Limpet shell oxygen isotopes as markers of seasonality in shell middens: The case of Molène Archipelago (Brittany, France) from Late Neolithic to Early Middle Age

The stable oxygen isotopes ratio (δ18O) from marine mollusk carbonated shells is widely used as a palaeo-thermometer, as the main driver of this ratio is the temperature (coupled with the salinity) at which the carbonate precipitated. This method is also used on anthropogenic shell middens, as a proxy for past human practices and their use of marine resources: the Sea-Surface Temperature reconstructed from the shell margin can be interpreted as the season during which the people who produced the midden collected the shells. To better understand the occupation patterns and protohistoric practices of shellfish collection in the Iroise Sea and the Molène archipelago (Finistère, France), we analyzed seasonality data of limpets (Patella sp.) from Late Neolithic (LN, 2570 – 2140 cal. BCE), Early Bronze Age (EBA, 2140 – 1740 cal. BCE) and Early Middle Age (EMA, 620 – 820 cal. CE) occupations within shell middens of two islands: Molène and Béniguet.

The methodology allowed us to discriminate seasonal and permanent occupations for Béniguet Island site, enriching archaeological observations. Our results also show that the largest shell middens yield all year round collection, on both islands, confirming the continuous occupation of these territories, despite uneven intensity of collection throughout the year. The most represented seasons are late winter and spring, both on Béniguet and Molène islands, and for LN and EBA suggesting an intensification of collection to compensate resource depletion toward the end of winter. These results complete and enhance the previous seasonality data existing on these sites demonstrating here that not only the number of analyzed shells but also their spatial distribution within the midden can impact the seasonality interpretation. This approach now needs to be completed by the determination of seasonality indicators on the other resources present in the middens, to truly grasp the domestic economies of these past insular populations.