Shells of Littorina littorea (Gastropoda)-an archive for palaeoclimate and seasonal shellfish collection in medieval and early modern Orkney?

Littorina littorea-the common periwinkle-is biogeographically widely distributed and occurs frequently in archaeological shell middens along the European Atlantic coast. Yet, its potential use as a recorder of palaeoclimate 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, UK, and the adjacent intertidal zone, and more than 3000 shell oxygen isotope data. Shell δ(18)O-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 late 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 the lifetime, the season of harvest can be faithfully determined from apertural δ(18)O 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.This article is part of the theme issue 'Shifting seas: understanding deep-time human impacts on marine ecosystems'.