Imports and Outcrops: Characterising the Baantu Obsidian Quarry, Wolaita, Ethiopia, Using Portable X-ray fluorescence

By Benjamin D. Smith, University of Florida and Winner of the RE Taylor Poster Award

The deep history of obsidian procurement in the Ethiopian rift makes it an ideal region to study long-term variation in raw material procurement, and portable X-ray fluorescence spectrometry offers a rapid, non-destructive means of identifying and comparing obsidian sources and lithic artifacts. 

The Baantu obsidian quarry was the primary source of toolstone for Mochena Borago rockshelter (Brandt et al.2012). At Baantu, quarried outcrops, quarrying debris, and lithic artifacts cover a ~1km2 area resting within eroding quaternary alluvium at the edge of a large caldera (Fig. 1). Surface artifact types suggest the area has been exploited from at least middle Pleistocene times onward. 

Figure 1. The Baantu quarry showing the locations of sampled outcrops ("Z") and surface artifacts.

My goal in this preliminary characterization was to identify the geochemical signature of the Baantu obsidian outcrops and to compare this to surface materials at the quarry (Fig. 2).

Figure 2. An obsidian outcrop and surface artefacts from Baantu.

Methods

I struck large flakes from 32 outcrops and collected 45 bags of surface materials along transects at regular intervals.  

In the lab I recorded thirty-two XRF spectra using a Bruker IIIeV Tracer + portable XRF spectrometer (SN: K0437). It is equipped with a rhodium tube. The filter is composed of 1μm Ti, 12μm Al, 6μm Cu. It records MnKa1, FeKa1, ZnKa1, GaKa1, ThLa1, RbKa1, SrKa1, Y Ka1, ZrKa1, and NbKa1. The machine was recalibrated on 11/6/2017 using the Bruker/MURR 40 standards, a Lucas Tooth regression method.

Spectra were recorded at the Ethiopian Authority for Research and Conservation of Cultural Heritage (ARCCH) in Addis Ababa. These included 25 outcrop flakes, 6 surface artifacts, and 1 artifact from Mochena Borago. Spectra were recorded once for each sample using Bruker’s accompanying software S1PXRF at 40kV in 180-second intervals. Each sample was >3mm thick with a fresh break where possible and large enough to cover the 3x4mm X-ray spot size.

Results

K-Means cluster analysis (LMP) revealed five clusters (Fig. 3). Obsidian outcrops clustered together, and most surface artifacts fell within this range of variation. Artifacts from cluster 1 likely represent another obsidian source and suggest that obsidian was both quarried at Baantu and imported.

Figure 3. The largest 95% confidence ellipse is the grouping of all "outcrop" samples encompassing clusters 2-5, likely representing minor sub-groups among the outcrops. Cluster 1 likely represents a different source. Figures by L.R.M. Johnson.

Conclusions & Future Research

The Baantu obsidian outcrops cluster well, and this preliminary comparison suggests that they are distinguishable from imported obsidians. More data are needed to 1) evaluate cluster integrity, 2) identify possible sub-sources, and 3) assess the variety of imported obsidians represented on the surface. Other obsidian sources in the region should also be sampled and compared to the Baantu outcrops using consistent calibration standards. By identifying obsidians and reduction strategies at quarries and in well-dated archaeological sequences, archaeologists can address questions of temporal and spatial variation in obsidian procurement across major periods of evolutionary, environmental, and social change

Acknowledgements

Thanks to Dr. Steven Brandt and the student members of the Southwest Ethiopia Archaeological Project. Thanks also to the Ethiopian Authority for Research and Conservation of Cultural Heritage (ARCCH) and the Wolaita Zonal Tourism offices, especially Ato Girma Dubusho.

References

Brandt, S. A., Fisher, E. C., Hildebrand, E. A., Vogelsang, R., Ambrose, S. H., Lesur, J., & Wang, H. (2012). Early MIS 3 occupation of Mochena Borago Rockshelter, Southwest Ethiopian Highlands: implications for Late Pleistocene archaeology, paleoenvironments and modern human dispersals. Quaternary International, 274, 38-54.