Magnetic Sourcing of Obsidians in Southern South America: Some Successes and Doubts

Journal of Archaeological Science (2001) 28, 613–618doi:10.1006/jasc.2000.0611, available online at http://www.idealibrary.com on

Magnetic Sourcing of Obsidians in Southern South America:Some Successes and Doubts

Carlos A. Vasquez*

CONICET-Laboratorio de Paleomagnetismo Daniel A. Valencio, Departamento de Ciencias Geologicas,Universidad de Buenos Aires, Pabellon 2, Ciudad Universitaria, 1428, Buenos Aires, Argentina

Hugo G. Nami†


Augusto E. Rapalini


(Received 19 November 1999; revised manuscript accepted 20 June 2000)

Sourcing of archaeological obsidians is of great importance in unravelling the cultural, social and economicdevelopment of many ancient societies. Use of magnetic properties of obsidian fragments has been reported as a cheap,fast and versatile tool for these purposes. One hundred and seventy-six obsidians from archaeological sites and sourcesin Argentina and southern Chile were analysed magnetically using the weight-normalized intensity of JNRM (Intensityof Natural Remanent Magnetization), the intensity of JSIRM (Intensity of Saturation of Isothermal RemanentMagnetization) at 0·35 T and the bulk susceptibility. The method allowed identification of at least two different sourcesfor archaeological materials found respectively to the north and south of the Lago Argentino, in SW Santa Cruzprovince, Argentina. Comparison of values from most samples in south and central Santa Cruz with those from aknown source may lead to the interpretation that most of them belong to that source. In contrast, a critical analysis ofthe resolution of this using a larger than usual number of samples from three well distant obsidian sources in Argentinashowed that two of the sources showed an almost complete overlapping of these three parameters, and all displayed avery large dispersion of values. This different result indicates that the magnetic sourcing of obsidians may not alwayshave the resolution previously portrayed, but is applicable in certain localities. � 2001 Academic Press

Keywords: ROCK MAGNETISM, SOUTH AMERICA, OBSIDAN SOURCING, PATAGONIA,NW ARGENTINA.

*Present address: Ciclo Basico Comun. Universidad de BuenosAires.†Present address: Programa de Estudios Prehistoricos (Programa deEstudios Prehistoricos, CONICET).

Introduction

O bsidians are volcanic glasses formed duringrapid cooling of certain types of lavas,generally with high silica content (>65%).

Apart from minor impurities, most obsidians arehomogeneous and isotropic in composition. As anatural resource, they were very much appreciatedin our prehistoric past (Torrence, 1986; Clark &McFadyen Clark, 1993; Smith, 1999) as they are very

6130305–4403/01/060613+06 $35.00/0

useful for making stone tools and were very widelyused to manufacture a large variety of artifacts asarrowheads, knives, mirrors, sculptures, etc. (SerraPuche & Solıs Olguın, 1994). The supply of obsidiansdeveloped in different ways in prehistory. These variedfrom simple local collection to wide and complexsystems of long distance commerce and interchangeboth by land and sea. In this way, it is possible to findsome obsidians in Europe that travelled over 2500 kmfrom their sources (Ambrose & Green, 1972). In south-ern South America, obsidians have been found up to300 km from their sources (Stern & Curry, 1995). Toadequately determine the source of archaeologicalobsidian artifacts, it is essential to characterize

� 2001 Academic Press

614 C. A. Vasquez et al.

unambiguously some physical or chemical propertiesof the source, in order to be compared with the sameproperties of the artifacts. At least three factors influ-ence the proper determination of the source: (1) howwell the sources of the region have been recognized, (2)how well this region is delimited, and (3) how possibleis it that the obsidians may belong to a more distantsource (Cann, Dixon & Renfrew, 1980).

Different methods are usually used to identify andcharacterize the obsidian sources. These include traceelements, X-ray fluorescence, optical spectroscopy, andneutron activation (Rapp, 1985; Rapp & Hill, 1998;Herz & Garrison, 1998). A new method, that is veryinteresting due to its low cost and rapidity, is the useof rock-magnetic properties (McDougall, Tarling &Warren, 1983; Tarling, 1983; Nami & Rapalini, 1994,1996; Urrutia Fucugauchi, 1999). This method wasapplied to determine the source of over 100 archaeo-logical obsidian artifacts collected in hunter–gatherers’sites in southern Patagonia (Argentina and Chile). Atthe same time a study of the rock-magnetic propertiesof three separate obsidian sources in Argentina wasundertaken. The results from these areas are quitedifferent and indicate that the magnetic method can beboth successful and unsuccessful in separating differentobsidian sources.

