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

Abstract

Stable sulfur isotope compositions (δ34 S values) are a useful marker of terrestrial (lower δ34 S) versus marine (higher δ34 S) diets. In coastal areas, 34 S-enriched sea spray can obscure these marine/terrestrial differences. We sought to establish whether δ34 S values of sea spray-affected terrestrial fauna can be distinguished from those of marine-feeding terrestrial fauna. We measured bone and dentine collagen δ34 S values, as well as stable carbon (δ13 C) and nitrogen (δ15 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 km2 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. Sheep bone collagen showed well-defined trends with highly significant correlations between δ13 C and δ15 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, δ34 S values were generally elevated for all sheep and were not significantly correlated with either δ13 C or δ15 N values. 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.