Luminescence Dating of Rock Surface. the Case of Monoliths from the Megalithic Sanctuary of Ossimo-Pat (Valle Camonica, Italy)

applied sciences

Article Luminescence Dating of Rock Surface. The Case of Monoliths from the Megalithic Sanctuary of Ossimo-Pat (Valle Camonica, Italy)

Anna Galli 1,2,* , Laura Panzeri 2,* , Paolo Rondini 3 , Raﬀaella Poggiani Keller 4 and Marco Martini 2

1 Istituto CNR-IBFM, via F.lli Cervi, 93, 20090 Segrate, Italy 2 Dipartimento di Scienza dei Materiali, Università degli Studi di Milano-Bicocca, via R. Cozzi 55, 20125 Milano, Italy; [email protected] 3 Dipartimento di Studi Umanistici, Università degli Studi di Pavia, Corso Strada Nuova 65, 27100 Pavia, Italy; [email protected] 4 Già Soprintendente per i Beni Archeologici Della Lombardia- Ministero dei Beni e Delle Attività Culturali e del Turismo, 20100 Milano, Italy; [email protected] * Correspondence: [email protected] (A.G.); [email protected] (L.P.)

Received: 30 September 2020; Accepted: 20 October 2020; Published: 22 October 2020

Featured Application: Dates obtained by optically stimulated luminescence (OSL) of rock artefacts and the underneath soils were shown to be in very good agreement with the archeological evidence. Highly satisfying results contribute to the understanding of a megalithic sanctuary.

Abstract: Ossimo-Pat megalithic sanctuary (Valle Camonica, BS, Italy) is one of the most relevant archaeological findings of the southern alpine region, for the variety of its structures and the quality of its engraved monoliths. Its unique state of preservation gives the opportunity to apply the luminescence dating of the rock surface method. Here, we investigate the use of optically stimulated luminescence (OSL) for dating five cobbles from the site and compare cobble-surface derived ages to quartz OSL ages from sediments and to archaeological evidences. The obtained ages confirm the archaeological studies and open the way to a new hypothesis.

Keywords: optically stimulated luminescence; surface dating; megalithic sanctuary

1. Introduction There are many examples of rock surfaces, rock art and stone structures of unknown age. In particular, in prehistoric archaeology, long-lasting building traditions can sometimes be diﬃcult to date, if the context does not oﬀer useful chronological tools, such as artifacts or organic remains associated to the structures. Moreover, this particular issue is not a marginal one, if we consider that the study of megaliths, drystone walls and other structures, such as cairns or mounds, is often crucial to our understanding of the society and traditions of very ancient cultures, devoid of other forms of communication. In this scenario, the Ossimo-Pat (Valle Camonica, BS, Italy, see Figure1) megalithic sanctuary is an exemplary case.

Appl. Sci. 2020, 10, 7403; doi:10.3390/app10217403 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7403 2 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 9

