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Obsidian in archaeological sites on Mocha Island, southern Chile:Implications of its provenience

Roberto Campbella,⁎, Charles R. Sternb, Ángela Peñalozac

a Programa de Antropología, Instituto de Sociología, Facultad de Ciencias Sociales, Pontificia Universidad Católica de Chile, Santiago, Chileb Department of Geological Sciences, University of Colorado, Boulder, CO 80309-0399, USAc El Dibujo s/n, Culiprán, Melipilla, Chile

A R T I C L E I N F O

Keywords:ObsidianMocha IslandSouthern ChileSocial complexity

A B S T R A C T

Results are presented of both a technological analysis and an identification of the sources of origin, by chemical(XRF and ICP-MS) fingerprinting, for 35 samples of obsidian recovered from three archaeological sites on MochaIsland attributed to El Vergel Complex Late Ceramic prehistoric period and early historic indigenous groups insouthern Chile (1000–1700 CE; 950–250 cal BP). These results permit us to identify the presence of artifacts anddebitage manufactured from obsidian from two different mainland sources corresponding to Nevados deSollipulli near Melipeuco (obsidian type MEL), Chile, and Portada Covunco in Neuquén (obsidian type PC),Argentina. Both these types of obsidian are non-local to Mocha Island, which is consistent with their lowpresence in the sites. Their local and regional social context suggests that this lithic material formed part of thephenomena of social differentiation that characterized the late prehistory of southern Chile.

1. Introduction

A variety of visually and chemically distinct obsidian types havebeen described from archaeological sites in southern Chile (36–42°S;Fig. 1), and the majority of their sources have been located (Stern et al.,2002, 2008, 2009, 2012). However, to date the record is scarce inregards to the presence of obsidian in sites along coastal areas, as wellas for late prehistoric and historic periods after 1000 CE (950 cal BP;Table 1). In this article, based on a study of 35 obsidian pieces fromMocha Island, we present data to help make up this deficit in the recordand propose a hypothesis regarding the social context for the presenceof this material.

2. Background

2.1. Obsidians in Southern Chile

“Southern Chile” is the territory that extends from the Itata riverbasin (36.5°S) to the northern part of Chiloé Island (43°S; Fig. 1). Inlatitudinal terms, this territory can be subdivided at approximately39.5°S into a “northern sector” and a “southern sector” (Aldunate,1989). In longitudinal terms, southern Chile is divided into coastal,central valley and cordilleran (mountain) zones (Campbell and Quiroz,2015). As such, Mocha Island is located in the coastal zone of southern

Chile's northern sector.Various studies have identified visually and chemically distinct

types of obsidian from archaeological sites of southern Chile andnorthern Argentine Patagonia. In most cases the respective sources oforigin for these materials have also been identified (Fig. 1). The mostimportant obsidian sources, from north to south, are as follows: Lagunadel Maule (LM: Seelenfreund et al., 1996; Giesso et al., 2011), CerroHuenul (CoH; Barberena et al., 2011), Portada Covunco (PC; Salazarand Stern, 2013; Stern et al., 2012), Nevados de Sollipulli (MEL; Sternet al., 2008, 2009), Cerro de las Planicies/Lago Lolog (CP/LL; Lópezet al., 2009), Quilahuinto/Arroyo Pocahullo (QU/AP; López et al.,2009), Yuco (YC; López et al., 2009), and Chaitén (CH; Stern et al.,2002). There are also a few obsidian artifacts from sites in southernChile that have not been traced to any known sources, but the numberof these samples is small relative to the number of artifacts from theknown sources (6 among 87; Table 1).

A review of the literature on the analytically confirmed origins ofobsidians found at archaeological sites in southern Chile shows that themajority of the data come from sites in the cordillera zone of thenorthern sector of the region, while there is no useful data of any kindfor the northern coast (Table 1). Exactly the opposite occurs in thesouthern sector, where the only data available come from two sites inthe coastal zone (Chan Chan 18 and Puente Quilo). Lastly, the analyzedsites correspond almost exclusively to the Archaic period

http://dx.doi.org/10.1016/j.jasrep.2017.05.005Received 24 October 2016; Received in revised form 1 May 2017; Accepted 1 May 2017

⁎ Corresponding author.E-mail address: roberto.campbell@uc.cl (R. Campbell).

