Seaweed fertilisation impacts the chemical and isotopic composition of barley: Implications for analyses of archaeological skeletal remains

Fertilisation with animal manure has been shown to affect crop chemical and isotopic composition, indicating that if manuring effects are not taken into account, there is a risk of overestimating consumer trophic levels in palaeodietary studies. The effect of fertilisation with seaweed, a common fertiliser in the past in coastal areas, has been the subject of several hypotheses, but until now has not been studied in this particular context. In this study the impact of fertilising bere, an ancient type of Scottish barley (Hordeum vulgare L.), with 25 t/ ha and 50 t/ha seaweed, in comparison to a modern commercial mineral fertiliser and to no fertilisation, was investigated in a field trial on the Orkney Islands, Scotland. Stable isotope ratios (delta C-13 and delta N-15) and elemental concentrations (B, Mg, K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Sr, Mo, Cd and Pb) of grain, husk and straw samples were determined. Significant differences were found between treatment groups, including increases in delta N-15 values of 0.6 +/- 0.5 parts per thousand (average +/- 1 sigma for five replicate plots) in grain, and 1.1 +/- 0.4 parts per thousand in straw due to seaweed fertilisation. Elevated concentrations of Sr in grain and husk samples (factors of 1.2-1.4) indicate the geographic tracer Sr-87/Sr-86 may also be affected. Fertilisation with seaweed thus needs to be considered for archaeological interpretations of chemical and isotopic compositions of crop and skeletal material for accurate palaeodietary and provenance reconstructions, particularly in coastal areas. Further implications of these results for studies concerning the effects of sea spray, radiocarbon-dating, and for dietary reconstructions using trace elements are also identified.