In modern Europe, for most the idea of eating seaweed may seem strange. Outside of sushi, and memories of beach holidays where rotting algae spoiled the fun, most people today have no particular connection to seaweed. It may therefore come as a surprise that a fellow mammal, namely sheep (Ovis aries), can subsist nearly exclusively on seaweed. The most well-known breed of seaweed-eating sheep today are located on North Ronaldsay, the northernmost island of the Scottish Orkney archipelago. Elevated sheep δ13C values attributable to seaweed consumption were common in Neolithic Orkney as well, as the last two decades of archaeological research have shown (cf. Balasse et al. 2019, 2005 for a review).
Since the interpretation of archaeological data relies on modern comparative data to gain an understanding of what individual values mean in the archaeological context, analyses of modern material are required. Our study, recently published in the Journal of Archaeological Science, was conceived to collect further modern baseline δ13C and δ15N data for seaweed, terrestrial vegetation, and sheep bone collagen on the Orkney Islands. The results showed extremes of −10.7‰ for δ13C and +10.4‰ δ15N for modern seaweed-eating sheep bone collagen, and a δ13C average of −17.4 ± 1.4‰ for the analysed seaweeds (n = 20; mostly kelps). This modern dataset will help with future identification of seaweed-consumption, and allows an improved assessment in archaeological contexts of how much seaweed was likely consumed when the analysed bone formed.
In addition to the consumption of seaweed, the use of seaweed as a fertiliser for terrestrial crops has also been suggested to influence consumer δ13C and δ15N values. To test this, we performed a field trial on Orkney where we fertilised bere barley (a six-row, hulled barley landrace) with seaweed. We followed traditional, historically attested practices with respect to the time and amount of fertilisation, but used a tractor for power-harrowing, sowing and rolling to reduce the still considerable amount of manual labour.
Isotope ratio analysis showed significant elevation in δ15N values for seaweed-fertilised barley compared to barley from unfertilised control plots, but no significant difference in δ13C. This can be explained by the differing sources of carbon and nitrogen: Since barley takes up nitrogen from the soil (as nitrate, NO3–, and ammonium, NH4+), the change in δ15N values upon fertilisation is due to the elevated δ15N values in the seaweed compared to the bioaccessible nitrogen in the unfertilised soil. In contrast, the lack of any major effects of seaweed fertilisation on δ13C is due to barley taking up most carbon by photosynthesis from the air (as carbon dioxide, CO2): Carbon stemming from the fertilisation at the roots has little likelihood of being taken up by the plant.
These results show that fertilisation of terrestrial crops with seaweed does not lead to these crops having more “marine” δ13C values, but that, similar to fertilisation with animal dung, δ15N values are affected by seaweed fertilisation. Further detail on the seaweed fertilisation field trial is available in Blanz et al., 2019 and Brown et al., 2020.
This research was undertaken as part of my PhD (2016-2020) at the Archaeology Institute, Orkney, supervised by Ingrid Mainland, Philippa Ascough, Mark Taggart and Jörg Feldmann. It was funded by the European Social Fund and Scottish Funding Council as part of Developing Scotland’s Workforce in the Scotland 2014–2020 European Structural and Investment Fund Programme.