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(10,500–1550 cal BP), while very few obsidians associated with theCeramic period (400–1550 CE; 1550–400 cal BP) or contexts associatedwith this period are available.

This said, the northern coast of southern Chile – that is, to the north

of Chan Chan 18 (Fig. 1) – shows a complete absence of obsidians inarchaeological sites during the Archaic (10,500–1550 cal BP) and EarlyCeramic periods (400–1000 CE; 1550–950 cal BP) (Massone et al.,2011). On the other hand, while obsidians are present in the northern

Fig. 1. Archaeological sites (circles) and obsidian sources (squares) referenced in the text, including on Mocha Island [B. Sites P5-1, P29-1, P31-1]. A: Sites in Purén-Lumaco [Pu-165,Trentrenkuel]. B: Los Riscos-1 and Fundo Tres Arroyos-1. C: Sites in Melipeuco [Alero Cabeza de Indio 1, Rumiñañe 1, Mallín Alto, Entre Mallines, Mallín Bajo, Laguna Fuentes, CasaAbandonada, Mirador 1, Bajo Mirador, Molulco 2]). Obsidian sources, including LM (Laguna del Maule), CoH (Cerro Huenul) PC (Portada Covunco), MEL (Nevadas del Sollipulli), CP/LL(Cerro de las Planicies/Lago Lolog); QU/AP (Quilahuinto/Arroyo Pocahullo, YC (Yuco), and CH (Chaiten).

Table 1Summary of the sources of obsidian artifacts from archaeological sites in southern Chile.

Sector Zone Site Chronologya Obsidian typeb Referencesc

MEL PC CH Unknown Not Analyzed TOTAL

Northern Coast Mocha Islandd Late Ceramic, Historic 27 8 35 1La Candelaria Late Ceramic 2 2 2Coronel-2 Late Ceramic X X 7

Central Valley Granaderos-2 ? 1 1 11Pu-165 Ceramic, Historic 1 1 3Trentrenkuel Historic 2 2 3Quillen 1 Archaic 1 1 2 11Villa JMC-1 Early Ceramic 8 8 4, 5

Cordillera Cabeza de Indio 1 Archaic, Ceramic 7 8 15 11Los Riscos-1 Ceramic 4 4 11Fundo Tres Arroyos-1 Ceramic 3 3 11Melipeuco areae Ceramic X X 6, 10Flor del Lago 1 Archaic 3 1 4 11Casa Fuerte Santa Sylvia Ceramic, Historic 11 4 1 16 8

Southern Coast Chan Chan 18 Archaic 2 4 1 7 9, 11Puente Quilo Archaic 3 1 2 6 9, 11

61 21 5 6 13 106

a Chronology. Archaic 10,500–1550 cal BP; Ceramic 1550–400 cal BP, 400–1550 CE; Early Ceramic 1550–950 cal BP, 400–1000 CE; Late Ceramic 950–400 cal BP, 1000–1550 CE;Historic 400 cal BP-today, 1550 CE–today.

b Obsidian type. MEL: Nevados de Sollipulli; PC: Portada Covunco; CH: Chaitén.c References. 1. This paper; 2. Bahamondes et al. (2006); 3. Dillehay (2014); 4. Mera (2014); 5. Mera et al. (2015); 6. Navarro (2012); 7. Quiroz et al. (2005); 8. Sauer (2012); 9. Stern

et al. (2002); 10. Stern et al. (2008); 11. Stern et al. (2009).d Mocha Island. Sites P29-1, P31-1 and P5-1.e Melipeuco area. Sites Rumiñañe 1, Mallin Alto, Entre Mallines, Mallin Bajo, Laguna Fuentes, Casa Abandonada, Mirador 1, Bajo Mirador, and Molulco.

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central valley and cordillera zones at sites dated or assigned to theArchaic period (Navarro, 2012; Stern et al., 2009), they seem todisappear from the central valley zone at the time of the Early Ceramicperiod.

In fact, for the Early Ceramic period, the presence of obsidian has sofar been recorded only at the funerary Pitrén site Villa JMC-1 (northerncentral valley; Mera, 2014; Mera et al., 2015). These samples come inthe form of four pieces of production byproduct/debitage of blackobsidian; a stemmed projectile point made from translucent blackobsidian; and a scraper, knife and piece of byproduct/debitage, allfrom black and red “tiger-striped” obsidian. It should be noted that thissite has produced other pieces of evidence that are so far unique for theEarly Ceramic Pitrén Complex, such as the presence of metallic artifactsand alpaca fibers within a site that dates to 1100 CE. To this site, weshould add, also from the northern central valley, the obsidiansrecovered at the domestic site Pu-165 (Purén-Lumaco) – including aprojectile point – in a context that can be situated chronologically aspost-400 CE (Dillehay, 2014: 192–203), although the same site hasother dates that extend its occupation to at least 1550 CE.