FigureFigure 1. 1. LocationLocation of the Ossimo-PatOssimo-Pat site site in in Middle Middle Valle Valle Camonica Camonica (GIS (GIS elaboration elaboration on LIDAR on LIDAR DTM base). DTM base). Its construction started in the first half of the fourth millennium BCE and was structured and used during the Copper Age (late 4th–3rd millennium BCE), with ceremonial structures and engraved monolithsIts construction articulated started in ain north-south the first half alignment. of the fourth The southernmostmillennium BCE structure and iswas a sequencestructured of and usedceremonial during the mounds, Copper built Age upon (late a concentric 4th–3rd seriesmillennium of stone BCE), circles with enclosing ceremo an innernial structures rectangular and engravedspace, monoliths which usually articulated contained in a objects, north-south such as alig flintnment. weapons The and southernmost tools or ornaments. structure The is centrala sequence of ceremonialpart of the mounds, sanctuary built was upon an alignment a concentric of monoliths series of of stone various circles lithologies, enclosing mainly an inner sandstones rectangular of space,diverse which granulometry, usually contained carved withobjects, figures such and as symbols flint weapons and probably and tools depicting or ornaments. mythical ancestors The central part orof gods.the sanctuary The alignment was ledan toalignment a monumental of monoliths tomb and of then various to a series lithologies, of at least mainly five votive sandstones stone of diversecircles, granulometry, which contained, carved similarly with tofigures the southern and symbols mounds, and a number probably of flint, depicting stone, metal mythical and ceramic ancestors or gods.artefacts, The probablyalignment the led result to a of monumental unknown cult tomb activity and [1,2 then]. The to site a series was frequented of at least until five its votive sudden, stone partial, dismantling and further abandonment, at the very beginning of Bronze Age, at the end of circles, which contained, similarly to the southern mounds, a number of flint, stone, metal and the 3rd millennium BCE. Later frequentation during the final Bronze Age and the whole Iron Age ceramic artefacts, probably the result of unknown cult activity [1,2]. The site was frequented until its is attested by a series of fireplaces that were lit in front and at the backside of the monoliths still sudden,vertically partial, standing. dismantling Ossimo-Pat and further is one of abandonment, the most relevant at the archaeological very beginning findings of Bronze of the southern Age, at the end ofalpine the 3rd region, millennium for the variety BCE. ofLater its structures frequentation and the during quality the of final its engraved Bronze monoliths,Age and the as wellwhole as Iron Age foris attested its unique by statea series of preservation, of fireplaces which that were gives lit the in opportunity front and at for the a documentationbackside of the of monoliths the exact still verticallymoment standing. of its disuse. Ossimo-Pat As part is of one the of UNESCO the most World relevant Heritage archaeological Site n.94 “Rock findings Drawings of the in Vallesouthern alpineCamonica,” region, for the the sanctuary variety occupiesof its structures an area ofand more the than quality 4000 of square its engraved meters, and monoliths, it is located as well on the as for its uniqueright hydrographic state of preservation, flank of the medium which Valle gives Camonica, the opportunity at an altitude for of 810a documentation m asl. The site is arrangedof the exact momenton the of edgeits disuse. of a terrace As part overlooking of the UNESCO a steep drop Wo intorld theHeritage Valle dell’Inferno Site n.94 “Rock (the strikingly Drawings named in Valle Camonica,”“Valley ofthe Hell”), sanctuary placed o onccupies a wide an elevated area of plateaumore than comprised 4000 squa by twore meters, dominant and peaks: it is located the Pizzo on the Camino (2491 m asl) to the southwest and the Concarena (2549 m asl) to the north-east. The rocky right hydrographic flank of the medium Valle Camonica, at an altitude of 810 m asl. The site is substrate of the area is mostly made of Triassic (from the Scitic to the Carnic) limestones, while the arranged on the edge of a terrace overlooking a steep drop into the Valle dell’Inferno (the strikingly surfacing lithology is often covered by morainic deposits, left by the glaciers, which can be encountered namedup “Valley at an altitude of Hell”), of 1650 placed m asl. on (source: a wide F.elevated Franzoni, plateau Piano dicomprised Governo delby two Territorio, dominant Comune peaks: di the PizzoOssimo, Camino 2012 (2491 (http: m// www.comune.ossimo.bs.itasl) to the southwest and/Allegati the Concarena/all_50012_GEO_Relazione%20Geologica.pdf (2549 m asl) to the north-east.)). The rockyThe substrate monoliths of the chosen area tois bemostly engraved made are of veryTriassic likely (from coming the Scitic come to from the theseCarnic) deposits, limestones, but the while the surfacinggeological surveylithology and is study often is stillcovered ongoing. by Excavationsmorainic deposits, and analyses left are by being the carriedglaciers, on which since 1994 can be encounteredand are still up ongoing, at an altitude under theof direction1650 m asl. of Ra (source:ffaella Poggiani F. Franzoni, Keller Piano (Figure di2). Governo del Territorio, Comune di Ossimo, 2012 (http://www.comune.ossimo.bs.it/Allegati/all_50012_GEO_Relazione%20Geologica.pdf)). The monoliths chosen to be engraved are very likely coming come from these deposits, but the geological survey and study is still ongoing. Excavations and analyses are being carried on since 1994 and are still ongoing, under the direction of Raffaella Poggiani Keller (Figure 2). Appl. Sci. 2020, 10, 7403 3 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 9

Figure 2. Ortophotograph of the Ossimo-Pat site.

