Strontium isotopes (⁸⁷ Sr/⁸⁶ Sr) in terrestrial ecological and palaeoecological research: empirical efforts and recent advances in continental-scale models

Abstract

Strontium (Sr) isotope analysis can provide detailed biogeographical and ecological information about modern and ancient organisms. Because Sr isotope ratios ( Sr/ Sr) in biologically relevant materials such as water, soil, vegetation, and animal tissues predominantly reflect local geology, they can be used to distinguish geologically distinct regions as well as identify highly mobile individuals or populations. While the application of Sr isotope analysis to biological research has been steadily increasing, high analytical costs have prohibited more widespread use. Additionally, accessibility of this geochemical tool has been hampered due to limited understanding of (i) the degree to which biologically relevant materials differ in their spatial averaging of Sr/ Sr ratios, and (ii) how these differences may be affected by lithologic complexity. A recently developed continental-scale model that accounts for variability in bedrock weathering rates and predicts Sr isotope ratios of surface water could help resolve these questions. In addition, if this 'local water' model can accurately predict Sr/ Sr ratios for other biologically relevant materials, there would be reduced need for researchers to assess regional Sr isotope patterns empirically. Here, we compile Sr/ Sr data for surface water, soil, vegetation, and mammalian and fish skeletal tissues from the literature and compare the accuracy with which the local water model predicts Sr isotope data among these five materials across the contiguous USA. We find that measured Sr isotope ratios for all five materials are generally close to those predicted by the local water model, although not with uniform accuracy. Mammal skeletal tissues are most accurately predicted, particularly in regions with low variability in Sr/ Sr predicted by the local water model. Increasing regional geologic heterogeneity increases both the offset and variance between modelled and empirical Sr isotope ratios, but its effects are broadly similar across materials. The local water model thus provides a readily available source of background data for predicting Sr/ Sr for biologically relevant materials in places where empirical data are lacking. The availability of increasingly high-quality modelled Sr data will dramatically expand the accessibility of this geochemical tool to ecological applications.

Using Carbon, Oxygen, Strontium, and Lead Isotopes in Modern Human Teeth for Forensic Investigations: A Critical Overview Based on Data from Bulgaria

“…However, in contrast to C and O, the ultimate source of Sr in a given region is the local bedrock and therefore can be directly related to a geographical location. Recent work by Crowley et al showed that Sr isotopes in surface water, soil, vegetation, mammals, and fish across the continuous USA are generally close to the expected Sr isotopes based on the local water model. Although Sr geo‐referencing can be useful in modern ecological and archeological studies (e.g., ), the increasing globalization of the modern food supply is making 87 Sr/ 86 Sr less variable in modern human populations .…”

“…Recent work by Crowley et al showed that Sr isotopes in surface water, soil, vegetation, mammals, and fish across the continuous USA are generally close to the expected Sr isotopes based on the local water model. Although Sr geo‐referencing can be useful in modern ecological and archeological studies (e.g., ), the increasing globalization of the modern food supply is making 87 Sr/ 86 Sr less variable in modern human populations . In particular, Keller et al stated that the available Sr isotope data for modern US teeth show very little correlation with expected regional differences.…”

“…For example, model‐based Sr isotopic maps are available for continental USA . Although these model maps provide useful information for modern ecological works , Keller et al showed that the available US tooth enamel data do not correlate with the expected Sr isotope variations across the continental USA. This is likely because the modern human Sr in the USA contains a major component that is independent of local Sr sources .…”

Strontium and nitrogen isotopic evidence of food import to Tell Ashara–Terqa, a Bronze Age city on the Euphrates, Syria

“…Although strontium sources are not completely homogenous in most environments, since more than 30 years (Ericson, ), 87 Sr/ 86 Sr values are widely used in ecology and bioarchaeology for reconstructing mobility of animals and humans (Crowley, Miller, & Bataille, ). Within archaeology, 87 Sr/ 86 Sr values are compared between early developing tissues (e.g., enamel) and the local environments where human bodies were buried, or tissues composed not long time prior to death of an organism (e.g., bone areas with high turnover rate).…”

Multi‐isotope provenancing of archaeological skeletons including cremations in a reference area of the European Alps

“…The laboratory processing protocol for the measurement of 87 Sr/ 86 Sr in archaeological skeletons has been long established, and the isotopic variability per site well exceeds the isotopic variability of local individuals (±0.0003), with only a few exceptions . Moreover, it has been shown recently that the strontium isotopic signature of local water instead of soil leads to a far better predictability for the respective signature in the bones of large vertebrates. Therefore, we conclude that the measured 87 Sr/ 86 Sr isotopic ratios are also free of contaminants.…”