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Article Excerpt
Abstract
We report recent X-ray Fluorescence analysis of samples from neighboring basalt adze production locales situated in western Tutuila, American Samoa. These data allow for the successful reassignment of source locations to locally and regionally transferred adzes with greater precision, and highlights the relevance of intra-island source analysis when addressing inter-archipelagic exchange models.
Keywords: Samoa, Adze, X-ray Fluorescence, Quarry, Exchange
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The volcanic high island of Tutuila, American Samoa, was an intensive production locale of basalt adzes destined for inter-island distribution during the late prehistoric period (Best et al. 1992; Weisler 1997; Di Piazza and Pearthree 2001). Previous provenance research has focused on one production site in Tutuila, the Tataga-Matau Quarry (TTM), which is often cited in discussions of long distance interaction spheres within Polynesia and eastern Melanesia (Best et al. 1992; Clark et al. 1997; Di Piazza and Pearthree 2001; Leach and Witter 1990; Sheppard et al. 1997; Sinton and Sinton 1997; Walter and Sheppard 1996; Weisler 1993a, 1993b, 1997, 1998; Weisler et al. 1994; Weisler and Woodhead 1995; Weisler and Kirch 1996). However, Tutuila contains at least 13 other known prehistoric basalt adze sources that vary significantly in acquisition techniques and scales of production (Ayres and Eisler 1987; Clark and Herdrich 1993; Clark et al. 1997; Winterhoff 2003; Winterhoff et al. 2006). These other sources have seen only limited geochemical analysis and have engendered little subsequent mention in the larger interaction debate. Because of earlier debates creating a false sense of homogeneity in stone sources on islands, the cultural relationship among communities and societies involved in adze distribution has been obfuscated. This is a timely juncture for researchers in West Polynesia to more closely examine provenance differences among Tutuila's other quarries and to assess their role in exchange networks. In this paper, we report the geochemical data for four new basalt sources found in two valleys adjacent to TTM in western Tutuila to address the question whether varying levels of socio-political inclusiveness is reflected in the scale of transport and exchange relationships. To address this question, we compare the geochemistry of these new sources against the previously published geochemical signatures of TTM basalt. We then re-evaluate geochemically analyzed adzes recovered from both Samoa and surrounding archipelagos, and we reassign the origin of these transported materials with greater precision while addressing issues of levels of exchange in West Polynesia and East Melanesia.
Geological background
Variations in the chemical composition of Oceanic lava flows result from temperature fluctuations in the crystallization rates and from the original magma plume's mineral composition under the Pacific Plate (Weisler and Sinton 1997: 174). Continuous eruptions from a single volcano produce rock with similar geochemical signatures, while intermittent eruptions from the same volcano result in lava flows that differentiate as a result of varying crystallization rates. Intermittent eruption rates are consistent with Tutuila's volcanic history and provide a positive setting for recognizing geochemical differences among rock samples (Natland 2004).
Tutuila is composed of five separate volcanic formations; Pago, Alofau, Olomoana, Taputapu and Leone (Figure 1). The island formed during multiple eruptions from the same magma dome in a short geologic period during the Pliocene and the early Holocene. Although formed relatively close together in geological time, the five basaltic masses are internally and stratigraphically distinct. All five are highly variable with dikes, plugs, and extra-caldera lavas that transect the island (Natland 1980; Stearns 1944). The Taputapu volcanic, the focus of this research, lies in the northwest end of the island and contains TTM, Malaeloa and Maloata. This specific volcanic formation consists of flows of alkalic olivine basalts similar to the other Pliocene volcanics on Tutuila. Under the thin soils in the mountainous uplands of Tutuila, the Taputapu basalt beds are typically two to eighteen meters in depth, and consist of the fine-grained, homogeneous tool-quality material utilized in prehistoric adze manufacture.
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Sampling strategy
To examine adze source locales within western Tutuila, the authors collected and analyzed 76 basalt samples from Malaeloa and Maloata valleys (Figure 1). To control for different raw materials and methods of acquisition, three types of samples were investigated. First, geologic samples were collected in different locations in each valley from outcrops, streams and the soil matrix. These samples enable us to determine if there is a direct connection among adze manufacture and available rock sources. Second, waste flakes were collected from cultural strata at archaeological sites. Flake samples from sites tie the geologic sources to the prehistoric settlement pattern. Third, samples from finished adzes and adze flakes were collected to examine intra-island distribution. For comparative purposes, this study also includes the already published data on 43 geochemical samples obtained from TTM, eight adze samples from other Samoan sites, and 29 adze samples recovered...
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