By the time of the Late Ceramic period (1000–1550 CE; 950–400 calBP), and including historic times (1550 CE onwards), we can identifythe presence of obsidians at sites on the northern sector coast (Coronel-2 and La Candelaria), central valley (Trentrenkuel), and cordillera(Casa Fuerte Santa Sylvia). At the site Coronel-2, black and “brown-veined” obsidians were recorded. Although the authors do not identifythe specific artifact categories of the obsidians, they do mention thediscovery of lithic flakes and debitage (Quiroz et al., 2005). Thesesamples came from a test pit of a funerary context that was previouslydisturbed, resulting in a date of 1150 CE. At La Candelaria site,researchers recorded the presence of small non-stemmed projectilepoints with notched bases, one of grey obsidian and another in anobsidian described as “light brown with parallel black and dark-browninclusions, which give it a striped appearance” (Bahamondes et al.,2006). This site is a disturbed El Vergel burial context, dated to 600 CEand 1450 CE. In the case of Trentrenkuel, investigations produced “asmall obsidian flake with red bands on a black background and a blackobsidian projectile point” (Dillehay, 2014: 159–170). This context is amound of anthropic origin, and the obsidians come from a stratum thatis datable to post-1680 CE. Lastly, for Casa Fuerte Santa Sylvia, sixteenobsidians were geochemically analyzed (Sauer, 2012); these weredescribed as black, grey, red and black/red. The first two were sourcedto Nevados de Sollipulli, the last two to Portada Covunco, while anotherone – no color indicated – comes from an unknown source. This site hasdates from 1000 CE to 1850.

Unfortunately, for La Candelaria, Coronel-2, Pu-165, Trentrenkueland Villa JMC-1, there are not obsidians geochemical determinationsyet. As a result, the information we supply for Mocha Island providesnew data in terms of both territory (the northern coast) and period(Late Ceramic and Early Historic), for which we currently have almostno previous information on the distribution of obsidian. These datamake an interesting contribution to the discussion of social dynamicsduring the El Vergel Complex of the late Ceramic period and the earlyhistoric indigenous groups of southern Chile.

2.2. Mocha Island and its settlement history

Mocha Island is 52 km2 and is located at 38.35°S, around 30 km offthe coast of Chile (Fig. 1). Geologically, it dates from the PleistoceneEpoch and is formed today by a central mountain chain (between 50and 400 m asl) surrounded by Holocene marine terraces that arebetween 200 and 2000 m wide (Fig. 1B; Kaizuka et al., 1973; Nelsonand Manley, 1992; Prieto, 1997). Its status as an island facilitates thedistinction between local and non-local resources. In terms of lithicresources, both obsidian and cryptocrystalline siliceous materials aretotally non-local to the island.

The history of human occupation on Mocha Island may be organized

into three different periods (Campbell, 2015; Goicovich and Quiroz,2008; Quiroz and Sánchez, 1997, 2005; Sánchez et al., 2004). The firstperiod involves groups of Archaic maritime hunter-gatherers, dated toaround 3400 cal BP, who left a limited material record. Following thisperiod, Mocha Island appears to have been unoccupied for the next1500 to 2000 years. The second period of occupation began at~1850 cal BP (100 CE) and continued until 1687 CE, aligned withcultural developments on the adjacent mainland. This second periodencompasses an initial ceramic occupation that is not fully defined,then the Pitrén Complex of the Early Ceramic period (400–1000 CE;1550–950 cal BP), followed by El Vergel Complex of the Late Ceramicperiod (1000–1550 CE; 950–400 cal BP), and finally the ethnohistoricReche-Mapuche indigenous groups (1550–1687 CE). This secondoccupation period ended with the forced resettlement of the island'sinhabitants onto the mainland, after which Mocha Island remainedvacant for nearly 160 years. The final occupation period is the presentone, which began in the 1840s with the progressive arrival of Chileantenants and small landholders (“parceleros”). Since then, the inhabi-tants of the island have developed an economy based on small-scaleagriculture and herding livestock.

