Homogeneous Glacial Landscapes Can Have High Local Variability of Strontium Isotope Signatures: Implications for Prehistoric Migration Studies

Abstract

Increasingly, strontium (Sr) isotopes are used to distinguish locals and migrants in prehistoric studies, by measuring 87Sr/86Sr in human remains and comparing these values to the distribution of the bioavailable 87Sr/86Sr in the study area, often in surface water. However, it has recently been shown that agricultural lime can have a substantial impact on the 87Sr/86Sr ratio and strontium concentration in surface water in areas where soils are low- to non-calcareous. Agricultural lime is rich in strontium with low 87Sr/86Sr ratios, such that interpretations of prehistoric migration based on surface waters affected by agricultural lime often overestimate the number of migrants in a given area. However, the impact of agricultural lime was questioned in a new study, which argues that strontium derived from agricultural lime is retained in the topsoil of the fields and therefore do not contaminate the surface water. In the present study and in a companion study in this volume, we show that strontium derived from agricultural lime is highly mobile in soils, and so contaminate surface waters extensively. We also show that the 87Sr/86Sr ratios are consistently higher in waters from “pristine areas” (where no agricultural lime has been applied within a distance of 150 m from the sample locality) than in water from farmland, thus confirming that it is of vital importance for accurate mapping of isoscapes to avoid sampling waters contaminated by agricultural lime. Our new measurements of 87Sr/86Sr ratios in central Jutland, Denmark, raise the highest measured values to 0.7186. High values between 0.7140 and 0.7156 occur repeatedly and it is apparent that nearly all prehistoric human finds in Jutland, previously believed to have journeyed from afar are more likely of local origin. Furthermore, we show that carbonate-rich areas along the coast of southwest Zealand carry high 87Sr/86Sr values (0.7112–0.7132), where we would expect low values. This surprising result indicates that nearly all humans buried at the Viking Age site, Trelleborg could well have originated locally, in contrast to past studies, which have suggested that about 50% of the burials were of individuals who came from afar.

The forest effect: Biosphere 87Sr/86Sr shifts due to changing land use and the implications for migration studies

“…Frei et al (2015) argue that the elevated 87 Sr/ 86 Sr measured from this adolescent female (up to 0.7155) exclude her from having an origin on mainland Denmark based on current mapping Frei, 2011 andFrei et al, 2022). However, it has been suggested that the biosphere map of Denmark, which is based on surface waters, was contaminated with agricultural lime (Thomsen and Andreasen, 2019;Thomsen et al, 2021). These authors argue that surface waters at a "pristine" forest site, provide sufficiently high 87 Sr/ 86 Sr to account for the human values obtained.…”

“…They concluded that any animals eating plants from, or drinking from water sources that percolate through, agricultural land applied with fertilisers, would have the same negligible effect on their skeletal strontium values. Conversely, Thomsen and Andreasen (2019) and Thomsen et al (2021) have more recently suggested that the biosphere map of Denmark was based on samples already contaminated with agricultural lime. Sr-isotope analyses of tree cores from agricultural land in Ireland and a forest in Germany have shown a change of ±0.0003-0.0016 over the last century, these shifts being compatible with anthropological influences such as the commencement of the application of modern commercial fertilisers (Crowley, 2015), forest liming and/or mining activities (Maurer et al, 2012).…”

Paleodemography: From archaeology and skeletal age estimation to life in the past

“…What a nonlocal signature really indicates about physical distance has given rise to misleading interpretations of lifetime movement (Frei et al, 2015; cf. Thomsen & Andreasen, 2019; Thomsen et al, 2021). Physical distance is also not the same as social distance when identifying the population one believes is being sampled.…”

Problems with Strontium Isotopic Proveniencing in Denmark?

“…These are (a) Sr derived from pre-Quarternary carbonaceous sediments ( 87 Sr/ 86 Sr = 0.7078-0.7082) and (b) Sr derived from a radiogenic component in Pleistocene glaciogenic soils with Precambrian granitoid components ( 87 Sr/ 86 Sr> 0.712). Apparently, Thomsen et al (2021) have also discovered this mixing of different sources of strontium iso-topes. This seems to be exactly the case in Djursland and is likely the situation throughout most of Denmark.…”