Determining the Sr isotopic composition of waterlogged wood – Cleaning more is not always better

Abstract

Sr isotopes represent a potential means to trace the provenance of archaeological timber. Such tracing allows us to examine the transport, by past people, of wooden objects and of wood and timber as a raw material. However, issues exist with the mobility of Sr and addition of exogenous Sr during waterlogging. This paper presents a systematic assessment of cleaning methods to remove exogeneous Sr from waterlogged wood. Neither a large number of Milli-Q washes, a combination of MQ and Hydrofluoric acid (HF) or alpha-cellulose extraction were able to retrieve the original signature. It was also shown that ashing leads to higher uncertainties due to the smaller amount of Sr available for analysis, this method will only be really useful when large samples are available, is not recommended for small archaeological samples. Our studies also highlight that the distribution of Sr in waterlogged wood is highly heterogeneous.

Testing Various Pre-treatments on Artificially Waterlogged and Pitch-Contaminated Wood for Strontium Isotope Analyses

“…The samples pre-treated with acetic acid without prior crushing or with HCl provide unsatisfactory results. The alpha-cellulose extraction method applied to experimentally water-logged oak samples by Van Ham-Meert et al (2020), modified from Andreu-Hayles et al (2019) to avoid equilibration between wood samples, also involves treatment with acetic acid. This method led to what the authors termed "enigmatic results, " proposing a possible explanation the presence of various Sr reservoirs within the wood (in alphacellulose, lignin, and beta-cellulose, etc.)…”

“…While a large number of studies have focussed on measuring Sr isotopes in archeological human and animal remains (e.g., Price et al, 2006;Bentley, 2013;Laffoon et al, 2014) or charred/carbonized grains (e.g., Benson et al, 2010; though not always successfullysee Styring et al, 2019), as well as on modern plants to establish a biologically available Sr baseline (e.g., Evans et al, 2010;Snoeck et al, 2016Snoeck et al, , 2020, less has been done with archeological wood remains. Exceptions include a study of the well-preserved desiccated structural timbers at Chaco Canyon (English et al, 2001), and of desiccated prehistoric willow and tule textiles in the Great Basin (Benson et al, 2006), as well as some more recent work on pre-Columbian wood sculptures from Florida (Ostapkowicz et al, 2017a) and Trinidad (Ostapkowicz et al, 2017b), waterlogged shipwrecks (Rich et al, 2016;Hajj et al, 2017;Van Ham-Meert et al, 2020), and South American/Lesser Antillean wooden clubs from museum collections (Ostapkowicz et al, 2018). Little targeted research has focused on pretreatments for archeological wood.…”

“…In waterlogged contexts and waterlogging experiments, 87 Sr/ 86 Sr measured on shipwrecks showed the impact of water on the Sr isotopic composition of the wood (Rich et al, 2016;Hajj et al, 2017;Van Ham-Meert et al, 2020). Two Mediterranean shipwrecks were investigated by Rich et al (2016): the Bronze Age Uluburun shipwreck and the Athlit Ram galley from the Hellenistic period.…”

“…Further work using HCl (6 M at room temperature) to remove exogenous soil Sr from charred grain was only partially successful (Styring et al, 2019). Recently, various pretreatments including successive MilliQ TM washes, a combination of HF and MilliQ TM and alpha-cellulose extraction (which is used in oxygen isotope studies) were tested as means to remove exogeneous Sr from experimentally waterlogged oaks (Van Ham-Meert et al, 2020). However, none of these methods were successful, the different procedures removed both endogenous and exogeneous Sr and did not succeed in retrieving the original Sr isotopic composition.…”

“…However, none of these methods were successful, the different procedures removed both endogenous and exogeneous Sr and did not succeed in retrieving the original Sr isotopic composition. A sample from a shipwreck was also included in this study and behaved similarly (Van Ham-Meert et al, 2020).…”