Western Pacific Obsidian

Obsidian is a naturally occurring glass formed during volcanic eruptions producing high-silica lava that ha
s cooled so rapidly that no internal crystallization takes place.  It was highly valued in non-metal using societies world-wide for tool production because of its easy workability, sharpness, and visual beauty—because obsidian is limited in its occurrence geographically, it was often traded over substantial distances.  In some cases, obsidians from different source flows are visually distinct and can be identified on that basis.  However, many are visually indistinct, but can be traced back to their original collection or quarrying location by measuring the relative levels of certain trace elements contained within them. 

In the western Pacific region, obsidian is found in several primary source locations (Figure 1)—in Papua New Guinea, sources are located in the Admiralty Islands (Lou and Pam Islands an at Mt. Hahie on Manus), on New Britain (Willaumez Peninsula and Mopir), and on Fergusson, Sanaroa, and Dobu Islands in the Massim region.  Additional sources in the Banks Islands of Vanuatu were exploited by people living further east, but have to date never been identified in Papua New Guinea.

As part of an ongoing initiative to build up obsidian source collections world-wide, the Field Museum Anthropology Department has recently acquired a source collection of obsidian pieces from major sources in Papua New Guinea, including prehistorically utilized sources on Lou and Pam Islands (Admiralty Group), New Britain (Gulu, Kutau/Bao, Baki, and Mopir) and the D’Entrecasteaux Group (eastern and western Fergusson Island).  We have chemically characterized their composition using portable X-ray Fluorescence Spectrometry (PXRF) and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS), two techniques capable of measuring concentrations of trace elements in obsidian such as iron, rubidium, strontium, and zirconium, that are important for distinguishing between different obsidian source flows. 

We have used these geological samples to identify the sources of obsidian pieces collected by curator John Terrell on the Sepik coast of northern Papua New Guinea (Sandaun Province) during his fieldwork there between 1990-1996.  In figure 2, five separate geological sources of obsidian can be identified among the 1602 pieces of obsidian analyzed from the Sepik coast.  The majority of the material was quarried from sources on Lou and Pam islands in the Admiralty group, and transported some 600 km to the north coast.  Approximately 10% of all the analyzed pieces derive from the Kutau/Bao source on the Willaumez Peninsula of New Britain, nearly 800 km from archaeological sites on Tarawai Island, where the eastern most analyzed assemblages were collected. 

Our research has for the first time also identified the presence of obsidian from Fergusson Island on the north coast—previously, the distribution of this material was limited to the Massim and Papuan coast regions.  This material was transported over 1000 km.  By comparing assemblages of different ages, we are examining how connections between communities on the coast fluctuated over time, and how the structure of these prehistoric social networks relates to the modern patterning of language and culture on the coast.

Figure 1.  Location of obsidian sources in Papua New Guinea with the Sepik coast region indicated.

Figure 2.  Bivariate plot of Zirconium and Iron concentrations measured in raw material samples and archaeological specimens suing PXRF.  Filled symbols are source samples collected at obsidian outcrops in Papua New Guinea.  Ellipses denote the statistical limits of inclusion for different geographically and chemically discrete obsidian sources.  Note the presence of five chemical signatures in the data.

For further discussion please see:

Golitko, M. (2011)     Provenience Investigations of Ceramic and Obsidian Samples Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry and Portable X-Ray Fluorescence.  In Exploring Prehistory on the Sepik coast of Papua New Guinea, edited by J.E. Terrell and E.M. Schechter, pp. 251-287, Fieldiana Anthropology New Series No. 42.  Field Museum of Natural History, Chicago.

Golitko, M., J. Meierhoff, and J.E. Terrell. (2010) Chemical characterization of sources of obsidian from the Sepik coast (PNG).  Archaeology in Oceania 45: 120-129.