There are twotwo potentialpotential methodsmethods forfor datingdating thethe surfacessurfaces ofof suchsuch artefacts:artefacts: the measure of cosmogenic radionuclidesradionuclides concentrations concentrations [3] and[3] theand applicationthe application of luminescence of luminescence dating (in dating particular (in Opticallyparticular StimulatedOptically Stimulated Luminescence, Luminescence, OSL) [4,5]. OSL) The [4,5]. first approach The first approach depends ondepends cosmogenic on cosmogenic nuclides 10 26 36 (e.g.,nuclidesBe, (e.g.,Al, 10Be,Cl) 26 build-upAl, 36Cl) build-up with time with in minerals time in exposedminerals to exposed cosmic rays.to cosmic Therefore, rays. measuringTherefore, theirmeasuring concentrations their concentrations allows determination allows determination of how long of rocks how havelong beenrocks exposed have been at orexposed near the at surfaceor near ofthe the surface Earth. of In the this Earth. paper, In wethis focus paper, on we the focus alternative on the approach alternative based approach on OSL. based This on method OSL. This was originallymethod was developed originally to developed date the depositionto date the deposition of sediments of sediments [6] and it depends[6] and it ondepends the ability on the of ability some minerals,of some minerals, such as feldspar such as and feldspar quartz, toand absorb quartz, and storeto absorb energy and from store environmental energy from ionizing environmental radiation. Whenionizing the radiation. light stimulates When these the ligh minerals,t stimulates they release these theminerals, stored energythey release in the formthe stored of light, energy from whichin the theform term of light, luminescence from which is the derived. term luminescence Measuring the is derived. amount Measuring of energy released the amount in conjunction of energy released with a determinationin conjunction ofwith the ratea determination at which the energyof the rate was accumulatedat which the allowsenergy to was calculate accumulated an age, indicatingallows to thecalculate time thatan age, has indicating elapsed since the thetime storage that ha ofs elapsed energy began. since the storage of energy began. Thus, ifif thethe OSL OSL signal signal from from mineral mineral grains grains is normally is normally used used to date to howdate longhow thelong sediment the sediment grains weregrains buried, were buried, recent recent works works have shownhave shown that luminescence that luminescence signals signals can also can bealso used be used to determine to determine the durationthe duration of daylight of daylight exposure exposu forre rockfor rock surfaces surfaces [4,7 –[4,7–9].9]. The OSL surface dating technique exploits the depth-dependencedepth-dependence of the resetting (bleaching) of the geological luminescence signal when exposed toto daylight.daylight. Sohbati et al. [4,10] [4,10] proposed a model that reproduces thethe dependencedependence of the OSL signal on depth and exposure-time and predicts that the longer thethe exposure exposure duration, duration, the the deeper deeper the the resetting resetting of the of luminescence the luminescence signal signal into the into rock the surface. rock Analogously,surface. Analogously, this dependence this dependence has been has evaluated been evaluated for renaissance for renaissance bricks [11 bricks]. [11]. Thus, there there exists exists chronological chronological information information in the in theluminescence-depth luminescence-depth profile, profile, but despite but despite some someapplications applications of the OSL of the surface OSL surfacetechnique technique [8,9,12], [the8,9, 12bleaching-depth], the bleaching-depth model is modelyet to be is validated yet to be validatedexperimentally; experimentally; this is critical this isfor critical further for develo furtherpment development of the technique. of the technique. The emerging The emerging OSL surface OSL surfacedating technique dating technique has the has potential the potential to answer to answer many many new new questions questions in inthe the fields fields of of geo- geo- and archaeological sciences. Dating the sanctuary was mostly done through the combination of stratigraphy observations, typo-chronological study of the artefacts and withwith the use of aa certaincertain numbernumber ofof radiometricradiometric measurements (14C) on charred coals from didifferentfferent parts of the site [[1],1], mostly from thethe southernsouthern mounds andand the the upper upper layers layers of theof the monoliths monoliths alignment. alignment. Therefore, Therefore, to test to the test capability the capability of the surface of the datingsurface technique, dating technique, five pebbles five were pebbles collected were from collected three di fromfferent thre areas.e different Three samples areas. consistedThree samples of the pebbleconsisted itself of andthe pebble the sediment itself and collected the sediment directly underneathcollected directly the pebble. underneath Dating the was pebble. attempted Dating on bothwas theattempted underside on ofboth the pebblethe underside and on theof sedimentthe pebble under and theon pebble,the sediment on the assumptionunder the thatpebble, neither on hasthe beenassumption exposed that to lightneither since has the been placement exposed of to the light rock. since In this the way, placement we checked of the the rock. predictability In this way, of thewe reconstructedchecked the predictability luminescence of profiles the reconstructed into the buried lumi rocknescence surface. profiles into the buried rock surface. The measured ages obtained from buried surfaces are in good agreement with those obtained from the sediments and with those obtained through archaeological methodology. Moreover, in some cases the ages obtained with luminescence dating are consistent with available 14C ages. Appl. Sci. 2020, 10, 7403 4 of 9

