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Basalt Pb Isotope Analysis and the Prehistoric Settlement of Polynesia MARSHALL I

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Basalt Pb Isotope Analysis and the Prehistoric Settlement of Polynesia MARSHALL I Proc. Natl. Acad. Sci. USA Vol. 92, pp. 1881-1885, March 1995 Anthropology Basalt Pb isotope analysis and the prehistoric settlement of Polynesia MARSHALL I. WEISLER*t AND JON D. WOODHEADI *Historic Preservation Office, P.O. Box 1454, Majuro, Marshall Islands 96960; and tResearch School of Earth Sciences, The Australian National University, Institute of Advanced Studies, Canberra ACT 0200 Australia Communicated by Patrick V Kirch, University of California, Berkeley, CA, November 28, 1994 ABSTRACT The prehistoric settlement of the Pacific sociocultural change has drawn increased attention (10-12). Ocean has intrigued scholars and stimulated anthropological Since isolation is a fundamental condition fostering divergence debate for the past two centuries. Colonized over a few (13), understanding how the relative extremes of isolation have millennia during the mid to late Holocene, the islands of the influenced the evolution of human diversity in the Pacific Pacific- displaying a wide diversity of geological and biotic becomes a basic problem that must be examined for each variability-provided the stage for endless "natural experi- island sequence. The scale, frequency, and temporal span of ments" in human adaptation. Crucial to understanding the imported artifacts is our best single measure of empirically evolution and transformation of island societies is document- documenting the relative degree of isolation of island societies. ing the relative degree of interisland contacts after island Studies throughout the Pacific have sought to document colonization. In the western Pacific, ideal materials for ar- settlement routes and subsequent interarchipelago communi- chaeologically documenting interisland contact-obsidian, cation by identifying similarities in language (14), stylistic pottery, and shell ornaments-are absent or of limited geo- affinities of portable artifacts and architecture (15, 16), con- graphic distribution in Polynesia. Consequently, archaeolo- current changes in adze and ceramic styles in the Fiji-Tonga- gists have relied increasingly on fine-grained basalt artifacts Samoa region (17), parallel histories in technological and as a means for documenting colonization routes and subse- subsistence transformations in the Marquesas (18), time pro- quent interisland contacts. Routinely used x-ray fluorescence gressive radiocarbon dates plotted from the western to eastern characterization of oceanic island basalt has some problems Pacific, and mtDNA sequences (19). More recently, archae- for discriminating source rocks and artifacts in provenance ologists have devoted increasing interest to identifying the studies. The variation in trace and major element abundances movement of actual commodities, rather than inferring inter- is largely controlled by near-surface magma-chamber pro- island contact from stylistic similarities of artifacts that may cesses and is broadly similar between most oceanic islands. merely signal the movement of ideas (12, 20-25). We demonstrate that Pb isotope analysis accurately discrim- The islands of the western Pacific have favored archaeolog- inates rock source and is an excellent technique for charting ical interaction studies because of their geological complexity, the scale, frequency, and temporal span of imported fine- larger size, and closer spacing, which is conducive for coloni- grained basalt artifacts found throughout Polynesia. The zation and subsequent interisland voyaging. Obsidian has a technique adds another tool for addressing evolutionary mod- restricted natural occurrence yet was transferred great dis- els of interaction, isolation, and cultural divergence in the tances (26, 27). Likewise, pottery and shell ornaments were eastern Pacific. manufactured in limited local settings and were exchanged regionally. Consequently, on islands on the continental side of Stretching more than 10,000 km from west to east, the the Andesite Line (Fig. 1), archaeologists have been very settlement of the vast Pacific Ocean was the greatest overwater successful in tracking colonization routes and subsequent migration in human history. The colonization of the western interisland communication by identifying exotic obsidian, pot- Pacific occurred during the late Pleistocene (1) as small groups tery, and shell ornaments in habitation sites found throughout with rudimentary ocean-going crafts ventured from the the western Pacific. greater Australian mainland of Sahul (2). Even at this early Polynesia presents a different challenge for tracking the stage of Pacific colonization, return voyages between islands occurrence of imported artifacts and