The present article concentrates specifically on the time period thatbegins with the El Vergel Complex in 1000 CE (950 cal BP) and extendsto the island's depopulation in 1687 CE. The occupations during thisperiod have produced the most ubiquitous, abundant and representa-tive archaeological record of Mocha Island. In fact, the El VergelComplex marks a new level of social complexity for the entire southernChile, represented by a series of novel elements, such as the construc-tion of public architecture (mounds), the development of a metalwork-ing tradition, a clear utilization of cultivated resources, diversifiedfunerary patterns, and the presence of denser and larger archaeologicalsites (Aldunate, 1989, 2005; Campbell, 2004, 2014; Campbell andPfeiffer, 2017; Dillehay, 2007, 2014; Roa et al., 2015; Silva, 2014).

2.3. Mocha Island's archaeological sites and lithic resources

For Mocha Island, during El Vergel and Early Historic times we haveproposed the existence of eight communities. They are represented bythe presence of domestic areas ranging between 4 and 15 ha in size.They do not provide evidence of a social hierarchy; rather they seem tobe autonomous in terms of subsistence. Still the construction of aplatform and two mounds above it (dated to 1000 CE, Campbell andPfeiffer, 2017) as well as the slight differential occurrence in these sitesof certain goods such as metallic adornments, beads, decorated pottery,and obsidian, points to some level of wealth differentiation (Campbell,2011, 2015; Campbell et al., 2017).

The materials used in the following analysis come from three ofthese communities: sites P5-1, P29-1 and P31-1, located on the marineterraces along the northeastern part of Mocha Island (Fig. 1B). Theywere studied using surface collection, the excavation of test pits, andflotation columns. The dates obtained for the three sites (Table 2)indicate they were occupied between 900 CE and 1650 (cal BP 1050and 350).

Donald Jackson (1997) was the first to described systematically theMocha lithic industry of El Vergel Complex, based mainly on the recordof P31-1 and on replication experiments of bipolar flaking that hecarried out with the basalt cobblestones dispersed on the island'sbeaches. According to Jackson, and to our studies, the locally sourcesraw materials consist mainly of basalt, sandstone, quartz and othersunidentified rocks, while obsidian and cryptocrystalline siliceous rocks(such as chalcedony) are non local to the island. These last are found invery low frequencies in archaeological sites, without cortex and often asbifacial tools. Isla Mocha's El Vergel lithic assemblages are expedientand multifunctional, with an overwhelming preference for localresources (more than 95% of local rocks per site). This means thatpeople used mostly flakes or cores made on available cobblestones, withno or scarce retouching on the edges, often for several tasks and

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promptly discarded after use. Stone tools are focused on exploiting andprocessing wood, other plant resources (cultivated or not) and othertypes of soft materials, since most of the use edges shows end roundingand have angles of 60° or more; nevertheless, a part of the domesticlithic assemblage also indicates the exploitation and processing of

resources such as fishes, mollusks, marine mammals or birds.In this context, the only formal tools made by knapping technique

are stemmed and non-stemmed triangular projectile points withstraight or slightly convex edges and concave or notched bases, andother stemmed triangular points with barbs and straight or slightly

Table 2Dates from the sites P29-1, P31-1 and P5-1 on Mocha Island.

Site Code Material Conventional BP D13Ca Cal BP date rangeb Cal CE date rangeb

P29-1 AA 89420 Charcoal 1105 ± 36 −24.1 1056–917 893–1029AA 89419 Charcoal 964 ± 36 −26.6 919–748 1029–1200AA 89416 Charcoal 895 ± 38 −26.4 901–680 1049–1271AA 89417 Charcoal 825 ± 36 −25.0 761–663 1190–1287AA 89418 Charcoal 759 ± 38 −25.6 724–566 1226–1384

P31-1 AA 89424 Charcoal 826 ± 27 −25.7 670–736 1214–1280AA 89422 Charcoal 519 ± 37 −24.0 551–490 1399–1460AA 89421 Charcoal 408 ± 37 −26.0 499–324 1451–1627AA 89423 Charcoal 334 ± 34 −24.0 455–298 1496–1654