The measured ages obtained from buried surfaces are in good agreement with those obtained from the sediments and with those obtained through archaeological methodology. Moreover, in some casesAppl. Sci. the 2020 ages, 10 obtained, x FOR PEER with REVIEW luminescence dating are consistent with available 14C ages. 4 of 9

2.2. Materials and Methods AllAll experimentsexperiments in in this this study study were were performed performed using using rock rock samples samples and and sediments sediments collected collected from from the Ossimo-Patthe Ossimo-Pat sanctuary sanctuary (Figure (Figure3). The Sample3). The “3Sample/2016” is“3/2016” a pebble is taken a pebble from thetaken stone from accumulation the stone behindaccumulation the big behind monolith the known big monolith as “Pat 2,”known presumably as “Pat relative2,” presumably to the ﬁrst relative stage ofto thethe abandonmentfirst stage of the of theabandonment site. “1/2016” of andthe site. “US “1/2016” 178” are samplesand “US from 178” the are northern samplesvotive from the stone northern circles, votive which arestone generally circles, placedwhich inare the generally central phaseplaced of in the the site-life, central around phase the of ﬁrstthe halfsite-life, of the around 3rd millennium the first half BCE, of while the the3rd samplesmillennium “Pat12” BCE, and while “Pat15”were the samples collected “Pat12” from and the “Pat15”were housing pits collected of the fallen from monolithsthe housing “Pat12” pits of and the “Pat15.”fallen monoliths All the cobbles “Pat12” have anda “Pat15.” diameter All in the the cobbles range of have 100–150 a diameter mm. in the range of 100–150 mm.

FigureFigure 3.3. (a) SiteSite mapmap withwith samplingsampling points;points; ((b)) detaildetail ofof samplesample US178;US178; ((cc)) duringduring thethe samplingsampling operationoperation underunder thethe sunlightsunlight andand underunder thethe darkdark ((dd).).

ToTo measure measure the the OSL OSL depth depth profiles profiles and and the the absorbed absorb doses,ed doses, the the cobbles cobbles have have been been drilled drilled under under dim reddim light red inlight laboratory. in laboratory. The reached The reached depth was depth checked was with checked a digital with precision a digital caliber. precision The polymineralcaliber. The finepolymineral grain fraction fine grain with fraction grain sized with between grain sized 4 and between 10 micrometers 4 and 10 wasmicrometers used. was used. ForFor thethe sediments,sediments, thethe quartz quartz grains grains have have been been extracted extracted using using the the conventional conventional procedure procedure [13 –[13–16] under16] under dim dim red red light light and and the the luminescence luminescence measurements measurements were were conducted conducted on on coarse coarse quartz quartz grainsgrains (180–250(180–250 µμm).m). AllAll samples,samples, bothboth cobblescobbles andand sediments,sediments, werewere mountedmounted onon stainlessstainless steelsteel discs.discs. TheThe OSLOSL measurementsmeasurements were were performed performed with with a Risøa Risø TL /TL/OSLOSL DA20 DA20 reader. reader. The The samples samples were were stimulated stimulated by blue by LEDSblue LEDS (470 (47030 ± nm, 30 nm, 54 mW 54 mW/cm/cm2) or2) byor by IR IR LEDs LEDs (λ (=λ 830nm,= 830nm, 360 360 mW mW/cm/cm2)2) and and the the emitted emitted photonsphotons ± werewere detecteddetected withwith an an EMI EMI 9235QB 9235QB photomultiplier photomultiplier coupled coupled with with a UVa UV filter filter (7.5 (7.5 mm mm Hoya Hoya U-340) U-340) in casein case of blueof blue stimulation stimulation or with or with a blue a blue filter filter (Schott (Schott BG39 BG39/Corning/Corning 7–59 7–59 filter filter combination) combination) in case in of case IR stimulation.of IR stimulation. Blue stimulation Blue stimulation was used was for used the fo samplesr the samples in which in thewhich main the dosimeter main dosimeter was the was quartz the andquartz IR stimulationand IR stimulation [13] for [13] the for samples the samples rich in rich feldspars. in feldspars. To evaluate the absorbed dose the Single Aliquot Regenerative dose protocol (SAR) was applied for all samples. It is a well-known standard procedure for the optically stimulated luminescence dating in which the same aliquot is irradiated, heated and illuminated, during experimental cycles that are repeated (Table 1). The obtained signal is integrated, normalized and plotted in function of dose and the absorbed dose is calculated from interpolation of the natural signal on the constructed curve. Appl. Sci. 2020, 10, 7403 5 of 9

