Floating Obsidian and Its Implications for the Interpretation of Pacific Prehistory

Article excerpt

A piece of pumice among drift material on Nadikdik Atoll, Marshall Islands, in far Micronesia had a large chunk of flakeable obsidian attached. As the atoll had been devastated by a typhoon and associated storm surge in 1905, the piece must have arrived by sea within the last 90 years. This and similar incidences of raw materials distributed by ocean drift show how sea-borne dispersal cannot be excluded offhand in the occurrence of obsidian in far-flung places, commonly attributed to human transport.

Origin

This paper discusses a rather large piece of flakeable obsidian attached to an even larger piece of pumice which was encountered on occasion of a pedestrian survey on Aelon-kan Islet, Nadikdik ('Knox Atoll', 5 [degrees] 45[minutes]N, 172 [degrees] 05[minutes]E), the southernmost atoll in the (eastern) Ratak Chain of the Marshall Islands. The item was found among numerous pieces of pumice in a zone of drift material (driftwood trees, cut logs, fishing-net floats and bottles) up to 20-30 m inland (for more detail see Spennemann 1996). The significance of the item rests in the fact that obsidian, volcanic or sedimentary rocks other than coral limestone or beachrock (= cemented sand) are not accessible at or near the surface of any of the Marshall Islands.

Description

The oblong - rounded shaped pumice boulder has a flattish, smooth slab of flakeable obsidian attached to one side [ILLUSTRATION FOR FIGURE 1 OMITTED]. The pum-ice side shows some flaking, with worn edges, suggesting that the flaking occurred early on and that subsequent erosion took place. The obsidian consists of a continuous section, separated by a groove. The piece had been suspended in seawater only for a comparatively short time (absence of marine growth) but rested in shaded supra-tidal localities for some time (fungal growth consistent with fungal and algal growth encountered on nearby coral boulders and rubble).

The lump of pumice, measuring about 32x22x20 cm, is of dark grey to blackish colour, medium-to coarse-grained, with air cavities ranging in size from 0.5 mm to 15 mm. The slice of obsidian is about 22 cm long, 15 cm wide and 4-5 cm thick. About 3.5 cm of this thickness would yield obsidian eminently suitable for flaking. The amount of obsidian represented in this single float is more than the total amount of obsidian recovered from many Lapita sites: approximately 1150 cu. cm of flakeable obsidian.

Dating the deposition of the obsidian

On 30 June 1905 a severe typhoon struck Nadikdik Atoll (Jeschke 1905). The impact of the 12-15-m high cyclonic surges was such that the entire atoll was flooded, reducing several inhabited islands to sand banks, others down to the bare reef platform and killing all but two inhabitants. The typhoon provides a terminus post quem for the deposition of the piece of pumice which could not have survived the typhoon event in situ as all floatable matter would have been resuspended and washed off.

Origin of the obsidian

The current patterns in the Marshalls are complex. During the northern summer Nadikdik is located within the north equatorial counter current (NECC) (west to east), and during winter at or near the interface between the NECC and the northern equatorial current (east to west; Barnes et al. 1948). In addition, the El Nino-induced changes to currents and typhoon frequency in the Marshalls (Spennemann & Marschner 1995) may bring material from other destinations to Nadikdik.

Drift material of known origin encountered in the Marshalls ranges from southeast Asian bamboo to saw-logs and driftwood trees from North America, the latter occasionally carrying sizeable chunks of volcanic and sedimentary [TABULAR DATA FOR TABLE 1 OMITTED] rock entangled in the root systems. Canoe hulls found washed up on the beaches have been sourced to the Solomon Islands and Kiribati. Historic accounts document drift arrivals of people from Kiribati, Lamotrek and Yap (see Spennemann 1996 for review). …