Discovering fire events in the HAS1 settlement on the Dhofar coast (Oman) by a multi-methodological study of mollusk shells

Mollusk shells from archeological deposits are often exposed to high temperatures through human-caused or natural heating events. While heat exposure affects reliability of mollusk shells for environmental reconstructions based on geochemistry, it can provide a valuable source of information on past human behaviors and human-environment interactions. We analyzed burned and not-burned bivalve and gastropod specimens collected within two megalithic circular structures in the HAS1 settlement in Oman (Late Iron Age and Classical Period). Through a multi-methodological approach, we investigated shell microstructure using scanning electron microscopy (SEM), shell mineralogy using X-ray diffraction (XRD), and shell stable-isotopic composition (delta O-18, delta C-13) using isotope-ratio mass spectrometry (IRMS) to infer the temperatures these specimens were exposed to and to reconstruct the processes responsible for heating the shells. Thermal response of aragonite and calcite shells having different microstructures were also determined. We found that mollusk shells at this site were exposed to three temperature ranges: a) no exposure or <300 degrees C, b) between 250 degrees C and 500 degrees C, and c) >= 500 degrees C. The heat source was likely a fire which engulfed the entire settlement, which is also supported by evidence of carbonized wooden poles found in situ inside the circular structures.