P5-1 UB 24526 Zea mays (seed) 992 ± 30 −9.7 925–793 1025–1157UB 24523 Chenopodium quinoa (seed) 816 ± 27 −27.9 669–565 1218–1282UB 24525 Zea mays (seed) 796 ± 25 −9.0 725–662 1225–1288UB 26213 Camelidae (bone) 751 ± 35 ND 721–564 1229–1386UB 24524 Chenopodium quinoa (seed) 718 ± 22 −28.6 732–668 1281–1385UB 26215 Zea mays (cobb) 635 ± 25 −10.2 641–451 1309–1409UB 24528 Camelidae (bone) 668 ± 26 −20.9 651–555 1299–1395UB 24529 Camelidae (bone) 683 ± 26 −21.1 656–559 1294–1391UB 26216 Camelidae (bone) 605 ± 26 −21.2 631–524 1319–1426UB 26214 Zea mays (seed) 552 ± 26 −10.0 550–507 1400–1443

a D13C — ND: not determined.b Calibrated at two sigma, with Calib7.0 (Stuiver et al., 2005), using the SHCal13 curve (Hogg et al., 2013).

Table 3Obsidian pieces analyzed from Mocha Island archaeological sites.

Sample # Site Recovery methoda Type of pieceb Weight (g) Color Obsidian typec

1 P29-1 SC Projectile point 0.50 Black MEL2 SC Debitage 1.40 Black MEL3 SC Debitage 0.10 Black MEL4 SC Debitage 0.30 Black PC5 SC Debitage 0.60 Black MEL6 SC Debitage 0.20 Grey MEL7 P5-1 SC Knife 9.00 Black MEL8 P31-1 SC Debitage 0.20 Black MEL9 SC Debitage 0.10 Black MEL10 SC Debitage 0.90 Grey MEL11 P5-1 SC Debitage 0.90 Black MEL12 P31-1 TP Scraper 1.30 Black PC13 P29-1 TP Projectile point 0.40 Striped PC14 TP Multifunctional tool 3.20 Black MEL15 TP Projectile point 0.30 Striped PC17 TP Debitage 0.20 Grey MEL18 P5-1 TP Multidirectional core 0.32 Grey MEL20 TP Multidirectional core 2.20 Black MEL21 TP Debitage 0.11 Black MEL22 TP Debitage 0.06 Black MEL23 TP Debitage 0.09 Grey MEL24 TP Debitage 0.08 Black MEL25 TP Multidirectional core 0.94 Black MEL26 TP UET for cutting 0.32 Black MEL27 TP UET for cutting-scraping 0.18 Grey MEL28 TP Debitage 0.03 Black MEL29 TP Projectile point (fragment) 0.12 Striped PC30 TP Debitage 0.13 Black MEL31 TP Debitage 0.22 Black MEL32 TP Projectile point 0.84 Striped PC33 TP Projectile point 0.71 Grey PC34 TP Multidirectional core 1.11 Black MEL35 TP UET for scraping 0.46 Black MEL36 TP UET for cutting-scraping 0.23 Grey PC37 FC Debitage 0.25 Grey MEL

a Recovery method. SC: surface collection; TP: test pit; FC: flotation column.b Type of piece. UET: unretouched edge tool.c Obsidian type. MEL: Nevados de Sollipulli; PC: Portada Covunco.

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convex edges. The majority of those island's projectile points – eitherfound in our and previous research projects and by local landowners –are manufactured with obsidian, while some debitage of this rawmaterial is also found. In particular, for the three sites presented here,we recovered 10 projectile points: six of obsidian, two of basalt, one of acryptocrystalline siliceous rock, and one of quartz. In addition, obsidianand cryptocrystalline siliceous rocks are found in other forms besidesbifacial projectile points; they have been recognized as knifes, scrapers,drillers, multidirectional cores (usually exhausted) and flakes.

3. Materials and methodology

The 35 samples we studied represent the full set of the obsidiansrecovered from the sites P5-1 (n= 21), P29-1 (n = 10) and P31-1(n = 4) between 2009 and 2013 (Table 3). Visually, these obsidians canbe described as reddish-brown with blacks streaks (“tiger-striped”,sometime referred to as “mahogany obsidian”) (n = 4), grey (n = 9)and black (n= 22). All except one (black obsidian sample #34) arecrystal-free obsidians.

