Seaweed‐eating sheep show that δ 34 S evidence for marine diets can be fully masked by sea spray effects

Rationale Stable sulfur isotope compositions ( δ 34 S values) are a useful marker of terrestrial (lower δ 34 S) versus marine (higher δ 34 S) diets. In coastal areas, 34 S‐enriched sea spray can obscure these marine/terrestrial differences. We sought to establish whether δ 34 S values of sea spray‐affected terrestrial fauna can be distinguished from those of marine‐feeding terrestrial fauna. Methods We measured bone and dentine collagen δ 34 S values, as well as stable carbon ( δ 13 C) and nitrogen ( δ 15 N) isotope compositions via continuous flow elemental analysis/isotope ratio mass spectrometry of 21 sheep ( Ovis aries ) raised on an island (North Ronaldsay, UK) of <7 km 2 that had widely divergent access to marine (seaweed) and heavily sea spray‐affected terrestrial (grass) food sources. We also analyzed the bone collagen of marine and terrestrial fauna from this island. Results Sheep bone collagen showed well‐defined trends with highly significant correlations between δ 13 C and δ 15 N values indicative of feeding along a continuum of fully terrestrial to fully marine diets, consistent with other modern baseline data from marine and terrestrial animals in the same area. In contrast, δ 34 S values were generally elevated for all sheep and were not significantly correlated with either δ 13 C or δ 15 N values. Conclusions Our findings demonstrate that δ 34 S values are poorly suited for differentiating marine and terrestrial diets in terrestrial animals in areas with pronounced sea spray effects. Care must be taken to characterize the isotopic compositions of potential food items before δ 34 S values are used as a marker for reliance on marine protein in modern and ancient contexts.