ppm and for therocks U¼ 0.32� 0.03, Th¼ 0.21�0.09, and K¼ 0.04� 0.02.

Samples were swiftly wrapped in black plastic bags to avoid sunexposure. As an added precaution against light exposure, thesampling took place late in the evening while the adherent soil onthe surface helped to block sunlight from reaching the surface (seealso Vafiadou et al., 2007). This wall (of about 30 cm thick) isconsisted of a double wall infilled with sediment and debris ofa thickness of about 30 cm. Initially, a surface layer of about 50 mm(measured by a micrometer), which included organic material,dust, and adherent paste, was removed from the inner surface bybriefly inserting it in dilute hydrochloric acid. Subsequently, a thinlayer of surface powder was acquired by gently scraping the inter-block surface to a depth of less than 0.5 mm (making a series ofreadings with a micrometer) and transferred to an acetone bathwhere grains were collected, washed in dilute acetic acid (0.5%) for1 min, and dried. Fine grains of calcium carbonate were sieved toa diameter of 2–8 mm prior to TL measurements.

The TL reader used for the natural ED was a Littlemore Co 711,containing a glow-oven with the pressure reduced to 0.1 Torr and

Fig. 3. a) Engraved ships on the surfaces of the fortified wall, b) representation of sucha ship by Televantou (2005) (In my first visit to the site I indicated the first engravingsto the excavator, and later the excavator identified the engravings of many more ships,four of which form a convoy and is analogous to a miniature wall-painting from Thera).

Fig. 4. Sampling for luminescence dating in two locations of the fortified wallaugmented with small arrows and encircled areas. The one with the chisel (STROF-1) isfor after sampling and the other before removing lower part of large stone.

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a flow of N2. PM type EMI 9635QA bialkali was used with both clearand blue filters. TL curves were stored in a 1024-channel ADC,operating in MCA mode. The heating plate was nicochrom, 0.8 mmthick, with a Cr-Al thermocouple and a heating rate 4 �C/s. Theradiation used was beta source 90Sr–90Yt at a rate of 0.6 Gy/min.The original surface of the sample was cleaned under red-lightconditions with diluted HCl acid to remove dust, and any organicresidues. Then gently remove a thin layer from the surface in theform of powder, deposit in acetone bath and collect fine grains,wash in dilute acetic acid, and dry (Liritzis and Vafiadou, 2005).

The ED dose was determined for successive temperature inter-vals between 200 and 350 �C, applying the plateau test (Aitken,1985; Zacharias et al., 2006; Liritzis et al., 1997). The principle ofsubtraction and the use of a dose plateau are based on the notionthat the effect of bleaching causes readings to surpass the equiva-lent unbleached readings on TL curves by percentages that dependon the length of exposure. Calcite, unlike quartz and feldspar, is notan easily bleached mineral, and in most limestones the unbleach-able residual is reached after a prolonged period of some dozens ofhours (Habermann et al., 2000; Liritzis and Bakopoulos, 1997).Whatever the unknown unbleachable residual TL is, it serves as the‘non-zero clock’ level upon which subsequent radiation builds up.However, the bleaching is not reduced proportionately for varioustime exposures for all temperature ranges of a TL curve. Thus theplateau obtained for different residuals are of variable length. Thestarting non-zero residual TL is determined as the residual TL levelwhich, when subtracted from the additive dose growth curve,produces the longest plateau in the temperature-dose plateau test(Liritzis and Galloway, 1999; Liritzis et al., 1997)

Powder was removed, 2–8 mm in diameter, and was adhered todiscs with silicone oil. In the series of bleaching tests, the powderremoved to determine NTL was used for solar exposures at30 minutes, 90 minutes, 4, 6, 12, 24, and 36 h (Fig. 5). An additivedose curve was made with doses of 28 Gy, 57 Gy, and 85 Gy (Fig. 6).A background count followed every measurement, against a testdose normalization of 6 Gy. The constructed growth curve followedthe additive procedure of multiple aliquots (Liritzis, 1994; Liritziset al., 1997)

The bleached TL curves were subtracted from the natural andNþ beta TL curves, after which a dose- temperature plateau (Fig. 7)and a built-up growth curve were constructed (Fig. 8).

