Shells of Littorina littorea (Gastropoda) – An archive for 1 paleoclimate and 2 seasonal shellfish collection in medieval and early modern Orkney?

Littorina littorea is biogeographically widely distributed and occurs frequently in archaeological shell middens along the European Atlanticc coast. Yet, its potential use as a recorder of paleoclimate and the season-of-collection has not been rigorously assessed. Here, gaps of knowledge are addressed with a comprehensive dataset of 78 specimens collected from Skaill Farm, a medieval and post-medieval archaeological site on the island of Rousay, Orkney, U.K., and the adjacent intertidal zone, and more than 3,000 shell oxygen isotope data. Shell δ18O-based coastal summer water temperature deviated by only +0.18 °C from the 1992/93–2022 average, but winter temperatures were overestimated by 2–5 °C, provided proper sampling was applied. Modern summers are 0.7 °C warmer than during the final centuries of the Little Ice Age, but statistically indistinguishable from such during two medieval intervals (14th–15th century). As the timing and rate of seasonal shell growth remains unchanged through lifetime, the season of harvest can be faithfully determined from apertural δ18O profiles. During studied time intervals in the past, snails were predominantly collected in early spring and possibly functioned as a starvation food. Findings of this study may encourage future shell midden archaeological studies at sites where the common periwinkle occurs.