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

Abstract

Strontium isotope ratios (87Sr/86Sr) are commonly used in archeological and forensic studies to assess if humans and fauna are local to the place they were found or not. This approach is largely unexplored for wooden artifacts recovered in archeological contexts, as wood – in the rare instances it does survive – is often poorly preserved. One of the most common ways wood is preserved is through the anoxic conditions found in waterlogged contexts. A more unusual form of preservation is through submergence in natural pitch. These depositional media contribute their own strontium values to the in vivo87Sr/86Sr wood values, which needs to be removed prior to analysis. Here we test several pre-treatment methods to remove potential strontium contamination from wood samples that were artificially immersed in seawater and pitch from Trinidad’s Pitch Lake. Water rinses and acid-leaching tests were carried out with hydrochloric acid and acetic acid to remove exogenous strontium from experimentally waterlogged wood. These tests removed large amounts of strontium from the samples and did not enable the recovery of the endogenous 87Sr/86Sr signal. For samples artificially immersed in pitch, the pre-treatments tested were based on radiocarbon dating procedures and carried out with and without the aqueous-based acid-base-acid (ABA) step. The use of organic solvents alone (methanol and toluene) removed exogenous strontium originating from the pitch. However, the ABA step eliminates large amounts of in vivo strontium from the samples. These tests show that 87Sr/86Sr values of wood are altered by the presence of pitch and water. With adequate pre-treatment using exclusively organic solvents, it may be possible to remove this contamination for samples immersed in pitch. However, the aqueous-based ABA pre-treatment should be avoided. The removal of contamination from waterlogged samples was unsuccessful with the current pre-treatment protocols and more research is needed. More importantly, and unexpectedly, 87Sr/86Sr values may extend outside of the mixing line between the wood’s endogenous strontium and the water. These results indicate the need for extreme caution when attempting to determine the provenance of waterlogged wood.

Provenancing 16th and 17th century CE building timbers in Denmark–combining dendroprovenance and Sr isotopic analysis

“…If an indication of geological origin for these trees could be attained through Sr analysis, the issue of which of the dendrochronological groups are Swedish timber could be solved. Extensive tests for Sr isotopic analysis of timbers from shipwrecks and other waterlogged wood were carried out in this project, and by others [30][31][32][33]. Unfortunately, Sr isotopic analysis of waterlogged wood has proven impossible [30][31][32][33].…”

“…Obtaining a signature comes with challenges in the form of exogenous material. As has been shown recently, Sr isotopic analysis of waterlogged wood is not possible due to the influence of Sr present in the waterlogging environment which cannot be removed [ 31 , 32 ]. This seriously diminishes the potential material available for Sr isotopic study of wood.…”

Dating Coastal Archaeological Wood From Pingusugruk (15th–17th CE), Northern Alaska:

“…Strontium 87Sr/86Sr ratios in wood are largely dependent on the geological substrate and, thus, can be characteristic of a tree's growth locality (English et al 2001;Kagawa & Leavitt, 2010;Rich, 2013;Rich et al 2015;Hajj et al 2017). However, recent research has challenged the use of strontium geochemical isotopes to determine the biogeographic origin of wood that has been in contact with seawater (Van Ham-Meert et al 2020;Snoeck et al 2021), as in the case of shipwrecks (Rich et al 2016;Hajj et al 2017), and potentially Alaskan archaeological coastal wood. As seawater has its own 87Sr/86Sr signature, wood that has been in its contact can have its 87Sr/86Sr signature contaminated and modified within a few weeks/days (Hajj et al 2017;Snoeck et al 2021).…”

“…However, recent research has challenged the use of strontium geochemical isotopes to determine the biogeographic origin of wood that has been in contact with seawater (Van Ham-Meert et al 2020;Snoeck et al 2021), as in the case of shipwrecks (Rich et al 2016;Hajj et al 2017), and potentially Alaskan archaeological coastal wood. As seawater has its own 87Sr/86Sr signature, wood that has been in its contact can have its 87Sr/86Sr signature contaminated and modified within a few weeks/days (Hajj et al 2017;Snoeck et al 2021). As of today, there is no chemical protocol or pre-treatment that can reliably recover the original 87Sr/86Sr signature (endogenous) of a tree that has been in contact with seawater.…”

“…As of today, there is no chemical protocol or pre-treatment that can reliably recover the original 87Sr/86Sr signature (endogenous) of a tree that has been in contact with seawater. As a result, it is not yet possible to confidently accept provenance results (Van Ham-Meert et al 2020;Snoeck et al 2021).…”