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 macroand 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, 14 C AMS 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.

Internal Water Facilitates Thermal Resetting of Clumped Isotopes in Biogenic Aragonite

“…This reaction rate is considerably accelerated and occurs at lower temperatures if a powdered aragonite is used in the alteration experiment (Guo et al., 2019). Dry heating (or “roasting”) experiments revealed that the transition to calcite requires more time and/or higher temperatures, but is nevertheless accompanied by an oxygen isotopic shift in the carbonate (Milano et al., 2018; Müller, Staudigel, et al., 2017; Staudigel & Swart, 2016). This isotopic shift, which occurs in absence of interaction with an outside medium, is believed to be caused by a set of exchange reactions that occur with internal fluids (Moon et al., 2020; Pederson, Mavromatis, et al., 2019; Pederson, Weiss, et al., 2019; Uemura et al., 2019), and/or internal organic material (Li et al., 2020).…”

Chronology, time averaging, and oxygen isotope composition of harvested marine mollusk assemblages from Ifri Oudadane, northeast Morocco

“…This observed ~200 yr offset may be explained by diagenetic alteration of the outer layer. High temperatures derived from cooking or burning activities (>300°C) may cause significant changes in the geochemistry of the shell (Milano et al, 2016, 2018). However, the shells analyzed here did not show visual evidence of heat exposure, and further research is needed to evaluate the potential effects of diagenetic alteration in radiocarbon results.…”

Marine Biogenic Carbonates and Radiocarbon—a Retrospective on Shells and Corals With an Outlook on Challenges and Opportunities

“…One study, which included several shell species from the Gulf of Oman, demonstrated that burned shells not only provided reliable 14 C ages, but also elucidated some patterns of shell processing in the past (Lindauer et al 2018). Microstructure patterns of the different shell layers helped to identify processes and mineral transitions from aragonite to calcite of a semi-burnt shell (Lindauer et al 2018;Milano et al 2018).…”