Mollusk carbonate thermal behaviour and its implications in understanding prehistoric fire events in shell middens

Abstract

Archaeological shell middens are particularly important for reconstructing prehistoric human subsistence strategies. However, very little is known about shellfish processing, especially when related to the use of fire for dietary and disposal purposes. To shed light on prehistoric food processing techniques, an experimental study was undertaken on modern gastropod shells (Phorcus lineatus). The shells were exposed to high temperatures (200-700 °C) to investigate subsequent mineralogy and macro- and microstructural changes. Afterwards, the three-pronged approach was applied to archaeological shells from Haua Fteah cave, Libya (Phorcus turbinatus) and from shell midden sites in the United Arab Emirates (Anadara uropigimelana and Terebralia palustris) to determine exposure temperatures. Results indicated that shells from the Haua Fteah were exposed to high temperatures (600 - 700 °C) during the Mesolithic period (c. 12.7 - 9 ka), whereas specimens from the Neolithic period (c. 8.5 - 5.4 ka) were mainly exposed to lower temperatures (300 - 500 °C). The thermally-induced changes in A. uropigimelana and T. palustris shells from the South East Arabian archaeological sites were similar to those seen in Phorcus spp. suggesting a broad applicability of the experimental results at an interspecific level. Although heat significantly altered the appearance and mineralogy of the shells, 14CAMS ages obtained on burnt shells fit within the expected age ranges for their associated archaeological contexts, indicating that robust radiocarbon ages may still be obtained from burnt shells. Our study indicates that the combination of microstructural and mineralogical observations can provide important information to infer shellfish processing strategies in prehistoric cultures and their change through time.