The Compilation of a British Lowland Heathland and Agricultural Grassland Phytolith Reference Database and its application at the archaeological site of Wytch Farm, Poole Harbour, Dorset.

Abstract

Phytoliths are inorganic, microscopic silica bodies formed within and between living plant cells. They can be used to identify plants to different taxonomic levels and have been utilised to address archaeobotanical and palaeoecological questions in multiple regions of the world. Phytoliths are rarely utilised as a proxy within British archaeology and that makes this project unique as it explores how phytoliths can be incorporated into archaeobotanical research frameworks. The project attempts to do so by building a foundation with the creation of a habitat related photographic phytolith database. The aim of this project was to compile the photographic phytolith reference database for comparative purposes and then apply the database and additional proxies as an analytical tool on a Late Anglo-Saxon archaeological site, Wytch Farm. The site investigated at Wytch Farm is a small promontory jutting out into the Poole Harbour area, Isle of Purbeck, Dorset, southwest England. The archaeological excavation at Wytch Farm had revealed salt working hearths and brine tanks. Three British Isle habitats, agricultural grassland, lowland heathland, and an experimental agricultural field, were used for the phytolith reference collection. Plants collected from these habitats in late autumn led to the compilation of a website that shows the plant’s phytoliths (httms://phynd.online). The website and phytolith soil analysis conducted were then combined to interpret the 1000-year-old soil deposit (baulk), accumulated, and overlying the late Anglo-Saxon saltern site. Additional proxies were used: pollen analysis, geochemical analyses using portable x-ray spectroscopy (pXRF), magnetic susceptibility, soil pH and loss on ignition. The combined analysis has led to methodological observations on the extraction of phytoliths from British native plants and an interpretation of the Wytch Farm soil deposits and anthropogenic use of the site during and beyond the Late Anglo-Saxon period. As the baulk contains modern A horizon soil samples from across the field, an experiment was conducted at the Wytch farm site to establish the link between the site’s vegetation and the phytoliths detected in the modern A horizon soil. This investigation did not prove successful, and the results did not improve when pollen data was added. Most of the archaeological soil samples analysed contained many single phytoliths. The analysis suggested that due to a lack in multicell phytolith numbers in all soilsamples the identification to plant species is difficult. The dry ashing process of the modern plants used for the reference data base showed that plants did produce multicells. The pXRF results showed that silica is available in the Wytch Farm soil, and therefore multicells should be expected within the soil samples. A possible answer to this discrepancy might be a weaker silicification of phytoliths due to low transpiration rates within a temperate climate zone. This means that post depositional taphonomic conditions account for the lack of multicelled forms preserved in the archaeological samples. Due to the lack in multicells the analysis used the available single phytoliths and concentrated on a broader interpretation related to plant parts. This application of single phytolith counts at the Wytch Farm site over time and through different contexts (core samples and micromorphology block subsamples) together with the other environmental proxies pointed to a site that since Late Anglo-Saxon times has been used for agricultural and industry related activities (salt production and metal working) and did not revert back to a lower heathland landscape over that 1000-year time period. Agricultural practices that could be inferred were grazing and cereal production. There was clear indication for a water edge clearing event right at the start of the saltern production site and the presence of repeated flooding events for the lowest soil strata. A rise in burnt phytoliths in certain areas together with the magnetic susceptibility results seems to indicate areas with high temperature burning processes. Using phytoliths and the accompanying phytolith database has proved a successful proxy at a site where, apart from pollen and charcoal, no other organic remains could be detected due to the acidic and well aerated nature of the soil.