To evaluate the absorbed dose the Single Aliquot Regenerative dose protocol (SAR) was applied for all samples. It is a well-known standard procedure for the optically stimulated luminescence dating in which the same aliquot is irradiated, heated and illuminated, during experimental cycles that are repeated (Table1). The obtained signal is integrated, normalized and plotted in function of dose and the absorbed dose is calculated from interpolation of the natural signal on the constructed curve.

Table 1. Single Aliquot Regenerative dose protocol applied (Li and Ti were calculated starting from the optically stimulated luminescence (OSL) curve, integrating the ﬁrst 0.8 s minus a background estimated from the integral of the last 8 s).

Step Rock Samples SAR Cycle Sediments SAR Cycle a a 1 Give dose, Di Give dose, Di b 2 Preheat at T0 for 10 s Preheat at T0 for 10 s 3 Stimulated for 100 s at 125 ◦C (OSL) or 50 ◦C (IRSL), get Li Stimulated for 40 s at 125 ◦C (OSL) 4 Give test dose, Dt Give test dose, Dt 5 Cut heat to 220 ◦C for 0 s Cut heat to 180 ◦C for 0 s 6 Stimulated for 100 s at 125 ◦C (OSL) or at 50 ◦C (IRSL) Stimulated for 40 s at 125 ◦C (OSL) 7 OSL or IRSL at 290 ◦C for 40 s OSL at 290 ◦C for 40 s 8 Return to 1 Return to 1 a For the natural sample i = 0 and D0 = 0. Four regenerative doses plus a zero-dose measurement and a repeat dose b were used; T0 depends on the sample.

The pre-heat value was experimentally derived on the basis of the results of a dose recovery pre-heat plateau test [17] (see Table2).

Table 2. Pre-heat temperature for rock and sediment samples.

Rock Samples Pre-Heat Sediment Samples Pre-Heat Sample Code Temperature (◦C) Temperature (◦C) 1/2016 260 US178 220 220 3/2016 200 Pat12 240 260 Pat15 200 220

Laboratory irradiations were given using a calibrated 90Sr/90Y beta source (5.50 GBq) mounted on the Risø TL/OSL reader (dose rate: 0.23 Gy/s for fine grain; 0.12 Gy/s for coarse grain). The absorbed dose is calculated from the weighted mean of values obtained from the analyzed aliquots (at least 10 for sample). The annual dose-rate absorbed by the samples is due to the contribution of the internal radioactivity of the material and the radioactivity of the surrounding environment. The radioactive families 238 U, 232 Th and 40 K are, in both cases, responsible for the natural imparted dose. There are also minor contributions from 87Rb and from cosmic rays. To take into account any non-homogeneity in the external contribution, the radioactivity of a sphere of the surrounding material (30 cm diameter centered at the sampling point) has to be measured [9]. The heterogeneity effect was considered by measuring the radioactivity concentrations of the surroundings materials, i.e., sediments and cobbles. The contribution of each material to the annual dose-rate was evaluated applying the infinite matrix approximation, with updated conversion factors [18]. After radioactivity measurements, it is possible to assess the contribution of each layer calculating the relative volume of every material within the interaction sphere and normalizing the annual dose fractions in the age equation [19]. 40 K concentrations were measured through atomic absorption spectrometry; 238 U and 232 Th concentrations were obtained from total alpha counting technique. Although the water content of the cobbles is negligible, the surrounding sediment material does contain water, and thus, a water content correction is needed. This was based on the one calculated using the sediment material, specifically a mean water content equal to 75% of saturation value (20 10%) was used. Attenuation of the beta ± Appl. Sci. 2020, 10, 7403 6 of 9 dose was taken into account in case of sediments for which the coarse grains fraction was used to

Appl.determine Sci. 2020 the, 10, absorbedx FOR PEER doses REVIEW [20 ], while alpha contribution was eliminated by an HF etching. 6 of 9 3. Results 3. Results 3.1. Luminescence—Depth Profiles 3.1. Luminescence—Depth Profiles To test the application of luminescence rock surface dating to the surface of buried cobbles, To test the application of luminescence rock surface dating to the surface of buried cobbles, we first we first assessed the degree of bleaching of the surface itself. To investigate this, we measured the assessed the degree of bleaching of the surface itself. To investigate this, we measured the absorbed absorbed dose as a function of depth into all the five cobbles. As an example, the Figure4 shows the dose as a function of depth into all the five cobbles. As an example, the Figure 4 shows the luminescenceluminescence-depth profiles from two different samples. Each point in the profile is obtained by the depth profiles from two different samples. Each point in the profile is obtained by the mean of the mean of the absorbed doses measured on 10 aliquots. absorbed doses measured on 10 aliquots.