A sensitivity test (for testing any lattice disruption) performedused two doses (5 and 20 Gy) in repeated irradiation andmeasurement cycles. For STR1 an increased 1st TL read-out isobserved at ca. 7% compared with the rest TL curves. For STR2 TL

sensitivity shows moderate variation (ca. 3–5%). Both tests weremade to check the variation of the adopted TL peaks at ca. 270 �Cand 330 �C

The ED was calculated from eq. (2)

ED ¼ ððNTL � NblÞ=ðN þ b� NTLÞÞ � b (2)

where NTL the natural TL, Nþb the natural TL added beta dosescurves, b the administered beta dose in Gy, Nbl the bleached TL.Application of this formula assumes a linear response; and thisassumption is valid and provided an average dose of 5.0� 0.2 Gy.

Indeed, the above procedure for ED determination wasstrengthened with a simulation dose recovery experiment repeat,administering a dose of 7 Gy employing the Riso OSL/TL systemwith heating rate 1 �C/s. Fig. 9 shows the additive dose proceduredose curves after administering 3.5, 7 and 14 Gy, Fig. 10 illustratesthe equivalent dose plateau after using the 12 h bleaching glowcurve, and, Fig. 11 the built-up curves, where the horizontal lineindicates the residual TL level after 12 h of bleaching. The arrowshows the equivalent dose of 6.51 Gy while the dose to be recov-ered was 7 Gy Equivalent plateau dose was 6.61�0.15 Gy while thedose to be recovered was 7 Gy.

The total equivalent dose was measured by the additive doseprocedure, but the extrapolation procedure is not exclusively usedhere. Instead, the residual TL level of unknown magnitude has to be

Fig. 5. Bleached TL curves after exposure to the sun for various times. Natural(squares) as the average of three curves, 12 h. (circles), 24 h. (triangles) and 36 h.(crosses).

Fig. 6. Additive dose TL curves: natural TL (squares) as the average of three naturalcurves, NTLþ 28 Gy (circles), NTLþ 57 Gy (triangles) and NTLþ 85 Gy (crosses).

Fig. 7. Dose–temperature plateau test. The symbols refer to natural (squares), 12 h(circles), 24 h (triangles) and 36 h (crosses) of outdoors sun bleaching. The longestplateau was for the 24–36 h exposure giving an average ED of 5� 0.2 Gy.

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calculated as described above. The longest plateau represents theoriginal (ancient) TL curve, from which the environmental dosebuilds up. Experimental simulations elsewhere, have shown thisplateau (Liritzis and Bakopoulos, 1997; Liritzis et al., 1997). Thismaximum plateau length was found for 24–36 h exposure, and the270 �C peak giving an average ED of 5.0� 0.4 Gy taking intoaccount uncertainties in subtraction of correct unbleachablecomponent and the limited extend of the plateau (Fig. 10). Theobservable difference in temperature plateaux and TL peaksbetween dose recovery and natural state is due to, a) the differentheating rates; 4 �C/s for the natural and 1 �C/s for the recovery test,and b) no bleached TL was taken for sun exposures> 12 h in thesimulated test. The large disturbances in TL shapes above 260 �Cmay be due to a) TL sensitivity changes in annealed calcite thew330 �C peak (Galloway, 2003), b) irreversible processes in calcites(Franklin et al., 1990).

Dose rates were measured individually for each radiationcomponent, converting concentration to dose rates (Grays/year)(Liritzis and Kokkoris, 1992; Liritzis et al., 2001). The sampleremoved from the wall was subjected to the following radiation

sources: 1) the vertical wall itself including the debris (breccia)from within the double fortified wall; 2) the rocky ground of similartype to the wall and soil cover over the centuries; and 3) cosmicradiation. In particular, U and Th content of the rock and the soilwere measured by alpha counting, employing the pairs technique(Aitken, 1985, 1990; Akber et al., 1983; Sjostrand and Prescott,2002), and by INAA for the rock, while K-40 and Rb-87 weremeasured by a portable EDXRF. In addition, beta particle dose rates(Db) were measured by a GM-25-5 beta counter (at Riso). Averagevalues of all methods used for each individual dose rate measuredfor the rock and soil are given in Table 1.

