Cereal Journey 2 – stable isotope analysis of Ness grain

Picture: Sigurd Towrie
Modern barley field in Orkney. (📷 Sigurd Towrie)

Sarah-Jane Haston is looking at the charred plant remains from Structure Eight as part of her PhD with the UHI Archaeology Institute. Here she brings us the second part of her latest update, looking at stable isotope analysis of cereal grains.

(Part one available here)

Sarah Jane Haston.
Sarah Jane Haston.

Stable isotope analysis of the cereal grains from the Ness of Brodgar will increase our understanding of past agricultural systems by directly investigating soil growing conditions. Carbonised cereal grains will preserve their isotopic composition following deposition (Fraser et al., 2013), reflecting the values of carbon and nitrogen of the soils in which they were cultivated.

Measurement of the nitrogen stable isotope (δ15N) values will indicate if any organic matter such as manure, household midden, seaweed or fish remains were added to enhance the fertility of the soil (Styring et al., 2022; Bishop et al., 2022). This has implications for the nature of the cultivation practices undertaken (permanent and intensive or low intensity/ shifting cultivation) and the integration of animal husbandry practices with the arable economy (Bogaard, 2012).

Manuring requires additional labour (gathering, transport and spreading) compared to plots which are only cleared and sowed. The nutrient benefits of the added materials might not be available for uptake in the next growing season but over following growing seasons showing that a long-term investment is being made in the land (Bogaard, 2012).   

High levels of soil enrichment through manuring would be shown by higher cereal δ15N values in the medium to high ranges (3-9‰) and these values reflect the frequency and intensity of manuring (Bogaard et al., 2013). Measurement of carbon stable isotope values in the grain (δ 13C) will reflect soil moisture (Bogaard et al.2013) and the degree of shading from vegetation around the plots (Styring et al., 2017; Bishop et al 2022). It thus provides information on the environmental conditions in the plots and the degree of variation in growing conditions, for example the level of rainfall as well as the soil conditions.

For example, similar isotopic compositions of different grains of the same or different species within archaeobotanical assemblages could suggest if they were grown in the same field (Styring et al., 2022). Analysis of the differences in the nitrogen levels of different crop species within the archaeobotanical assemblage from Structure Eight will give further evidence on how, and where, crops were produced in the landscape. This might have implications for understanding of the social provisioning of the site, such as whether the grain was grown locally or imported from different areas.

Any changes in the nitrogen levels through the chronology developed by the C14 dating could provide valuable information on changes in cultivation strategies. This will add to recent work by Rosie Bishop and others (2022) on early farming practices in mainland Scotland and Orkney which shows cultivation strategies vary greatly between different landscape settings (Bishop et al. 2022).

Selection of samples for isotopic analysis

With the assistance of Dr Rosie Bishop and Dr Scott Timpany, I selected suitable samples with well-preserved grains. I took these samples to the University of Durham, where I learnt to prepare individual cereal grains for isotopic analysis, with help from Professor Mike Church, Professor Darren Gröcke, and Masters student, Joanna Iosifidi.

Using a stereomicroscope, the individual grains were assessed and cleaned of any sediment and rootlets by gentle scraping with a clean scalpel. It is important the grains are as clean as possible for the isotopic analysis so that material homogenised in a pestle and mortar, is purely from the grain.

The individual grains are described it is noted whether they were symmetric or asymmetric grains. Asymmetric (twisted) grains indicates that six-row barley is present, whereas symmetric grains can come from six-row or two-row barley. The individual grains were measured in millimetres (mm) on the x, y and z axes. On grains that had the embryo remaining, the length along the x axis was measured with and without the embryo. The mass of the individual grains was weighed and recorded in grams (g).

External preservation was assigned to a preservation class, according to surface survival and distortion of the individual grain between P1 and P6 (Figure 1). Grains selected should both be of preservations classes P1 and P2, if that is possible because those lesser categories (P3-P6) are likely to have been fired at a higher temperature and will present a glassy or vitrified appearance, and have a less dense matrix (Stroud et al., 2023). To assess the internal preservation, the individual grains were cut in half with a scalpel and compared with examples of grain with good, bad, or borderline characteristics used by Stroud and others (2023).

Figure 1: (a) and (b) Structure Eight cereal grain being recorded and (c) cereal grains ready for isotopic analysis
Figure 1: (a) and (b) Structure Eight cereal grain being recorded and (c) cereal grains ready for isotopic analysis.

Stable isotope measurements of the samples were undertaken in the Stable Isotope Biogeochemistry Laboratory (SIBL) at Durham University using a Costech Elemental Analyser (ECS 4010) connected to a Thermo Scientific Delta V Advantage isotope ratio mass spectrometer.

It will be really interesting to see how these techniques will contribute to the understanding of the site, and by extension the wider Neolithic communities.

Thanks to the Ness of Brodgar Trust for their continued support in my research. Thanks also to the FRONTIERS research group at the University of Stavanger for funding the stable isotope analyses and to Professor Mike Church, Professor Darren Gröcke, and Joanna Iosifidi for their assistance with undertaking the stable isotope analysis of the grain.


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