Stable isotope evidence for seasonal consumption of marine seaweed by modern and archaeological sheep in the Orkney archipelago (Scotland)

The use of seaweed as fodder for domestic animals during winter has been suggested for the North Atlantic isles in prehistory with reference to a historically attested practice. We tested the use of stable isotope analysis in tooth enamel bioapatite to detect seasonal consumption of seaweed on modern sheep from the Orkney archipelago, with no contribution, seasonal contribution or exclusive contribution of seaweed to diet. There was no overlap between the δ13C of local terrestrial vegetation and marine plants available on the shore. Sheep relying on seaweed had enamel δ13C values as high as −3·1‰, differing from grazing sheep with enamel δ13C values comprised between −17·7‰ and −15·1‰. Seaweed consumption was also reflected in lower amplitudes of intra-tooth variation in enamel δ18O values (0·3‰ to 1·8‰ in seaweed-eating sheep; 3·6‰ to 4·1‰ in grazing sheep), likely due to ingestion of oceanic water. The protocol was applied to archaeological assemblages from the same archipelago. A winter consumption of seaweed was detected in one sheep from the Neolithic site of Point of Cott and one sheep from the Iron Age assemblage of Mine Howe. No contribution of seaweed to the sheep diet was shown at the Late Norse site of Earl's Bu.