documenting interisland are suggested from the transport of obsidian (3, 4), an impor- contacts. Obsidian and pottery have an extremely limited tant raw material for manufacturing sharp cutting tools. By the occurrence, being found on only a few islands (28), and shell second millennium B.C., a 4500-km region of the southwest ornaments-similar to those of the western Pacific-are com- Pacific stretching from the Bismarck archipelago to the Fiji- pletely absent. Archaeologists, then, have been forced to Tonga-Samoa region was settled by a people carrying a explore other material classes for documenting interisland distinctive dentate-stamped pottery known as Lapita (5-7). communication (20, 23, 29). This latter wave of migration was nearly instantaneous as Opportunities for external exchange in Polynesia are limited reasoned from the efficacy of the radiocarbon dating tech- because of much smaller island size and greater distances nique (8) and implies that colonization was purposeful and between islands. Since the relative isolation of an island is directed (9). Colonization culminated in the eastern Pacific related to the distance of its neighbors, which influences the where nearly every inhabitable island was occupied by A.D. frequency of external contacts, we might expect that there 1000 (Fig. 1). would be less voyaging in eastern Polynesia after colonization Identifying patterns of interaction after settlement is fun- than in the west. Wide distances between archipelagoes may be damental to understanding evolutionary models of cultural a deterrent to frequent contacts. That is not to say, however, variability throughout Oceania. Consequently, documenting that voyaging was not an important component to many island the changing role and significance of interisland contacts in societies. Because Polynesia generally lacks those artifacts that are routinely used to track interisland communication in the The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in tPresent address: Department of Anthropology, University of Otago, accordance with 18 U.S.C. §1734 solely to indicate this fact. P.O. Box 56, Dunedin, New Zealand. 1881 Downloaded by guest on October 1, 2021 1882 Anthropology: Weisler and Woodhead Proc. Natl. Acad Sci. USA 92 (1995) FIG. 1. Map of the Pacific showing Polynesia, the major island groups discussed in the text, and the Andesite Line, which separates islands composed of continental rocks (west) from those of oceanic island basalt (east). western Pacific, archaeologists are now relying on fine-grained In contrast, radiogenic isotope variations are a function of basalt-fashioned into adzes and other cutting tools-which is both parent/daughter elemental ratios in the mantle source of widespread in habitation sites and has a limited geological oceanic island basalts and the age of this source. The consid- occurrence. In Polynesia, then, fine-grained basalt is ideal for erable age of most oceanic island basalt sources (of the order documenting colonization routes and later interisland con- of 1-2 billion years) has resulted in a diversity of isotopic tacts. compositions and thus isotope ratios can be used with far more confidence as provenance indicators. Sr, Nd, and Pb isotopes The Problem are most frequently determined by Earth scientists, and thus a considerable database exists in these elements for most oce- Archaeologists have become increasingly interested in the anic islands. Of these the Pb isotope system is perhaps the most petrographic and geochemical characterization of Polynesian sensitive, consisting, effectively, of three individual decay basalt artifacts and source rock (29). Some studies have relied schemes: the daughter products of the radioactive parent exclusively on thin-section petrology (30, 31), but researchers isotopes of U and Th (namely, 238U, 235U, and 232Th) are, have come to recognize the limitations of relying solely on this respectively, 206Pb, 207Pb, and 208Pb. Abundances of the latter technique (20). X-ray fluorescence studies have gained wider are conventionally quoted as ratios relative to the stable Pb application due to the following factors: the reproducibility of isotope 204Pb-thus, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/ results, elemental abundances that can be determined with 204Pb. Additionally, many of the geochemical techniques that accuracy, precision parameters that can be reported, use of archaeologists routinely use are highly destructive (but see ref. standards that facilitates interlaboratory comparisons, and 23) and require relatively large samples for analysis. We report on a technique for identifying exotic artifacts and bulk chemical constituents that more closely represent the characterizing source rocks in Polynesia. The technique is whole specimen, in contrast to a two-dimensional thin section destructive,
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