Chemical characterization of sources of obsidian from the Sepik coast (PNG)

Abstract We report here on the chemical analysis by portable x-ray fluorescence (PXRF) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) of 438 obsidian flakes recovered from Sepik coastal sites spanning the last two millennia of prehistory. Our results demonstrate the continuous involvement of local people in exchange networks moving obsidian from both the Admiralty Islands and West New Britain to this coast, possibly as far back as the mid-Holocene. Suspected early assemblages contain significantly higher frequencies of obsidian from the more distant sources on New Britain. Assemblages dated to after ~2000 calBP contain obsidian predominately from the nearer Admiralty sources. Site by site differences in the relative frequencies of obsidian from different Admiralty sub-sources suggest that prehistoric patterns of interaction on the coast were complex, and cannot be accounted for by simple patterns of 'down the line' connections. Keywords: Papua New Guinea, Sepik coast, Obsidian, LA-ICP-MS, portable XRF Prehistory on the Sepik coast The Sepik coast, by which we mean the coastline and small offshore islands between the urban centers of Jayapura and Madang (Terrell 2001: 213, in press) is today the home of upwards of 60 languages belonging to 24 distinct language families. Even so, this coast is the setting for wide-reaching exchange relationships among communities structured by inheritable trans-generational friendships (sometimes also called trade or exchange 'partnerships') that cross linguistic and cultural boundaries (Terrell, et al. 1997: 549-557; Welsch and Terrell 1998)--relationships contrary to the commonsense idea that great linguistic diversity must be a sign of great isolation between people and places (Terrell 2001: 213, 2010). From a prehistorian's perspective, it is of interest to understand the long-term history of social networks on the coast, and whether the kinds of intensive connections that exist today, crossing ethnic and linguistic boundaries, may also have existed in the past. On current evidence, New Guinea and its major offshore islands have been settled for approximately 40,000 years, and there is some evidence to suggest that people may have been living on the north coast near modern day Vanimo as early as 35,000 years ago (Terrell 2002: 10-11). However, the Sepik coast for the most part probably did not support any sizable populations until around 6000-7000 years ago, when Holocene sea levels had stabilized sufficiently for productive lagoons to form. Prior to this, the coastline rose steeply from the sea to the Torricelli Mountains, providing little productivity for humans to exploit (Pope and Terrell 2007: 10; Terrell 2004: 603-605). During 1993 and 1994, Terrell and Rob Welsch undertook a program of combined ethnographic and archaeological survey work along the Sepik coast between the modern towns of Leitre and Wewak in the context of the Field Museum of Natural History Alfred B. Lewis Project (Fig. 1), and later returned to conduct excavations at archaeological sites in the vicinity of modern day Aitape on the New Guinea mainland (NGRP 16, 22, and 23) and on nearby Tumleo Island (NGRP 46) (Terrell and Schechter no date; Terrell and Welsch 1990, 1997). Their team's surface collections and excavations have revealed extensive evidence for continuous settlement along the Sepik coast dating back at least two millennia, although aceramic inland contexts near modern-day Kobom associated with the mid-Holocene shoreline are suspected to be far older, as are find spots on Ali Island, where a single sherd of Lapita style (~3500-2200/1600 BP) was recovered (Terrell and Welsch 1997: 558-559). We report here on chemical compositional analysis by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) and portable X-ray Fluorescence (PXRF) of 438 pieces of obsidian recovered from the Sepik coast. …