Mollusk shells are often found in archeological sites, given their great preservation potential and high value as a multipurpose resource, and they can often be the only available materials useful for radiocarbon (14C) dating. However, dates obtained from shells are often regarded as less reliable compared to those from bones, wood, or charcoals due to different factors (e.g., Isotope fractionation, reservoir effects etc.). The standard acid etching pretreatment for mollusk shells is the most used in many 14C laboratories, although another protocol known as CarDS (Carbonate Density Separation) was introduced just over a decade ago. We compare these two methods with two newly proposed methods for intracellular organic matrix extraction. We applied all four methods to samples selected from different archeological layers of the well-known Upper Paleolithic site of Vale Boi, rich in mollusk specimens throughout the stratigraphic sequence. Here we compare our results to previous dates to determine which of these pretreatment methods results in the most reliable 14C dates. Based on the results of this study, all four methods gave inconsistent ages compared to previous dates and the stratigraphic attribution.

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 (δ18O, δ13C) 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°C, b) between 250°C and 500°C, and c) ≥500°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.

Species of Haliotis (abalone) show high variety in structure and mineralogy of the shell. One of the European species (Haliotis tuberculata) in particular has an unusual shell structure in which calcite and aragonite coexist at a microscale with small patches of aragonite embedded in larger calcitic zones. A detailed examination of the boundary between calcite and aragonite using analytical microscopies shows that the organic contents of calcite and aragonite differ. Moreover, changes in the chemical composition of the two minerals seem to be gradual and define a micrometric zone of transition between the two main layers. A similar transition zone has been observed between the layers in more classical and regularly structured mollusk shells. The imbrication of microscopic patches of aragonite within a calcitic zone suggests the occurrence of very fast physiological changes in these taxa.