Developing pXRF soil analysis of preservation at Vindolanda

Student thesis: Doctoral Thesis

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 promoted Methylophilus to produce vivianite rather than alternative Fe species. Overall, elemental analysis rapidly distinguished zones that corroborated with layers of preservation and vivianite formation at Vindolanda, and was enhanced with microbial analysis to show potentially fundamental parameters of vivianite formation.

pXRF of Anthropogenic Activity
This thesis is accompanied with a case study at Boroughgate, Skelton UK to explore the potential for identifying anthropogenic activity zones with pXRF. Spatial clustering guided elemental heat maps to distinguish internal and external areas using Al, P, K, Ca and Fe, clean and refuse areas using Al, P and Ca, and waste accumulation zones using Mg, P, K, Mn and Fe. These zones corroborated well with locations of artefacts and the archaeological interpretations. Overall, pXRF has excellent capacity for visualising and interpreting the elemental composition of excavations for routine archaeological practice.
Date of Award19 Feb 2021
Original languageEnglish
Awarding Institution
  • Teesside University
SupervisorGillian Taylor (Supervisor), Tim Thompson (Supervisor) & Caroline Orr (Supervisor)

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