Figure 4. LuminescenceLuminescence depth-profiles depth-proﬁles ( a)) 1/2016 1/2016 sample; ( b) 3/2016 3/2016 sample.

In the left panel, the absorbed dose increases wi withth the depth and reaches a plateau (the so-called fieldfield saturation level) in the middle of the cobble. In the right panel, this plateau is reached in the first first layers below the surface. It It seems seems to to have have limited limited light light exposure, exposure, insufficient insufficient to fully reset the OSL signal in the layers layers below below 1mm 1mm from from the the surface. surface. Presumably, Presumably, light light exposure exposure has has not not been been sufficiently sufficiently long toto totally totally bleach bleach the the luminescent luminescent signal signal throughout throughout the cobble the orcobble this sample or this has sample a high has attenuation a high attenuationcoefficient. Forcoefficient. both samples, For both the sa absorbedmples, the dose absorbed at the surface dose at is the proportional surface is toproportional the time elapsed to the sincetime elapsedthe deposition. since the The deposition. calculated The ages calc areulated reported ages in are Table reported3. in Table 3.

Table 3. Cobbles and sediments calculated ages.

Sample Code Sample Type Technique AbsorbedAbsorbed Dose (Gy) DoseDose Rate Rate (mGy/a) Date Sample Code Sample Type Technique Date 1/2016 ROCK OSL 24.5Dose ± 3.5 (Gy) (mGy 5.15 ±/a) 0.15 2760 BCE ± 500 US1781/2016 ROCK ROCK OSL OSL 20.8 24.5 ± 3.53.5 5.15 2.95 0.15± 0.20 2760 2675 BCE BCE ±500 500 ± ± ± US178US178 SEDIMENT ROCK OSL OSL 15.4 20.8 ± 0.53.5 2.95 2.95 0.20± 0.07 2675 3220 BCE BCE ±500 330 ± ± ± 3/2016US178 ROCK SEDIMENT IRSL OSL 42.3 15.4 ± 7.50.5 2.95 10.750.07 ± 0.30 3220 1920 BCE BCE ±330 250 ± ± ± Pat123/2016 ROCK ROCK IRSL IRSL 30.5 42.3 ± 5.07.5 10.75 4.20 ±0.30 0.12 1920 5240 BCE BCE ±250 600 ± ± ± Pat12 ROCK IRSL 30.5 5.0 4.20 0.12 5240 BCE 600 Pat12 SEDIMENT OSL 17.2 ± 0.4± 2.40± ± 0.08 5090 BCE± ± 360 Pat12 SEDIMENT OSL 17.2 0.4 2.40 0.08 5090 BCE 360 Pat15 ROCK IRSL 22.9 ± 4.0± 6.50± ± 0.25 2470 BCE± ± 500 * Pat15 ROCK IRSL 22.9 4.0 6.50 0.25 2470 BCE 500 * Pat15 SEDIMENT OSL 12.8 ± 0.4± 3.00± ± 0.05 2270 BCE± ± 260 Pat15 SEDIMENT OSL 12.8 0.4 3.00 0.05 2270 BCE 260 * Fading-corrected± date. ± ± * Fading-corrected date. 3.2. Fading Correction and Cobbles Ages 3.2. FadingBy applying Correction the andwell-known Cobbles Ages equation age [6], the cobble burial ages were simply derived by dividing the cobbles surface absorbed dose values by the dose rate at the surface of the cobbles. By applying the well-known equation age [6], the cobble burial ages were simply derived by However, the luminescence signal from feldspars could be unstable: this phenomenon is known as dividing the cobbles surface absorbed dose values by the dose rate at the surface of the cobbles. ‘anomalous fading’ and can be taken into account by the conventional g value measurement [21]. For However, the luminescence signal from feldspars could be unstable: this phenomenon is known as the samples Pat12 and Pat15, we have quantified the fading rate as a percentage of IR signal loss with ‘anomalous fading’ and can be taken into account by the conventional g value measurement [21]. storage time after irradiation. Three aliquots from each sample were bleached in the reader using an For the samples Pat12 and Pat15, we have quantiﬁed the fading rate as a percentage of IR signal loss IR stimulation (for 500 s at 50 °C repeated after 60 s); a known dose has been imparted, and then the absorbed dose were measured, both immediately after irradiation and after a delay time of 6 h, 1 day, 19 days and 33 days. For the sample Pat12, the ratio between the imparted dose and the dose measured after the time delay are indistinguishable from unity, (Figure 5), thus, it possible to deduce that the phenomenon of anomalous fading is negligible. The cobble Pat15 is in a different condition: Appl. Sci. 2020, 10, 7403 7 of 9 with storage time after irradiation. Three aliquots from each sample were bleached in the reader using an IR stimulation (for 500 s at 50 ◦C repeated after 60 s); a known dose has been imparted, and then the absorbed dose were measured, both immediately after irradiation and after a delay time of 6 h, 1 day, 19 days and 33 days. For the sample Pat12, the ratio between the imparted dose and the dose measured after the time delay are indistinguishable from unity, (Figure5), thus, it possible to deduce that the phenomenonAppl.Appl. Sci. Sci. 2020 2020 of, ,10 anomalous10, ,x x FOR FOR PEER PEER fadingREVIEW REVIEW is negligible. The cobble Pat15 is in a diﬀerent condition:7 its7 of of signal99 fades by 10% in about a month. Therefore, we evaluate the fading rate with a g value of 2.4 0.5%, itsits signalsignal fadesfades byby 10%10% inin aboutabout aa month.month. TherTherefore,efore, wewe evaluateevaluate thethe fadingfading raterate withwith aa gg valuevalue ofof 2.42.4± and in±± 0.5%, Table0.5%, and3and, the inin Table fading-correctedTable 3,3, thethe fading-correctedfading-corrected date is reported. datedate isis reported.reported.