At the sampling location, a complex geometry was involved inthe estimation of the environmental gamma radiation due to itsheterogeneous environment. Due to the complex gradient geom-etry, in the dosimetry involved (see Figs. 4 and 12) the non existingparallel layer model of Aitken (1985, Appendix H) is not applied,Instead, the following corrections were made and compared with insitu measurements with a portable NaI scintillator. For the envi-ronmental gamma-ray dose-rate (Dg), assuming the spherical

Fig. 8. Built-up curve of natural TL. A representative dose–response curve (filledsquares) plot for the temperature of 270 �C of the glow curve for the sample STR1.Horizontal line indicates the residual TL level after 24 h of bleaching. The arrow showsthe equivalent dose of 4.91 Gy while the equivalent dose plateau yielded 5 Gy.

Fig. 9. Recovery test. Additive dose TL curves after administering 3.5, 7 and 14 Gy.

Fig. 10. Recovery test. Equivalent dose plateaus after using the 12 h bleaching glowcurve. Equivalent dose yielded was 6.61�0.15 Gy while the dose to be recovered was7 Gy.

Fig. 11. Recovery test. A representative NTLþ b (filled squares) plot for the tempera-ture of 325 �C of the glow curve. Horizontal line indicates the residual TL level after12 h of bleaching. The arrow shows the equivalent dose of 6.51 Gy while the dose to berecovered was 7 Gy.

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approach for self-gamma dose rate and the superposition principlewhere infinite dose rate comes from approximately 35 cm diametersphere (Liritzis, 1986), the computations made on gamma-ray doserates from soil to various thicknesses of calcite layer ofr¼ 2.71 g cm�3, using data generated with codes GAMP 1/FGX areemployed. In fact we used GAMMA BANK system through NuclearEnergy Agency, NEA for the case of an infinite soil medium overlaidby an infinite calcite medium and for calcite of 10 cm thickness slabwith infinite soil on both sides. The plane one-dimensional photontransport (Boltzmann’s) equation is solved for the scatteringgamma radiation flux in the case of two adjacent media, assumingradioactive equilibrium in the U–Th decay series and the naturalgamma radiation is predicted (Kirkegaard and Lovborg, 1980). Thismodel is a fairly good approximation of the present geometry andhas been presented elsewhere (Liritzis, 1989). This rule of thumb isan approximate one, but given the irregular geometries of mostcontexts there is no point in attempting to be more precise. Thus,around 75% derived from the wall (Dgw), the bedrock and infilldebris layer (the wall, the bedrock and infill debris layer comprisea 3/4ths or 75% of a hypothetical sphere for which the samplinglocation is at or near the center), and 25% from the soil mixed withdebris (Dgs) (i.e., one quadrant of the hypothetical sphere, whichhad covered the wall before the excavations) while applyinga 30%� 10% water uptake correction suitable for such coastal sites(Liritzis and Galloway, 1981, 1982). A similar value for humidity wasalso obtained in Spring 2007 after monitoring it for a period of oneyear (a HOBO H8 data logger downloaded the data onto a PC withthe appropriate Box Car software). A cosmic ray dose rate was alsoadded.

Furthermore, the sampling refers to a depth of around 3 cmfrom the outer surface of a 30 cm thick limestone wall, thus gammaradiation from soil exponentially attenuates by about 60% (0.4 gs inFig. 12; see also, Liritzis, 1989, Figs. 3 and 4). The additionalcontribution from the inner sediment debris double row of w30 cmthick passing through the 27 cm thick limestone block, attenuatesby 99% (0.01 gs in Fig. 12) – an almost insignificant amount. Direct insitu measurements with a portable g-scintillometer (ORTECMicroAce 32, 2� 2 inches in diameter, with an EMI 9814A PM andMAESTRO-32 software), well calibrated to radioactive pads, havealso provided a similar result, taking into account the soil cover.This enhances the detailed evaluation of Dg.

