Developing pXRF soil analysis of preservation at Vindolanda

Abstract

Background
Vindolanda is a Roman auxiliary fort on the Frontiers of the Roman Empire which sees excellent preservation of delicate artefacts that reform our understanding of life in Roman Britain. The slightly acidic, waterlogged soils produce anoxic conditions with P, S and Fe available for the formation of vivianite, an iron(II) phosphate mineral associated with preservation. Elemental analysis enhances archaeological interpretations via mapping, preservation analysis and zoning of anthropogenic activity. This thesis investigated whether elemental analysis with portable X-ray fluorescence (pXRF) can enhance the understanding of preservation at Vindolanda.

pXRF Method Development
Three standard preparations were compared: in‐situ (no preparation), in‐field (analysing soil through plastic bags) and ex‐situ (laboratory‐based preparation). Influential factors of calibration parameter, moisture, homogeneity, sieve size and soil type were also systematically investigated. The in-field method substantially reduced elemental concentrations and skewed proportional distributions. The ex-situ method significantly increased elemental concentrations and reduced variation. Matrix effects were accounted for with ex-situ preparation through complete drying, homogenisation, sieving to 2 mm, and storage in XRF cups. This approach ensured sample consistency for trace analysis in different burial layers at Vindolanda.

pXRF at Vindolanda
Two excavations at Vindolanda were analysed using ex-situ pXRF, showing significant interactions between P, S and Fe in vivianite formation layers. Elemental distribution heat maps showed clear preservation zones with increased P, S and Ca, decreased Al and K, and Fe for vivianite production. The aerobic transitional preservation layers had sufficient P, S and Fe but low artefact survival. Supplementary microbial analysis showed abundance of Acidobacteria and Actinobacteria in degenerative layers, whereas anaerobic preservation layers showed abundance of Bacteroidetes, Firmicutes and Proteobacteria. The delicate balance of chemical, biological and environmental mechanisms potentially

Date of Award	19 Feb 2021
Original language	English
Awarding Institution	Teesside University
Supervisor	Gillian Taylor (Supervisor), Timothy Thompson (Supervisor) & Caroline Orr (Supervisor)