Materials and MethodsAmong the numerous possible magnetic propertiesexamined by McDougall, Tarling & Warren (1986),they proposed that the most efficient for obsidiancharacterization were: (1) the intensity of natural rema-nent magnetization (JNRM), (2) the bulk magneticsusceptibility (k), and (3) the intensity of isothermalremanent magnetization (JSIRM) acquired at 0·3 T atroom temperature. These three parameters are gov-erned by the magnetic mineralogy of each sample, i.e.type, size and number of magnetic grains. Magnetite(or Ti-poor titanomagnetite) is usually the dominantmagnetic mineral in obsidian; the size and number ofmagnetite grains will generally determine these threeparameters. In addition the JNRM intensity will alsobe influenced by the intensity of the Earth’s magneticfield when the remanence was acquired and by themagnetic ‘‘history’’ of the sample between this timeand its collection. Different magnetic behaviours areobserved in magnetites of different grain sizes (Butler,1993; Dunlop & Ozdemir, 1997)—superparamagnetic(SP), single domain (SD), pseudo single domain (PSD)and multidomain (MD). This turns the issue in amultivariate statistical problem. Therefore, magneticcharacterization of obsidians has been based in asemi-quantitative procedure that consists in plottingthe three above-mentioned parameters by pairs inlogarithmic scale. Grouping of data in different areasof the diagram would indicate different magneticsources. Based on the apparently successful experience

of McDougall, Tarling & Warren (1983) systematicmeasurements have been made of these three diagnos-tic parameters for 107 samples of obsidians fromidentified archaeological sources and sites in southernPatagonia (Figure 1b). The samples consisted of smallarchaeological and experimental flakes (from 1 to 5 g)collected from archaeological sites that mostly belongto hunter–gatherers that inhabited these areas duringthe last 11,000 years. A slightly larger direct field of0·35 T was applied to produce the JSIRM. JNRMversus k and JNRM versus JSIRM for 16 samples ofgreen and black obsidians found as archaeologicalartifacts at the archaeological site of Don Ariel, in SESanta Cruz province in Argentina (Figure 1(b)) arecompared (Figure 2). Except for one anomalous greensample, the remainder cluster in two groups in bothdiagrams, with the green obsidians showing significantlower values of all three parameters than the blackones. This is clearly due to a correlation betweencolour and magnetic mineral content. Twenty-sevensamples of black obsidians from 15 differentarchaeological sites near Lago Argentino (SW SantaCruz province, Figure 1(b)) cluster into two separategroups in the JNRM versus k diagram (Figure 3). Onegroup, with the largest JNRM and k values, corre-sponds to all samples from sites on the northernmargin of Lago Argentino. The other group corre-sponds to the samples belonging to four archaeologicalsites on the southern margin, some 30 to 40 km furthersouth. This is a strong indication that obsidians foundat each side of the lake derive from different sources,probably to the north and south of it. As shown below,those samples from the northern margin probablybelong to the Pampa de la Chispa source in north-western Santa Cruz, while those from the other marginwould correspond to a source that has not yet beenfound.

Seventy-seven samples of obsidians from 17archaeological sites in central and southern Santa Cruzprovince and southern Chile (sites on the southernmargin of Lago Argentino were excluded, Figure 1(b))were analysed by the same magnetic sourcing method.These can be compared with results from five samplesof the well-known Pampa de la Crispa archaeologicalsource (Figure 1(a),(b)). While most sites are close tothe Andes in the west, the results also include thosefrom Locality No. 5, on the Atlantic coast. Except fortwo samples from Chorrillo Malo 2 site, located on thenorthern margin of Lago Argentino (Loc. No. 3,Figure 1) and one sample from El Volcan (Loc. No. 5),all others show no evidence for magnetic discrimina-tion (Figure 5). One simple explanation for theseresults is that most archaeological samples (blackobsidians) found north of Lago Argentino, and onthe southern Atlantic Coast of Santa Cruz province,belong to the Pampa de la Chispa source. Untilrecently, this was the only known archaeologicalsource of obsidian in the Santa Cruz province. How-ever, Aschero et al. (1995) found a second source in

