Inside a nutshell: a stable carbon and nitrogen isotopic charring and tissue offset for hazelnuts

Charred common hazel (Corylus avellana) nutshell fragments are frequently found at archaeological sites, sometimes in large quantities, but relatively little has been done with them in palaeodietary and palaeoecological studies beyond noting their presence. The oily hazelnut kernel in contrast rarely survives charring. This study provides baseline data for the incorporation of stable carbon (δ13C) and nitrogen (δ15N) isotope results on hazelnut shells in palaeodietary and palaeoecological models. Using modern hazelnut shells and kernels, we first calculate the offset between the hazelnut shell and the kernel, and then the offset caused by charring the hazelnut shells for four hours at low (∼290°C), medium (∼320°C) and high (∼350°C) temperatures. We find that δ13C and δ15N measurements on uncharred hazelnut shells are offset from uncharred kernels by 2.31 ± 0.21‰ and −1.18 ± 0.14‰ respectively. The effect of charring on the δ13C and δ15N values, both in terms of the magnitude and statistical significance, is dependent on the charring temperature. As such, a small adjustment based on the calculated offsets can be applied to the δ13C and δ15N values of archaeological charred hazelnut shells to predict the values of consumed kernels (for palaeodiet) or uncharred shells (for palaeoecology). These offsets can be applied to archaeological hazelnut shells from any region or period. As an example, we apply this offset to charred hazelnut shells from three Mesolithic sites from western Scotland to estimate the value of the archaeological hazelnut kernels.