Ratios of barium and strontium concentrations in skeletal samples (e.g. in the logarithmic form lg(Ba/Sr)), are a possible alternative or supplementary marker to stable carbon isotope ratios (δ$^{13}$C) for identification of marine food consumption. Previous studies have compared lg(Ba/Sr) values between different species of animals with differing diets, but few studies have been performed where animals of the same species consumed known diets ranging from completely terrestrial to completely marine. Additionally, how seaweed consumption affects dental and bone Ba, Sr and other trace element concentrations has not yet been directly investigated. In this study, tooth enamel from modern sheep (n = 15) that consumed known diets containing varying amounts of terrestrial grasses and seaweeds were analysed for their Sr, Ba, Ca, V, Mn, Co, Ni, As, and U concentrations. Additionally, δ$^{13}$C values were analysed to enable comparison of δ$^{13}$C and trace element ratios as markers of marine plant food consumption. The consumed vegetation types (grasses and seaweeds) were also analysed for trace element ratios, as were soils and sands from areas where the animals were pastured. To investigate how decay processes (i.e., diagenesis) may affect lg(Ba/Sr) in archaeological tooth enamel, teeth of 22 sheep from seven archaeological sites (ranging from ca. 5000 to 1000 years old) on the Orkney Islands, Scotland, were also analysed. The results show that tooth enamel from seaweed-eating sheep had significantly different lg(Ba/Sr) (−2.4 to −1.6) and δ$^{13}$C values (−6.7‰ to −3.3‰) when compared to terrestrial-feeding sheep (lg(Ba/Sr) 0.6 to −0.5; δ$^{13}$C −15.5‰ to −14.7‰), with a linear correlation between lg(Ba/Sr) and δ$^{13}$C (R2 = 0.94). Vegetation, soil and sand results confirmed the assumed dependence of enamel lg(Ba/Sr) values on the (bioaccessible) Ba and Sr concentrations of the consumed matter. The archaeological samples had elevated relative amounts of U, V, As, Mn, Co, and Ni, attributable to diagenesis. However, the lg(Ba/Sr) values of the archaeological enamel followed the trend established using the modern samples, indicating that diagenesis did not cause significant changes in lg(Ba/Sr) in these samples. In conclusion, lg(Ba/Sr) values in enamel appear to be a useful indicator of the relative amount of marine food consumed, including seaweeds. This may be particularly advantageous for samples and locations where δ$^{13}$C is unreliable or ambiguous as an indicator of marine food consumption.