Half of Daw derives from the wall rock itself with a measured k-value of 0.09� 0.015 (the other half is removed with the surfacelayer of 50 mm), while, for Db half of this derives from the rock andthe rest from the paste which is of similar content to the soil(attenuated by the removed layer of 50 mm), for which a correctionfor the water uptake values of 30% has been made. Thus, the TotalAnnual Dose Rate (DR) is:

DR ¼ ½3Dgw=4þ Dgs=4 þ Dc� þ ½Dbw þ 0:90� Dbs��

2

þ0:09� Daw (3)

The corrected annual dose rate was 0.913� 0.09 Gy/ka (ka¼ kilo-year) taking into account all parameters and an additional 3%uncertainty in the radiation geometry assumptions. The portable g-scintillometer data gave 0.907� 0.05 Gy/ka, making an average ofthe two values of 0.910� 0.08 Gy/ka, and an age for STR15,500� 530 B.C.

Another piece from a different block provided a similar ED(5.04� 0.40 Gy). The average dose of the two samples was 5.020�0.224 Gy and the mean dose rate of the two samples together withthe scintillometer value 0.909� 0.08 Gy/ka making a mean age of3520� 540 years B.C. (see Table 2). Laboratory irradiation andstorage for two months excluded any fading for both sub-samples.

4. Obsidian hydration dating SIMS–SS: the new approach

The OHD method is based on modelling the rate of water diffu-sion into the natural glass surface. A variety of strategies have beendeveloped over the years to calibrate the movement of ambientwater into glass. Many of these approaches have developed proce-dures for controlling the chemical composition of the glass andmodelling the environmental history of an artefact’s environment(e.g., temperature, humidity) (Ambrose and Stevenson, 2004;Anovitz et al., 1999; Friedman and Smith, 1960; Stevenson et al.,1998). However, the development of calibrations to compensate thevariation in external variables has proven to be difficult. This hasbeen the major impediment to making OHD a fully chronometric

Fig. 12. Schematic representation for STROF-1 showing the geometric relationship ofall parts: rock, soil, rubble, showing how radioactivity of each part was measured(same principles apply for the second sample too).

Table 2Equivalent Dose (ED), alpha (Da), beta (Db), gamma (Dg) and cosmic (Dc) dose-rate and corrected ones, and for soil and rock, and average age for two rock slabs.

Radiation data Soil Rock Corrected

Da Gy/ky 8.2� 0.41 0.73� 0.06 (0.75� 0.06) 0.066� 0.06 (0.068� 0.05)c

Db Gy/ky 1.61� 0.03 0.06� 0.03 (0.058� 0.03) 0.612� 0.05a (0.605� 0.05)c

Dg Gy/ky 0.83� 0.04 0.039� 0.005 0.0653� 0.003Dc Gy/ky – – 0.17� 0.03d

DR Gy/ky 0.913� 0.09 (0.910� 0.08)b (0.908� 0.09)c 0.909� 0.083f

ED (Gy) 5.00� 0.20 (5.04� 0.40)c 5.02� 0.224f

Age, years BP 5,500� 530 5,520� 540e

a Sum of two components: half from the rock and the other half from the paste similar to local soil. Beta ray dose rate for soil and rock were average from – alpha counting,PXRF, INAA and by GM-25-5 beta counter at Riso.

b Average of portable and individually measured.c data for the second sample..d Based on Prescott and Stephan (1982).e Average age of two samples including additional uncertainties in dose rates and rounded figures (see text).f Average of two samples.

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dating method comparable to radiocarbon or luminescence dating(Liritzis, 2002). The major controlling parameters that limit themethod is obsidian chemistry and environmental conditions., whiledoubts have been expressed on the validity of the rate of hydrationbeing proportional to the square root of time in the traditional ageequation (x2¼ K$t). (Anovitz et al.,1999; Liritzis, 2006). For the latter,Stevenson et al. (1998, 2004) have showed by laboratory studies thatthe square root of time could be valid for practical purposes. Thissquare root of time dependence in conventional OHD arises from theBoltzmann transformation, z¼ x/(4 t)1/2, employed in solving thediffusion equation. Inspite that conventional OHD under exceptionalcase sites, where several structural (chemistry, hydroxyl amounts)and environmental parameters are known and extensive labexperiments are made with same obsidian source and use of tradi-tional temperature cells used, and newer analytical techniques mayprovide useful results (see, Eerkens et al., 2009), yet it can not besafely applied to any other site where such detailed information persite and obsidian is lacking.

