Phosphatisation of seeds and roots in a Late Bronze Age deposit at Potterne, Wiltshire, UK

Abstract

The Late Bronze Age ‘midden-type deposit’ at Potterne, Wiltshire, yielded a diverse assemblage of seeds and roots preserved in calcium phosphate. The majority of seeds comprise bare embryos but a small number of taxa also preserve mineralised seed coats. Flash pyrolysis–gas chromatography/mass spectrometry (py-GC/MS) of examples of comparable living seeds revealed a significant variation in seed coat composition. The coats of several species analysed contain tannins and protein instead of, or in addition to, lignin and cellulose. The preferential phosphatisation of Urtica urens seed coats is attributed principally to their lack of lignin, although their thin undifferentiated structure may have allowed them to decay more rapidly than the more complex coats of the other seed taxa. The apparent bias towards mineralisation of dicotyledonous versus monocotyledonous roots at Potterne is attributed mainly to the greater robustness of dicot roots, allowing them to persist in the soil long enough for phosphatisation to take place.