The technological analysis of these 35 pieces (Fig. 2) was carried outby reviewing the same set of variables for every retouched piece, andanother set of variables for debitage. These variables permitted us tocharacterized lithic assemblages. For cores and tools we identified rawmaterial, the morphology of the piece's body, base and cross section andmeasured the maximum length, width and thickness (in millimeters)and the weight (in grams). We also identified morphology and angle ofthe used edges, determine the portion of the piece that was retouched(marginal, unifacial, bifacial, etc.) and the function of the artifact inquestion. For debitage, we identified raw material, type of debris (flake,bipolar flake, blade, etc.), size of debris, type of striking platform,percentage of cortex on the dorsal surface, and the correspondingproduction stage for each piece (core debris, bifacial flaking debris,etc.).

From the combined analysis of the pieces we can conclude that the“tiger-striped” obsidian is found solely and exclusively in the form ofnon-stemmed triangular projectile points with concave or notched

bases and straight or slightly convex edges with fine denticulatedending retouch. The retouching tends to be parallel or sub-parallel, andin some pieces reworking activity is visible. To date, not a singledebitage from this material has been recorded.

The black and grey obsidians, in contrast, are present in a variety offorms, which include stemmed and non-stemmed projectile points withnotched bases and barbs, exhausted multidirectional cores, bifacial andnon-bifacial debitage, and flakes used as unretouched edge tools. Thecortex percentage is practically nill.

The compositional evaluation of the 35 pieces, for its part, consistedprimarily of XRF analysis, using an Innov-x portable X-ray analyzerwith a silver X-ray source tube run at 35 kV. With this technique, wecould evaluate all of the obsidians in a non-destructive fashion, anecessary condition given the material's uncommon status in the lithiccontext of Mocha Island, as well as the exploratory nature of the presentstudy. The results of the non-destructive XRF test were qualitative (notquantitative) because the analysis was based on data obtained asgraphed intensity peaks for the energy of secondary X-rays producedby iron and the four trace-elements elements Rb, Sr, Y and Zr (Fig. 3).Fig. 3A shows the graphed results obtained in this fashion for geologicsamples of the three most common types of obsidian found inarchaeological sites in southern Chile (Table 1): MEL from Nevadosde Sollipulli (Stern et al., 2008, 2009), CH from the Chaitén volcano(Stern et al., 2002) and PC from Portada Covunco (Stern et al., 2012;Salazar and Stern, 2013). These three types of obsidian contain well-defined quantities of Rb, Sr, Y and Zr determined by ICP-MS analysis ofbulk samples (Fig. 3A; Table 4), which demonstrates that they arechemically distinguishable from one another, as is clearly visible in thegraphed results of their XRF analyses. The graphic representationsobtained in this fashion for geologic samples of these three obsidiantypes were used as standards of comparison in the XRF analysis of the35 obsidian artifact samples from Mocha Island. These samples thenproduced intensity peak graphs that were similar to either the MELobsidian (Fig. 3B, Sample 31) or to the PC obsidian (Fig. 3B, samples 29and 36). No samples were similar to the CH obsidian, which isconsistent with the fact that grey CH obsidian contains feldspar crystals

Fig. 2. Some of the pieces analyzed, representative of the sources from Portada Covunco (PC, upper row) and Sollipulli (MEL, lower row). The numbers of the pieces correspond with thenumbers in Table 1; pieces 29, 31 and 36 underwent destructive ICP-MS analysis (Table 4).

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(Stern et al., 2002) and is not crystal-free, as all the analyzed greysamples were. Only black obsidian samples #34 contained crystals andthis was identified chemically as MEL-type obsidian.

These qualitative results allowed us to classify the obsidian samplesunder study into two different groups: MEL and PC types. With this

information, we then proceeded to carry out a destructive butqualitative ICP-MS analysis in order to verify our classifications. Forthis analysis, we selected an obsidian sample from each group (MELtype and PC type; samples 31 and 36, respectively), as well as a “tiger-striped” obsidian sample (PC type sample 29). By comparison with thepreviously determined compositions of geologic samples of these twoobsidian types, the quantitative ICP-MS analysis supported the classi-fications and distinguishing characteristics that we had already ob-tained from the qualitative XRF analysis (see Table 5).