In the past few years an alternative approach to the OHD methodhas been proposed, based on the concentration-dependent waterdiffusion profile, by modelling the concentration-to-depth sigmoidshape of the diffused water as determined by secondary ion massspectrometry (SIMS). This age modelling of SIMS Hþ profiles issupposed to be independent of the former unknown factors, beinga purely intrinsic method. Whatever the environmental factors are,the obtained SIMS profile is in fact the end product of water diffusionthroughout the period of deposition, and includes all uncertainvariations affecting the rate of diffusion. The latter is reflected inslight variations of shapes of the sigmoid profiles between differentobsidians and environments as has been shown elsewhere (Liritzis,2006). Although the claim of being independent of environmentalfactors could be labelled an assertion, experience with a majority ofobsidian sources from many parts of the World and age span over thepast 30,000 years, so far, verifies steadily its validity (Liritzis andLaskaris, 2009). Moreover laboratory simulated hydration experi-ments running over 20 years seem to verify this assertion andprovide correct ages (Liritzis et al., in preparation)

We have evaluated a procedure for obsidian age estimation thatis based upon the depth and shape of the Hþ diffusion profile asdetermined by SIMS. We have termed this approach SIMS-SS sincethe primary input variable, which controls the water in the mass ofobsidian, is the saturation achieved in the surface layer (SS). Earlierwork outlines the evolution and derivation of this method in somedetail (Liritzis et al., 2004; Liritzis and Ganetsos, 2006; Brodkey andLiritzis, 2004; Liritzis et al., 2008b).

In summary, the steps taken towards SIMS-SS dating include: 1)polynomial fit: defining best polynomial fitting (R-squared) of the Cvs. X profile, usually of 3rd order (taken from TABLECURVE 2Dgraphics software) of the type C¼ exp(aþ bxþ cx2þ dx3) (Fig. 13);and, 2) defining the plateau layer (SS) from the Hþ profile to thusdetermine Csur¼ Cs and Xsur¼ Xs. This is achieved from the peaks ofthe 1st derivative of the profile and 1st derivative of the fitted initialdiffused part of the profile, combined with successive linearregressions. Subsequently, the software also provides all concen-tration values in this plateau layer and calculates (as outputs) theCsur, Cint and Xsur values.

That is, the new modelling is based upon, a) Fick’s law of diffu-sion, b) the fitting of the Hþ profile with a 3rd order polynomial, andc) the setting of initial and boundary conditions. Subsequently,making use of numerical solutions of diffusion and other mathe-matical transformations (see Crank, 1975), as described below (seeFootnote 1), the diffusion age equation or time is obtained (Eq. (3)):

T ¼hðCo � CsÞ2ð1:128=ð1� 0:177kCo=CsÞÞ2

i.

h4Ds;eff ðb� expðaÞÞ2

i(4)

when Co¼ Cint, this is the initial concentration of the intrinsic waterin obsidian; Cs is the constant surface concentration of water in thesaturated layer; k is a constant factor that corresponds to the non-dimensional curve which characterizes the measured diffusionprofile, according to Crank (1975), where k-values presented as ek

are derived from a family curves of ek versus X/Xs for certain C/Cs,and

D ¼ DsexpðkC=CsÞ (5)

The end point of SS layer Xs is used in the calculations of thegradient dC/dx of the C vs. X profile. The SS layer is located via thederivatives of the diffused part of the Hþ profile. When C¼ Cs andX¼ Xs the diffusion coefficient, D¼Ds. According to Fick’s 1st law,Ds is the inverse of the gradient. Thus:

Gradient ¼ ðvC=vxÞx¼0¼ b� expðaÞ (6)

Ds ¼ ðflux=gradientÞ � 10E� 11 years (7)

The factor 10E�11 is used to convert units of D from the calculatedmm2/1000 years to cm2/year, which are the units used in SIMS-SS,and flux assumed one. For X¼ Xs, Ds is computed from equation (6).The only empirical term, Ds,eff, is determined from 26 differentwell-dated samples of different origin given in equation (7). Thisequation was produced as a phenomenological way to overcomethe unknown flux. It should be noted that the different chemistrythat imply hydration at vastly different rates in the same condi-tions, it has been stated right from first publication (Liritzis andDiakostamatiou, 2002) that the Ds,eff contains the unknown andvariable environmental conditions subjected by the particularsample and site, via the shape of Hþ profile from the latter theequations (6)–(8).