1.41.4

1.31.3 PAT12PAT12 1.21.2 PAT15PAT15

1.11.1

11 Normalized absorbed dose absorbed Normalized Normalized absorbed dose absorbed Normalized 0.90.9

0.80.8 00 200 200 400 400 600 600 800 800 1000 1000 tt (h) (h)

FigureFigureFigure 5. 5. 5.Normalized Normalized Normalized absorbedabsorbed dose dose dose versus versus versus time. time. time.

3.3. Sediments’3.3.3.3. Sediments’Sediments’ Ages AgesAges The absorbedTheThe absorbedabsorbed dose dosedose histogram, histogram,histogram, relative relativerelative to toto moremoremore th thanthanan 5050 50 aliquotsaliquots aliquots forfor for eacheach each sample,sample, sample, asas reportedreported as reported inin in FigureFigureFigure6 for 66 sample forfor samplesample Pat15, Pat15,Pat15, shows showsshows a symmetricaa symmetricsymmetric distribution distridistributionbution with with ratherrather rather lowlow low dispersiondispersion dispersion ofof thethe of data, thedata, data, evidencing a similar dose absorbed by all the aliquots. This is an indication that the surface sediments evidencingevidencing a similar a similar dose dose absorbed absorbed by by all all the the aliquots.aliquots. This This is is an an indication indication that that the thesurface surface sediments sediments werewere fully fully bleached bleached before before the the cobbl cobbles’es’ deposition. deposition. This This allows allows to to use use the the weighted weighted mean mean to to calculate calculate were fully bleached before the cobbles’ deposition. This allows to use the weighted mean to calculate thethe absorbedabsorbed dosesdoses byby thethe samplessamples thatthat areare reportedreported inin TableTable 3,3, togethertogether withwith thethe annualannual dosedose ratesrates the absorbedandand thethe OSLOSL doses dates.dates. by the samples that are reported in Table3, together with the annual dose rates and the OSL dates.

2222 PAT PAT 15 15 2020

1818

1616

1414

1212

1010 Frequency Frequency 88

00 66 8 8 10 10 12 12 14 14 16 16 18 18 20 20 22 22 24 24 DoseDose (Gy)(Gy) Figure 6. Sample “Pat15.” Distribution of absorbed doses. FigureFigure 6. 6. Sample Sample “Pat15.” “Pat15.” Distribution Distribution of of absorbed absorbed doses. doses.