4. Results and discussion

The analyses of the sample origins indicate that obsidians from justtwo mainland sources – Nevados de Sollipulli (MEL) and Portada

Fig. 3. XRF energy spectrum results. A: the three standards taken from geologic samples of MEL, PC and CH obsidians; and B: artifact samples 29, 31 and 36. The numerical Rb, Sr, Y andZr values above the peaks in the spectrum of the standards represent the concentration in parts-per-million (ppm) of each element as determined by bulk-rock ICP-MS analysis (Table 4;Stern et al., 2002, 2008, 2012). The differences in the relative heights of the peaks, especially between Sr and Zr, distinguish these three obsidian types from one another, and this visualqualitative difference was used to determine the identity of the Mocha Island obsidian artifacts.

Table 4Trace-element contents in parts-per-million for geologic samples of different obsidiantypes from southern Chile and South-central Argentina.

Obsidian Rb Sr Y Zr

MEL 113 134 15 226PC 177 46 18 157CH 127 148 13 88

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Covunco (PC) – are present at the three analyzed archaeological sites(P29-1, P31-1, P5-1) on Mocha Island. MEL-type obsidians are visuallyidentifiable as both grey and black and PC-type samples as grey, blackand “tiger-striped”. As a result, visual inspection alone does not providesufficient data to determine with full certainty the source of an obsidiantype, except perhaps in the case of “tiger-striped” obsidian from PortadaCovunco. This situation reinforces the need for geochemical analyses.

The results of this analysis are significant for different reasons. Onthe one hand, the Mocha Island sites mark the westernmost distributionof the Nevados de Sollipulli and Portada Covunco obsidians, as well asthe northernmost distribution in the case of the latter source. Theanalyses indicate that no obsidian from Chaitén reached Mocha Islandand therefore Chan Chan 18 (Fig. 1) still is the northernmost site knownto date for the distribution of obsidian from the Chaitén volcano. Thepresence of MEL and PC obsidians on Mocha Island implies that theyhave been derived from sources located to the east by linear distances of220 km in the case of Nevados de Sollipulli and 340 km in the case ofPortada Covunco. The distance covered in the latter case is verysignificant since it entails crossing two major geographic barriers —the Andes mountain range and then the 30 km channel that separatesthe island from the mainland.

On a regional scale, these results, along with the informationpreviously presented (Section 2.1), suggest that the trans-Andeanobsidians – assuming that all the “tiger-striped” samples describedfrom these other sites originated from Portada Covunco – likelyappeared in the northern central valley during the Early Ceramic

period (site Villa JMC-1). In turn, only in the Late Ceramic periodwould these materials reach the northern coast (Coronel-2, La Cande-laria) and Mocha Island. It is noteworthy that this “tiger-striped”obsidian from Portada Covunco has been also reported on the northPatagonian Atlantic coast, what highlights its widespread distribution(Alberti et al., 2016). As for the black and grey obsidians, it is difficultto make a definitive assessment, given that they are present at bothNevados de Sollipulli and Portada Covunco.

The fact that the “tiger-striped” Portada Covunco obsidian is presenton Mocha Island only as projectile points with a well-defined morphol-ogy (non-stemmed triangular, with straight or slightly convex edgesand a concave or notched base) allows us to propose that the island'sinhabitants had restricted access to this non-local, sourced from far-away raw material. This situation could have been possible preciselybecause this obsidian can be visually distinguished from other potentialraw materials in circulation. As such, this obsidian appears very likelyto have had a socio-political dimension, beyond its also probableaesthetic appeal (cf. Flegenheimer and Bayón, 1999). Said in anotherway, the flow of “tiger-striped” obsidian from Portada Covunco couldbe controlled, to the point of allowing only finished artifacts – morespecifically, only projectile points – to enter Mocha Island. This controlwould have been exercised outside of the island.

This control situation could not have occurred in the case of theblack and grey obsidians (be they from Portada Covunco or Nevados deSollipulli), given that the difficulty of differentiating them wouldimpede control over their movement. In fact, the black and greyobsidians, irrespective of their source, appear as cores, debitage, andtools with minimal shaping (e.g., unretouched edge and multifunctionalinstruments), as well as tools.

Moreover, the presence of obsidian at Mocha Island's archaeologicalsites is an exceptional situation within the context of the lithic universethat has been recovered there (Table 6). The mere occurrence ofobsidian artifacts on Mocha Island lead us to interpret them as part ofthe more developed networks of communication that were becomingactive around the end of the Early Ceramic period (1000 CE; 950 calBP), connecting different areas of southern Chile with one another andthe territory itself with areas on the other side of the Andean mountainrange (Berón et al., 2012; Campbell et al., 2017; Hajduk et al., 2011;Mera, 2014; Mera et al., 2015; Musaubach, 2014; Musaubach andBerón, 2016; Salazar and Berón, 2013). Based on the temporaldistribution of obsidian artifacts summarized above, these networksappear to have progressively grown in importance or fluidity from theLate Ceramic period on, making it possible for exotic, scarce orexclusive goods to reach the central valley and coast of the northernsector of southern Chile.