Magnetic Sourcing of Obsidians 615

50°S

66°W

44°S(a)

72°W

1

Santa Cruz province(Argentina)

PC2

6

3

45

Magallanes(Chile)

(b)

AN

RepublicaArgentina

MM

CO

Figure 1. (a) Location of obsidian sources analysed in this study. AN: Antofalla, MM: Mariano Moreno, CO: Cerro Obsidiana. (b)Distribution of archaeological sites for this study. Identical symbols identify different sites at the same locality. 1. Alero Cardenas. 2. AleroDireccion Obligatoria (ADO). 3. North margin of Lago Argentino. 4. Cueva del Medio. 5. Gallegos-Chico basin. 6. South margin of LagoArgentino. PC: Pampa de la Chispa source.

NW Santa Cruz with different chemical characteristicsthan that of Pampa de la Chispa. The possibility thatmost archaeological specimens belong to the Pampa dela Chispa source would have important implicationsfor hunter–gatherers’ technological organization insouthern Patagonia (Stern et al., 1995a, b). While thisis consistent with 40Ar/39Ar dating and geochemicalanalyses of major and trace elements (Stern et al.,1995a, b; Stern, 1998), it is also possible that theobsidians belong to more than one source with similarmagnetic parameters. McDougall, Tarling & Warren(1983) failed to discriminate with magnetic properitesbetween Mediterranean obsidian sources in only a fewcases. If this happens more frequently, the utility of themagnetic method would be seriously hampered.

In order to test this, the same three characteristicparameters were measured using samples from well-identified archaeological sources located far apart fromeach other in Argentina. Only those samples withsimilar flaking properties i.e. texture, hardness, brittle-ness, fracture, etc., to the archaeological samples werecollected for this test. Sixty-seven samples from threedifferent sources were analysed: (1) the Cerro Onasource in Antofalla (Catamarca province) in NWArgentina (34 samples, Figure 1(a)); (2) the Mariano

Moreno source along the Covunco river (12 samples,Neuquen province, Figure 1(a)) in central westernArgentina (Figure 1); and (3) the Cerro Obsidianasource in central Chubut province (21 samples,southern Argentina, Figure 1(a)). The plots of theseresults (Figures 5 & 6) show a clear broad distributionof values for all three parameters. In particular, theJNRM and JSIRM values span four orders of magni-tude. The K values are generally somewhat betterconstrained but still range over three orders of magni-tude. There is a complete overlapping of the JNRMand JSIRM values, with no possible discriminationbetween these different sources. Furthermore, a goodlinear fit could be obtained between the log values ofthese two parameters (correlation factor: 0·94). Thisstrongly suggests that they cannot be used as indepen-dent variables. On the other hand, K can act as adiscriminant factor (Figure 6), since the three sourcesplot separately in the diagram. In particular, thesamples from the Cerro Ona source have clearly lowerK values. The JSIRM versus K plot could then be usedto discriminate between artifacts made from obsidiansobtained at these three sources.

Another factor that may be playing a role in themagnetic characterization of an obsidian source is


1010 000

100 000

JNRM (Am2/kg)

(a)

JSIR

M (

Am

2 /kg)

1

100

1000

10 000

10010 1000

× 10–6JNRM versus JSIRM (0.35 T)

0.0110 000

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JNRM (Am2/kg)

(b)

0.1

0.1

1

10

101 1000

JNRM versus K

K (

10–8

m3 /k

g)

100× 10−6

× 10−6

Figure 2. JNRM versus k and JNRM versus JSIRM discriminationplots for 16 obsidian samples collected at Don Ariel archaeologicalsite (Santa Cruz province). Note the correlation between colour andmagnetic parameters.

0.0110 000

100

JNRM (Am2/kg)

K (

10–8

m3 /k

g)

0.1

0.1

1

10

10010 1000

Northern margin

Southern margin

1× 10−6

Figure 3. JNRM versus k plot for 27 obsidian samples from severalsites located to the north and south of Lago Argentino. Note thesystematic distribution of samples according to the location of thesites. �, D. Amel (black); +, D. Amel (green).

