Tracing the provenance of Tuscan Extra Virgin Olive Oil using Sr isotopes and Rare Earth Elements

A promising method to trace the geographical provenance of products of the agro-food sector consists in using petro-chemical tracers derived from the geological substratum of the production area. The radiogenic isotope 87Sr is one of the most robust tracer used. Indeed, Sr is ubiquitous in rocks and soils at ppm levels. Its isotopic composition depends on the parent/daughter ratio (87Rb/86Sr), the age and the origin of the bedrock. Another possible geochemical tracer is represented by REEs (Rare Earth Elements) patterns, which varies depending on the geological and petrological setting. Pelacani et al. (2017) showed that the REEs and the 87Sr/86Sr are absorbed from soil and transferred to the olive pulp of Olea Europea, without any significant elemental and isotopic fractionation, meaning that 87Sr/86Sr of plants and fruits reflects the 87Sr/86Sr ratio of the bioavailable soil fraction. In order to assess if Sr isotopes and REE patterns can be used as geographical tracers also for olive oil, we investigate sixteen olive groves from different geological and geomorphological setting of Tuscany, located in Chianti, High Tiberina Valley and Maremma. Particular attention was paid to minimize local differences in climate, topography, and cultivar. Samples of soil, olives and extra virgin olive oil (EVOO) were collected from each olive grove. The main challenge is represented by the low content of REEs and Sr in EVOO, especially for Heavy REEs with concentrations < 4 pg g-1. We set up in our lab a procedure to extract REEs and Sr from olive oil and olives involving mechanical stirring and ultrasound assisted extraction (Turk et al., 2019), followed by Sr purification using standard chromatographic techniques. 87Sr/86Sr measurements in soil, olives and olive oil were performed by TIMS, while the REE content was determined by ICP-MS. Preliminary results suggest that this analytical procedure is able to concentrate more Sr and REEs from EVOO, allowing more precise measurements.