This phenomenon can be situated chronologically from around900–1100 CE, with examples in the unique discoveries made at theVilla JMC-1 site and the obsidians found at sites on the northern coastand Mocha Island. This situation, in turn, intersects with a series ofother phenomena that mark the transition from the Early Ceramic tothe Late Ceramic period. These changes can be articulated in socio-political terms by denoting the emergence of certain subjects or groupswho marked their differences from others (Adán and Mera, 2011;Dillehay, 2007). These differences would be reflected precisely by theiraccess to and control of exotic, scarce or exclusive goods such asobsidian, which goes well beyond the simple practical functions of thisraw lithic material.

5. Conclusions

Through technological and compositional analysis of 35 black, greyand “tiger-striped” obsidian pieces from Mocha Island, we havemanaged to establish that these came from sources at Nevados deSollipulli and Portada Covunco, at a distance of 220 km and 340 kmfrom the island, respectively. These obsidians were employed in theproduction of different artifacts, the black and grey obsidians appearing

Table 5Composition in parts-per-million of three obsidian artifacts from Mocha Island comparedto average analysis of geologic samples of MEL and PC types obsidian.

Sample # 31 MEL average 36 29 PC averageColor Black Grey StripedLab # CS 3803 CS 3802 CS 3801

Ti 1439 1083 1143 1029Mn 383 397 405 390Rb 111 113 162 171 177Sr 125 134 47 48 46Y 16 15 19 19 18Zr 210 226 156 158 157Nb 6 7 27 28 27Cs 5.5 5.5 7.9 7.8 7.1Ba 674 774 229 228 237Hf 5.5 5.2 5.1 5.0 5.1Pb 22.1 24.3 19.0 23.1 18.3Th 10.5 11.2 23.7 25.1 25.8U 3.3 3.5 7.3 7.6 7.0La 22.4 21.1 32.5 33.8 31.2Ce 42.9 41.0 59.0 60.7 56.8Pr 4.51 4.26 6.07 6.14 5.73Nd 16.1 15.8 19.6 20.5 18.3Sm 3.22 2.55 3.38 3.54 3.29Eu 0.80 0.66 0.44 0.40 0.52Gd 3.49 2.93 4.37 4.49 4.39Tb 0.38 0.43 0.45 0.43 0.49Dy 2.38 2.56 2.85 2.92 2.80Ho 0.47 0.57 0.59 0.56 0.58Er 1.56 1.60 1.91 2.01 1.95Tm 0.20 0.23 0.23 0.24 0.29Yb 1.84 1.69 2.16 2.33 2.28Lu 0.26 0.30 0.30 0.32 0.35

Table 6Frequency of obsidian relative to other lithics from test pits on Mocha Island.

Site Obsidians Totallithics

m3 excavated Percentage ofobsidians

Obsidians perm3 excavated

P29-1 5 244 1.19 2.0% 4.2P31-1 1 148 1.66 0.7% 0.6P5-1 18 1192 6.75 1.5% 2.7

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as debitage as well. Among the artifacts, it is worth highlighting theprojectile points; in particular, the “tiger-striped” obsidian fromPortada Covunco, which appears only as non-stemmed triangularprojectile points with notched bases, all of which are finely crafted.

Given the distance between Mocha Island and these sources, thescarce presence of obsidian on the island, and the chronological andcultural context of their presence in southern Chile, we propose thattheir appearance on Mocha Island reflects an intensification of thenetworks of exchange in this geographic area and is connected with theprocesses of social differentiation occurring from around 1000 CEonwards.

Acknowledgments

We would like to acknowledged the NSF BCS-0956229, FONDECYT3130515 and FONDECYT 11150397 research project grants; BruceGeller and Ed Raines for granting us access to and assisting us with theXRF facilities at the Geology Museum of the Colorado School of Mines,Golden, Colorado; and Donald Jackson for his meticulous and indis-pensable descriptions of the Mocha Island lithics. We thank tworeviewers for their constructive comments which helped improvedthe final manuscript.

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