0.0110 000

100

JNRM (Am2/kg)

K (

10–8

m3 /k

g)

0.1

0.1

1

10

10010 1000× 10–6

1

Figure 4. JNRM versus JSIRM plot for 84 samples from 17archaeological sites located in Santa Cruz province and southernChile, and samples from the Pampa de la Chispa source. Samplesfrom sites on the southern margin of Lago Argentino have beenexcluded. �, Pampa de la Chispa Source; �, Loc. 1; �, Loc. 2; �,Loc. 3; �, Loc. 4; �, Loc. 5.

whether this archaeological source is primary orsecondary (Luetke, 1979). A primary source is thatwhich can be assigned to a specific geologic event (flowemplacement) beyond doubts. This will generally meanthat the samples were collected at the obsidian outcropin situ. A secondary source, on the other hand, is thatin which the obsidian samples cannot be correlatedunequivocally with a single geologic event, such asthose represented by an accumulation of obsidianboulders or pebbles far from the geological outcrop

of the flow and after transport by geomorphologicprocesses (fluvial, gravity, etc.) (Nami, 1992). Someaccumulations of obsidian fragments can result,however, in a primary source, i.e. when they wereaccumulated by the same volcanic process that pro-duced the extrusion of the lava—as for example in an

Magnetic Sourcing of Obsidians 617

1E-60.01

1E-3

JSIRM (Am2/kg)

Jnrm

(A

m2 /k

g)

1E-4

1E-5

1E-4

1E-3

Figure 5. JNRM versus JSIRM plot for 67 samples from three well distant obsidian sources in Argentina: Cerro Ona (AN), Mariano Morena(MM) and Cerro Obsidiana (CO). Note the large within source dispersion of values. See Figure 1 for location of sources. More references inthe text.

1E-90.1

1E-5

JSIRM (Am2/kg)

K (

m3 /k

g)

1E-4

1E-8

1E-7

1E-3

1E-6

0.01

Figure 6. k versus JSIRM plot for 67 samples from the obsidian sources Cerro Ona (AN), Mariano Moreno (MM) and Cerro Obsidiana (CO).See comments in text and caption of Figure 5.

explosive volcanic eruption. However, in many cases todistinguish between a primary or secondary sourcemay require a detailed geologic study.

Cerro Obsidiana and Mariano Moreno are bothsecondary sources as the obsidians are found aspebbles or boulders, meanwhile Cerro Ona is anobsidian flow and therefore a clear primary source,although some secondary sources have also been foundin the same area (Escola, Vazquez & Momo, 2000). Inthe case of a secondary source it may be quite difficultto distinguish if all obsidians fragments come from a

single or multiple flows. In the second case a largerdispersion of the magnetic parameters would beexpected. This seems to be the case for the CerroObsidiana source in Chubut. The distribution ofJNRM versus K values of which tend to cluster intotwo not very well defined groups which might representtwo different ‘‘primary’’ sources. Taking into accountthe previous observations it may result necessary tocharacterize properly the magnetic parameters of thelikely sources (with a few tens of samples) before anyattempt of sourcing of archaeological material.


ConclusionsPositive provenance discrimination of archaeologicalobsidians was established in Patagonia based onsusceptibility, initial remanence and isothermalremanence at 0·35 mT. However, only bulk susceptibil-ity appeared to provide a partial discriminant for threeother very separate sources in Argentina. The mainfactor is the high within source dispersion of both thenatural and isothermal remanances. It is suggested thata reliable magnetic characterization of the potentialsources, probably based on a few tens of samples, maybe needed before extensive sourcing of archaeologicalmaterial is attempted. It might also be necessary toinvestigate whether the source is primary or secondary.Nonetheless, the cheapness and rapidity of the tech-nique still makes it a readily applicable technique inparticular situations.

AcknowledgementsThis study was done with the economic and institu-tional support of the National Geographical Society(Grant #5691-96), the University of Buenos Aires andthe Consejo Nacional de Investigaciones Cientıficas yTecnicas (CONICET, Argentina). A. M. Aguerre, J. B.Belardi, C. Belleli, P. Cardenas, M. T. Civalero, P.Escola, N. Franco, J. Gomez Otero, R. Goni, C.Gradın and A. Prieto kindly provided many of theanalysed samples. J. I. Garate Zubillaga, from Zapala,provided support and hospitality during obsidiansearches in Mariano Moreno. Critical comments byD. H. Tarling and an anonymous reviewer improvedthe final version.

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Magnetic Sourcing of Obsidians in Southern South America: Some Successes and Doubts