Identification of Fergusson Island obsidian on the Sepik coast of northern Papua New Guinea

Abstract During PXRF analysis of obsidian recovered from sites on the Sepik coast of northern Papua New Guinea, seven pieces were identified exhibiting a chemical signature inconsistent with sources in the Bismarck Archipelago previously identified on the coast. Comparative chemical analysis of obsidian from sources in the western Pacific confirms that these pieces originated on western Fergusson Island in the D'Entrecasteaux group. Our results represent the first identification of this source material on the north side of the New Guinea mainland beyond the Massim region. Keywords: Papua New Guinea, Sepik coast, Fergusson Island Obsidian, PXRF Obsidian distribution in the western Pacific The presence of obsidian from distant sources recovered in archaeological contexts has been the primary means by which the scale and arrangement of prehistoric exchange and social networks have been reconstructed in the western Pacific, where prehistorically utilized obsidian derives from four primary source areas (Fig. 1): the Admiralty Islands (primarily Pam and Lou Islands), western New Britain (Willaumez Peninsula and Mopir), the Banks Islands (Garua and Vanua Lava), and the D'Entrecasteaux group (Fergussou, Sanaroa, and Dobu Islands) of far eastern Papua New Guinea (Summerhayes et al. 1998). The prehistoric distribution of material from these different sources has been mapped with increasingly fine chronological and spatial resolution over the last four decades since methods of physically determining the provenience of obsidian were first applied, primarily PIGME-PIXE, and more recently INAA, LA-ICP-MS, and PXRF. While obsidians from sources in the Bismarck Archipelago (Admiralty Group and New Britain), particularly from the Kutau/Bao source flows on New Britain, were particularly widely distributed during the Lapita period, reaching as far west as Sabah, as far south as New Caledonia, and as far east as Fiji (Summerhayes 2009), other source types have more restricted known distributions. Banks Island obsidians, for instance, are found in Vanuatu, the Reef/Santa Cruz group and the southern Solomon Islands (Reepmeyer 2010). The prehistoric distribution of obsidian from the D'Entrecasteaux sources is perhaps the least well known at present. The earliest possible documented exploitation of D'Entrecasteaux obsidian is represented by a single flake assigned to the Kukuia West Fergusson sub-source recovered from Lapita period (~3100BP) deposits at the SERF-2 site in the Santa Cruz Group (Green 1987; Green and Bird 1989). Unfortunately, this piece has since been lost and was consequently not included in a recent chemical reanalysis of the obsidian from Santa Cruz Group Lapita sites by PXRF (Sheppard et al. 2010). In the recent past, D'Entrecasteaux obsidians were observed as minor components of the kula and hiri exchange systems that connected the Massim area with the south Papuan coast, and D'Entrecasteaux obsidian has archaeologically also been found primarily at sites in the Massim and along the Papuan coast as far east as Yule Island (Irwin 1991) in contexts dating to after 2000BP. Obsidian found in contexts dating from ~2500BP onwards at the recently reported Bogi 1 site in Caution Bay are also likely from Fergusson sources (McNiven et al. 2011), and suggest earlier movement along the south coast of Papua New Guinea. The furthest northwards along the New Guinea mainland coast D'Entrecasteaux obsidian has been archaeologically identified to date is Collingwood Bay, which is also the ethnographically documented approximate limit of distribution. The data on prehistoric and ethnographic distributions of western Pacific obsidians have to date led to two competing viewpoints on the evolution of exchange networks in the Pacific. On the one hand, many researchers posit that an initial Lapita "pulse" of settlement coincided with wide ranging exchange of obsidian ~3300-2000BP, followed by a period of contraction in network scope that eventually saw the development of ethnographically documented, locally intensive exchange networks (Kirch 1991; Specht 2002). …