Strontium isotopes in biological material: A key tool for the geographic traceability of foods and humans beings

In this contribution we will discuss the variation of the 87Sr/86Sr isotope ratio in the solid Earth and its link to that of biological material. The great potential of 87Sr/86Sr is that it represents a ratio between two isotopes of the same element, hence with identical geochemical behaviour. However, while 86Sr is stable, 87Sr is radiogenic being the product of the radioactive decay of 87Rb, an isotope of a different element, hence with different geochemical behaviour. Long lived isotope ratios of heavy elements of geological interest, such as 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb have gained importance in the last decades for assessing the issue of traceability in food, archaeological, environmental, medical and forensic sciences (Kelly et al. 2005; Podio et al. 2013; Voerkelius et al. 2010). These new lines of application stem from the extensive and successful use of such radiogenic isotopic ratios in several 'classic' field of the Geoscience, as precious tools for Geochronology (in both terrestrial and meteoritic samples) and for tracking petrological, geological and environmental processes (Horn et al. 1993; Capo et al. 1998; Tommasini et al. 2000). This chapter is focused on strontium isotopes and presents a review aimed at describing the variety of pathways and different source materials that characterize the cycle of strontium in the solid Earth and in the biological systems. First, we describe how strontium isotopes are used in geochronology and how 87Sr/86Sr differs in the major Earth's reservoirs and rock types, then how strontium isotopes are transferred to weathered materials (i.e., soils), plants, and animals, and eventually we highlight a number of examples pointing to the added value of strontium isotopes in the abovementioned studies.