Seaweed‐eating sheep show that δ³⁴S evidence for marine diets can be fully masked by sea spray effects

Abstract

RationaleStable sulfur isotope compositions (δ34S values) are a useful marker of terrestrial (lower δ34S) versus marine (higher δ34S) diets. In coastal areas, 34S‐enriched sea spray can obscure these marine/terrestrial differences. We sought to establish whether δ34S values of sea spray‐affected terrestrial fauna can be distinguished from those of marine‐feeding terrestrial fauna.MethodsWe measured bone and dentine collagen δ34S values, as well as stable carbon (δ13C) and nitrogen (δ15N) 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.ResultsSheep bone collagen showed well‐defined trends with highly significant correlations between δ13C and δ15N 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, δ34S values were generally elevated for all sheep and were not significantly correlated with either δ13C or δ15N values.ConclusionsOur findings demonstrate that δ34S 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 δ34S values are used as a marker for reliance on marine protein in modern and ancient contexts.

Diversity aboard a Tudor warship: investigating the origins of theMary Rosecrew using multi-isotope analysis

“…Within the group of likely migrants, FCS-85 has a high ‘coastal’ δ 34 S value whereas FCS-70 and FCS-81 have very low δ 34 S values suggesting ‘inland’ residence, potentially in areas of wetland and/or impervious lithology [ 58 ]. The three individuals with the highest δ 34 S values (FCS-09, FCS-12 and FCS-85) also had the highest δ 13 C values, supporting a coastal residence [ 59 ].…”

“…Biosphere δ 34 S has a wide range; relatively low and negative values are common in inland areas of waterlogged ground and/or impervious lithologies [ 58 ]. By contrast, marine resources have relatively high δ 34 S values, and food grown and grazed on coastal soils can be influenced by a ‘sea-spray effect’ which results in δ 34 S values which approach marine sulfate composition (+20‰) [ 59 ], although local geological and environmental factors can also play a role [ 60 ]. Therefore, humans and animals feeding at coastal sites generally exhibit higher δ 34 S values than those sourcing their diets from further inland, although estuarial regions also tend to have high biosphere δ 34 S [ 61 ].…”

A veritable confusion: use and abuse of isotope analysis in archaeology

“…• Sulphur isotope values vary mainly according to coastal proximity, with exposed coasts facing prevailing winds having the highest values, although diet and geology also have an impact (Guiry and Szpak 2020;Nehlich 2015;Zazzo et al 2011). It has become an increasingly useful provenancing tool in multi-isotope strategies (e.g.…”

Freshwater wetland–driven variation in sulfur isotope compositions: Implications for human paleodiet and ecological research

“…A second major use of δ 34 S relies on the distinctive and homogenous isotopic composition of δ 34 S of seawater sulfates (+20‰; Rees et al, 1978), which can differ from the (often lower) δ 34 S characterizing baselines and food webs in adjacent terrestrial environments (Fry, 1988;Dance et al, 2018). This means that sulfate contributions from sea spray, with a marine δ 34 S composition, can have a large impact on the δ 34 S values of consumers in coastal areas (Zazzo et al, 2011), even masking local terrestrial sulfate contributions entirely (Guiry and Szpak, 2020). For this reason, consumer δ 34 S is sometimes used to assess residence in coastal areas (Richards et al, 2001).…”