4.4. DiscussionDiscussion andand ConclusionsConclusions TheThe dosesdoses absorbedabsorbed byby thethe cobblescobbles werewere directlydirectly derivedderived byby thethe aliquotsaliquots takentaken fromfrom thethe firstfirst layer.layer. AlthoughAlthough thethe uncertaintyuncertainty inin thethe dosedose isis dederivedrived fromfrom thethe standardstandard errorerror ofof 1010 aliquots,aliquots, thethe Appl. Sci. 2020, 10, 7403 8 of 9

4. Discussion and Conclusions The doses absorbed by the cobbles were directly derived by the aliquots taken from the first layer. Although the uncertainty in the dose is derived from the standard error of 10 aliquots, the relative error spans between 15% and 30%, it is quite high, especially if compared with that obtained with sediments (5–6%). Due the scarcity of material for each considered layer, it has been impossible to extract quartz or feldspar grains from the samples. This may have led to luminescence signals with inadequate characteristics (slow decay rate, high background) that introduced a high error in the fitting to calculate the absorbed dose. For US178, Pat12 and Pat15, the age from cobble is in agreement with the sediment age at a level of 1 standard deviation, thus, they are consistent with each other. From a methodological point of view, these preliminary results show that it is possible to apply luminescence dating to the surfaces of the artefacts present in the Ossimo-Pat site. Despite the high error obtained on rock surfaces, the data obtained on sediments under the rock were more precise. This suggests that for future application of surface dating of megalithic structure, it is recommended to sample both rocks and the underlying sediments in order to reduce the age errors. Moreover, the results of the dating offer, from an archaeological point of view, an interesting feedback to the existing timeframe of the site. Even considering the range error of each date, we can compare the results with what was already established about the site, chronology-wise. The first sample (the rock “1/2016,” 2760 BCE 500) was taken from the northernmost of the votive stone circles. ± Its date is compatible with the other two samples from the nearby, similar, stone structures (samples “US 178,” rock, 2675 BCE 500, and sediments, 3200 BCE 330). There are no 14C dates available ± ± from these circles that have been dated to the first half of the 3rd millennium BCE based on the objects found inside the circles, namely a specific type of arrowheads, usually related this period. The three OSL dates from the area would seem to confirm the current chronological framework. The IRSL date from the rock sample “3/2016” (1920 BCE 250) seems equally aligned with the ± current chronology of the site. This rock was collected from the accumulation layer located behind the big monolith called “Pat 2,” one of the few still vertically standing. The small accumulation of soil and rocks is probably related to the first stage of abandonment of the site, right after its partial destruction and dismissal, and is usually dated around the 23rd–20th centuries BCE thanks to a charred coal 14C dating [1]. The samples taken from the monolith pits are the most intriguing ones, as we still do not have a full comprehension of the precise life cycles of these monuments. The samples taken from the housing pit of the monolith “Pat15” are related to its filling, which happened in time, after the dismissal of the site and the overthrowing of the monolith, which was pushed to the ground, out of its proper housing. The dating of rock and relative sediment (2470 BCE 500 and 2265 BCE 115, respectively) ± ± are perfectly coherent with the said action, which probably happened, as said, between the end of the 24th and the 21st centuries BCE. The dating of the samples from the housing pit “Pat12” (Rock: 5240 BCE 600; sediment: 5090 BCE ± 360), on the other hand, offer a very interesting result, related back to the end of the sixth millennium ± BC, before the foundation of the sanctuary. Nevertheless, these dates are perfectly coherent with another dating, obtained by 14C on a charcoal sample from the layer 181 [1], which was documented in this very sector of the site thanks to a deep stratigraphic trench. Layer 181, rich in charred coal and burnt wood, is believed to be the outcome of the very first action carried out in this place, namely a slash/burn activity, usually performed during the late Neolithic to open new pasture areas in medium and high-altitude locations. Evidently, the digging activity performed during the Copper Age to prepare the housing pit for the huge monolith “Pat12,” which is longer than 2 m and very heavy, led to reach the deeper and more ancient level of the site, to which our samples appear to belong.

Author Contributions: Conceptualization, A.G. and L.P.; methodology, A.G., L.P. and P.R.; writing—original draft preparation, A.G. writing—review and editing, A.G., L.P., P.R., R.P.K. and M.M.; supervision, R.P.K. and M.M. All authors have read and agreed to the published version of the manuscript. Appl. Sci. 2020, 10, 7403 9 of 9

Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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