The contamination of in situ archaeological remains: A pilot analysis of microplastics in sediment samples using μFTIR.

Background: Microplastics (MPs) are found in all environments: aquatic, airborne, and terrestrial. While their presence is not disputed, their potential impacts are not yet known.

Objective: To undertake a pilot analysis of MP contamination in archaeological sediment samples, taken in the late 1980s from two archaeological excavation sites in the historic city of York (UK) as well as contemporary sources close to the same sites, with respect to the presence (if any), levels, and characteristics of any particles identified.

Methods: This study analysed pre-digested sediment samples as follows: n = 3 from Queens Hotel (QH) site and n = 3 Wellington Row (WR) contemporary core-source, and n = 3 QH and n = 3 WR archival-source samples, alongside procedural controls (n = 8), using μFTIR spectroscopy (size limitation of 5 μm) to detect and characterise any MPs present.

Results: In total, 66 MP particles consisting of 16 MP polymer types were identified across both site and contemporary/archived samples. The highest levels of MP particles, 20,588 MP/kg was identified at the lowest sample depth (∼7.35 m) at archived WR, 5910 MP/kg in the mid depth layer (∼5.85 m) at the contemporary QH site. Of the MPs detected in sediment samples overall, polytetrafluoroethylene (PTFE), polybutylene sulfone (PSU), and polypropylene: polyethylene (PE:PP) copolymer polymer types were most abundant; mainly fragmented and irregular shape.

Conclusions: This is believed to be the first evidence of MP contamination in archaeological sediment (or soil) samples with polymers and size ranges measured and while accounting for procedural blanks. These results support the phenomenon of transport of MPs within archaeological stratigraphy, and the characterisation of types, shapes and size ranges identified therein. Through contamination, MPs may compromise the scientific value of archaeological deposits, and environmental proxies suspended within significant sediment, and as such represent a new consideration in the dynamism of, as well as arguments for preserving, archaeological deposits in situ.