Revisiting the potential of carbonized grain to preserve biogenic ⁸⁷Sr/⁸⁶Sr signatures within the burial environment

Abstract

Strontium (Sr) isotope analysis of archaeological crops is a potential method of provenancing and identifying the movement of crops in the past, but there remains uncertainty about whether original 87 Sr/ 86 Sr values can be obtained from carbonized buried grains. We have determined that hydrochloric acid (HCl) leaching removes some, but not all, exogenous Sr from carbonized cereal grains buried in soil for up to one year. We conclude that while further work could refine the leaching method, Sr isotope analysis of archaeological cereal grains can distinguish crops sourced from outside a particular (e.g., local) area if it can be shown that leaching moves grain 87 Sr/ 86 Sr values significantly away from the expected Sr signature.

Strontium isotope evidence for a trade network between southeastern Arabia and India during Antiquity

“…It should be noted that the textile samples were leached successively in HF and HCl to remove burial contaminants (see “Materials and methods” for details). As all (n = 7) bar one of the acid leachates are less radiogenic than the residual textiles (Supporting Information, Table S2 ), should the leaching procedure have been unsuccessful in entirely removing any exogenous strontium, the incomplete decontamination would result in an underestimation of cotton grown outside of the local area 46 , 47 . This is because burial contamination would skew the strontium isotope signature towards that of the local region in which they were found.…”

“…The entire acid digestion process and subsequent Sr purification were achieved under a class 100 laminar flow hood in a class 1000 clean room. Exogenous Sr was removed from carbonized cotton seeds by leaching them in 5 ml 6 M HCl for 24 h, an adapted procedure of that found to be the most effective of several different leaches at removing some but not all contaminants 46 . The solutions were centrifuged, the supernatant removed and the leached seeds were rinsed in ultrapure Milli-Q water three times and dried.…”

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

“…Soluble Sr is taken up by plants from the soil and hence reflects the geology on which the plants are growing. 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).…”

“…Desiccated remains should not present a problem, as long as dust particles are removed prior to analysis, as they have not been exposed to Sr in solution. For all other cases, however, pre-treatments are required, as it is clear from studies carried out on human and animal remains as well as charred grains that pre-treatment is required to remove depositional Sr contamination (Sillen and LeGeros, 1991;Budd et al, 2000;Benson et al, 2010;Snoeck et al, 2015;Styring et al, 2019).…”

“…Recent stable carbon and nitrogen isotope analyses of charred grain, however, highlighted possible issues with the use of ABA as it removed large amounts of material (Vaiglova et al, 2014), which is problematic when only small samples are available, as is often the case with archeological materials. 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).…”