Ds;eff ¼ a� Ds þ b=�

1022 � Ds

�(8)

where a¼ 8.051E�6, b¼ 0.0999, (r2¼ 0.999).Derivation of equations (4)–(8) is discussed and given elsewhere

(Liritzis and Ganetsos, 2006; Liritzis et al., 2004, 2005). The data ofTable 3, where a¼�6.88 and b¼�2605, shows the coefficients of

Fig. 13. SIMS Hþ profile and 3rd order polynomial fit of concentration vs. depth.

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the 3rd order polynomial fit of the SIMS sigmoid, with equation (3)providing the diffusion time, which includes the time required forthe formation of the saturated layer, the depth Xs and theconcentration Cs. Table 3 also includes the data of the secondsample. Needless to say, the SS layer formed with the highestconcentration and a progressive front during the diffusion time isdefined from Cs and Xs. SIMS profiles were taken by a PHI Model6300 and 6600 quadrupole-based SIMS (data obtained at EvansEast, USA; see Liritzis et al., 2004). The measurements were per-formed using a 300� 300 micron ion beam raster. For flat andsmooth areas, reproducible errors for depth are 2–3%. The sput-tering rate used was 24 Å/s, which is fairly typical. Conversion of ioncounts to concentrations was achieved using an ion implantedobsidian standard. This standard was used to calibrate the sput-tering rate, so that there was no direct crater depth measurementsof archaeological samples. The procedure was processed via a novelsoftware (Liritzis and Ganetsos, 2006; Liritzis et al., 2005; Liritziset al., 2008b; Liritzis and Laskaris, 2009).

The two obsidian blades were obtained from within the doublewall filled with debris and sediments, were parts of blades withoutany clear typology. The SIMS-SS OHD dating approach usedthroughout the calculations the Taylor’s rules of error propagation(Taylor, 1997) and provided an average age dated to 3400� 200years BC for both (for more information see the detailed descriptionof the dating and free use of its software at www.rhodes.aegean.gr/tms/sims-ss). Table 3 gives the measured and calculated data ofequation (3). Based on the criteria imposed for the suitability ofsamples from their sigmoid shapes and the involved respectiveerror from surface topography (investigated by Atomic ForceMicroscopy) the Strofilas samples are suitable enough providinga satisfactory overall age error (Liritzis and Laskaris, 2009). Bothpieces fall within the island of Melos sources, as Ti and Sr plottingalone have shown (Liritzis, 2008).

5. Discussion

A dating of 3400� 200 years BC for the two obsidian bladesprovides a similar result within errors with two samples of thefortified wall dated by luminescence to 3520� 540 years BC. Thisindicates the use of the settlement within a certain cultural phaseand with a possible overlapping time period, that spans between3200 and 3800 BC (i.e., mid to late Final Neolithic) taking intoaccount the standard error of the mean (�300 years).

Concerning the dating of the wall, it should be taken intoaccount that: 1) the sample was taken from the external side of thewall, half a metre above bedrock; and 2) in the upper level of thewall, ancient masonry which was always visible is intermingledwith later masonry build by contemporary cultivators.

Therefore, the date of the specific part of the fortification wallrefers to the original position and to the last time (terminus antequem) the stone was exposed to light. Such an event indeed couldhave occurred during some restoration activities of upper layers ofthe masonry by later occupants. Naturally, the age of the engravedships of this part of the wall is similarly a terminus ante quem of theage of the dated stone wall. However, the shape of the ship findsparallels elsewhere in the Aegean, Mesopotamia (Wadi Hammat),and Egypt (especially the eastern desert petroglyphs at Wadi

Barraniya (Wadi Abu Wasil) during the 3rd and 4th millennium BC(Renfrew, 1972: plates 3, 28, and 357).

