Isotope domain mapping of 87 Sr/ 86 Sr biosphere variation on the Isle of Skye, Scotland

Abstract: The potential of 87 Sr/ 86 Sr isotope composition for tracking and determining the origin of material, whether it is humans, animals, water or wine can be fully realized only if high-quality reference datasets are available against which to compare the unknowns. This is currently not the case. Despite the rapid increase in 87 Sr/ 86 Sr-based provenance studies of biosphere materials there are no well-documented maps available. Most researchers resort to generating reference maps from a variety of geological datasets plus the analysis of sparse ’environmental' samples. A major reason for this lack of reference material is the investment needed to undertake systematic sampling and analysis over large areas. A further problem is associated with the choice of proxy material, as organisms differ in their habitat and exploitation of resources. In this paper various approaches to isotope biosphere sampling are considered, some of the problems are assessed, the precision required for the study of human populations is discussed, and a first attempt is made to condense all of the available information into an ’isotope package' map. The work is based on a study undertaken on the Isle of Skye, in the Inner Hebrides, and reduces the complexity of geological and isotope data into five main 87 Sr/ 86 Sr packages: Package 1 (northern area) = 0.705–0.7092; Package 2 (NE coast) = 0.7082–0.7102; Package 3 (granitic rocks) = 0.716–0.720; Package 4 (metamorphic rocks) = 0.7092–0.7188; Package 5 (seawater) = 0.7092 ± 0.005% (2σ). The approach of trying to constrain the isotope composition of certain areas and/or lithologies offers the possibility of extrapolating to other similar areas without having to sample at the original density.