Seaweed-eating sheep show that δ34 S evidence for marine diets can be fully masked by sea spray effects.

Our findings demonstrate that δ34 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 δ34 S values are used as a marker for reliance on marine protein in modern and ancient contexts.