Regarding the obsidian age, this corresponds to the samecultural phase of the wall within the errors (i.e., Final Neolithic inthe Aegean ca. 4500–3200 BC) and is archaeologically acceptableregarding a habitation stage of the settlement as it relates to findscorresponding to that phase. It is important to note that these areonly two of the thousands of obsidian artifacts found thus far.

The dating of these obsidian artifacts from Strofilas indicates theuse of Melian obsidian (from both sources: Adamas or sta Nychiaand Demenegaki, as analyzed by portable EDXRF analysis; Liritzis,2008) during the middle of the 4th millennium BC. Relevantcharacterization of obsidian tools has been made for the Mediter-ranean and the Aegean, indicating long distance routes of contactsmade either with continental Greece or the islands via the sea(Kilikoglou et al., 1996; Tykot, 2002; Williams-Thorpe, 1995).

The dating of the wall by TL has greater significance with regardsto an artifact, essentially a non-stratified find, such as the two smallobsidian blades. That is, the dating of more samples along the longwall – a work in progress – should provide a better clue concerningthe construction and use of the fortification. At any rate, theoccupational period, at least with the present initial work inprogress, postulates a period between 3600 and 3200 BC.

Contact between Strofilas and other Aegean settlements asevidenced through ceramic trade is not yet confirmed (chemicalanalysis of ceramics and sources are in progress), although it iscertain that the settlement was an important marine center, withan extensive view out to sea, stretching from Syros and Youra to Keaand Makronisos, which enabled them to control routes betweenthe northern Cyclades to southern Attica and Euboea. Thesepeoples had well-developed navigational skills, based on theprevalence of obsidian blades acquired from Melos, two of whichwere dated, but also from the ship engravings on the surfaces of thefortification wall and bedrock, two samples from the latter of whichwere sampled and dated.

The mid-4th millennium BC date suggested by the analysesreported here is very plausible and makes good sense on widerarchaeological grounds. Within the overlapping error bars of bothmethods used we may rule out earlier FN dates as well as very earlyEBA (taking into account this fuzzy boundary anyway) that could beseen as a real advance.

6. Conclusion

The recently excavated prehistoric settlement at Strofilas onAndros Island, one of the best preserved settlements of this age inthe Aegean, has been dated to the Final Neolithic/Early Bronze Ageusing luminescence and obsidian hydration dating, confirming theinitial archaeological estimation

The first four independent dates, two by thermoluminescence ofthe fortified wall and two by obsidian hydration (SIMS-SS) of twoblades, are contemporaneous, spanning part of the Final Neolithicperiod in the Aegean. Although archaeological finds suggest thatoccupation may have extended earlier and later (Televantou, 2001,2006), the archaeometric dating reinforces the hypothesis that thesettlement was occupied during the mid-4th millennium BC. Littleis known of the Final Neolithic cultural phase in the Aegean, and

Table 3SIMS-SS data for the two samples and respective age.

Cint (grmol/cc) Cs (grmol/cc) Xs (cm) ek Ds (cm2/yr) Age (years BP)

9.05284E�005� 6.58068E�006 8.45302E�4� 8.63427E�006 4.12828E�005� 2.51379E�006 30 5.43081E�12 5,393� 2249.052844E�005� 6.58068E�006 8.5709E�4� 8.9568E�006 3.72545� 1.14759E�006 30 5.216E�12 5,760� 196

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because the unique unearthed archaeological finds have a variableand in some cases disputable typology, the present archaeometricalresults are highly significant for the Aegean and eastern Mediter-ranean Final Neolithic period.

Acknowledgements

I thank the excavator Dr. Christina Televantou for her valuablecomments and excellent collaboration, the 21st Ephorate and theMinistry of Culture for granting permission, Dr P.Sotirakopoulou foruseful discussions, Dr Scott Fitzpatrick, Dr Jon Erlandson andChristine Armstrong for editing and valuable comments, MrN.Laskaris for SIMS-SS computation, Drs A.Vafiadou and G.Poly-meris for dosimetry and OSL measurements, and Dr Jakob Wallingafor helpful comments on TL and four more anonymous referees forconstructive comments.

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